A Modern Commentary on W.F. Denning’s, “Telescopic Work for Starlight Evenings (1891).

The monumental work of William F. Denning; "Telescopic Work for Starlight Evenings."

A good quality  modern reprint of William F. Denning’s; “Telescopic Work for Starlight Evenings.”

 

 

 

 

 

 

 

 

 

 

A Work Dedicated to David Gray.

Humility is the fear of the Lord; its wages are riches and honour and life.

Proverbs: 22:4

William Frederick Denning (1848-1931) is not a name that trips off the lips of the modern amateur astronomer. Of all the sky watchers of that era, it is arguably the literary work of The Reverend Thomas William Webb and especially his, Celestial Objects for Common Telescopes, that is most celebrated by amateur astronomers today. But while a great work in its own right, Webb was by no means the only populariser of astronomy in England, nor was he necessarily the most knowledgeable and dedicated to his hobby. That accolade, in this author’s opinion, should be reserved for an obscure Bristolian, who emerged from relative obscurity, in what was the meritocracy of the Victorian astronomical tradition, to pen one of the loveliest treatise on the art of visual observation, both with and without a telescope.

William F. Denning (1848-1931), the doyen of amateur astronomy.

William F. Denning (1848-1931), the doyen of amateur astronomy.

In this essay, we shall explore Denning’s masterful tome: Telescopic Work for Starlight Evenings, first published in 1891, to bring to the modern reader, a distillation of late nineteenth century astronomical knowledge; presented in such a way as to captivate the widest possible audience; both young and old, rich and poor, novice and learned alike. As explained in the preface to the work, the book was conceived at the behest of some of his closest friends, to gather together the best nuggets from his published writings in The Journal of the Liverpool Astronomical Society (of which he served as President in the years 1887-88), The English Mechanic, and The Observatory, among many others.

In Denning’s own words:

The methods explained are approximate, and technical points have been avoided with the view to engage the interest of beginners who may find it the stepping stone to more advanced works and to more precise methods. The object will be realised if observers derive any encouragement from its descriptions or value from its references, and the author sincerely hopes that not a few of his readers will experience the same degree of pleasure in observation as he has done for many years.

No matter how humble the observer, or how paltry the telescope, astronomy is capable of furnishing an endless store of delight to its adherents. Its influences are elevating and any of its features possess the charms of novelty as well as mystery. Whoever contemplates the heavens with the right spirit reaps both pleasure and profit and many amateurs find a welcome relaxation to the cares of business in the companionship of their telescopes on “starlight evenings”.

iv-v

Chapter I:The Telescope, Its Invention and the Development of its Powers

Covering pages 1 through 19.

In this chapter, Denning sets forth his extensive knowledge of the history of the telescope and its development over time. With an engaging writing style, he offers the reader an excellent summary of the key inventions that led to the state of affairs at the end of the 19th century.  Burning glasses, carved into a convex shape, were known to the ancients and were used as magnifying glasses. One such example, Denning informs us, was recovered from the excavations of the ancient Roman town of Pompeii, which met its terrible demise in 79 AD in the aftermath of the eruption of Mount Vesuvius. The Roman writer and philosopher, Pliny the Elder (23-79 AD) also gave mention to globules of glass, which could focus sunlight so intensely that it could ignite combustible material. The development of spectacle lenses from the 13th century onwards is also mentioned, but despite having some grasp of the optical science underlying their prescription, Denning is somewhat perplexed as to why it took so long for their adoption into telescopic devices. That said, he does proffer some tantalising historical titbits that the principle might have been known as early as the fourth decade of the sixteenth century :

Francastor (most probably a one Girolamo Fracastoro) , in a work published at Venice in 1538 states:-

“If we look though two eye lenses, placed the one upon the other, everything will appear larger and nearer.”

pp 4

Denning wryly comments that despite attempts by some fame hungry individuals to claim the invention of the telescope as their own – in particular Galileo Galilei and Simon Marius  – or who pronounced they had ‘figured the principle out’ from basic axiom of physics, it was very likely the case that one of mankind’s most revolutionary devices was very probably elucidated through purely accidental means! Indeed, Denning entertains the notion that the children of the Middleburg spectacle maker, Zachariah Jansen, might have stumbled upon the telescope by placing two spectacle lenses along the line of sight of their eyes, and unwittingly hit on an ingenious way of seeing faraway objects as though they were much closer.

Having said this, Denning appears to align himself with the opinions of many contemporary scholars in  attributing the invention of the telescope to a certain Hans Lippersheim (also known as Hans Lapprey), who was in possession of a simple telescope in 1608. On page 5-6 he refers to a critical piece of research carried out by the professional astronomer, Dr. Doberck, who showed the Lippersheim had applied for a thirty year patent from the Dutch States, in exchange for an annual stipend:

“He solicited the States, as early as the 2nd of October 1608, for a patent for thirty years, or an annual pension for life, for the instrument he had invented, promising then only to construct such instruments for the Government. After inviting the inventor to improve the instrument and alter it so that they could look through it with both eyes at the same time, the States determined on the 4th October, that from every province one deputy should be elected to try the apparatus and make terms with him concerning the price. The committee declared on the 6th October that the invention useful for the country, and they offered the inventor 900 florins for the instrument. He had at first asked 3000 florins for three instruments of rock crystal. He was then ordered to deliver the instrument within a certain time, and the patent was promised him on the condition that he kept the invention secret. Lapprey delivered the instrument in due time. He had arranged it for both eyes, and it was found satisfactory; but they forced him, against the agreement, to deliver two other telescopes for the same money, and refused the patent because it was evident that already several others had learned about the invention.”

pp 5-6

Denning proceeds form here to give an excellent overview of the unwieldy non-achromatic telescopes devised by Huygens, Hevelius, Cassini and Campani, amongst others, who ground and mounted lenses up to 8 inches in diameter with enormous focal lengths (up to 212 feet in focal length) yet all still delivering powers of 150 diameters or less. From here, Denning discusses the development of the much more convenient reflecting telescopes – the Gregorian, Cassegrain, Newtonian and other compound designs – and the problems associated with the construction of metallic mirrors fashioned from speculum metal (an alloy of copper, tin and small amounts of arsenic and/or antimony), which tarnished quickly and were exceedingly heavy in large apertures.

The author also discusses the origin of the Herschelian reflector, which involved tilting the primary mirror so that it reached a focus at the side of the tube without the requirement for a secondary flat mirror. The design, so Mr. Denning informs us, dates to 1728, when Le Maire first presented it to the French Academie des Sciences. Herschel adopted the design to increase the telescopes space penetrating power (light grasp) since it avoided a second reflection and hence saved more light that would otherwise have been lost with the addition of a second mirror. But such a design could not deliver the ‘defining power’ (image quality) of a conventional Newtonian. This is the principal reason why Herschel’s major work on the study of the planets and double stars were conducted with smaller Newtonian reflectors which were much more easy to operate and afforded the greatest degree of ‘mileage’ under the starry heaven.

Denning chronicles the growth in telescopic aperture throughout the 19th century, discussing such telescopes as the 6 foot aperture speculum built by the Third Earl of Ross as well as those used by Lassell and the great Melbourne telescope, which housed a 4-foot diameter (48 inch) speculum metal mirror with a focus of 28 feet. The latter telescope (produced by Howard Grubb of Dublin) was found to have poor defining power but Denning seems to lay the blame squarely with the shoddy mechanical set up of the instrument and not the optician.

The chapter ends with a discussion of the invention of the achromatic doublet by Chester Moor Hall (1729) and John Dollond and its development by Joseph von Fraunhofer, culminating with the creation of the sensational Dorpat Refractor of 9.5 inch aperture, and its state-of-the-art German equatorial mount, which ushered in the age of astrophysics.

Throughout the 19th century, astronomers began to build larger and larger refractors, first in Europe and then in North America, housed in magnificent domes that opened on every clear night to advance our knowledge of the heavens, and culminating with the Great Lick refractor of 36-inch aperture atop Mt. Wilson, California, which saw first light just three short years before the publication of Denning’s book. And while the author was aware that still larger refractors would surely come into existence, he seems more interested in a new technological advance in the production of parabolic mirrors for Newtonian telescopes; enter the silver-on-glass-reflector.

Beginning on page 14 and continuing on page 15, Denning describes the exciting work of the French physicist Jean Bernard Léon Foucault (1819-1868), who published a valuable memoir in which he described an ingenious new method of parabolising a glass disk followed by the deposition of a thin layer of silver upon its surface, and which exhibited much higher reflectivity than metal. It marked the end of the employment of speculum metal in telescope mirrors and ushered in a new age which promised to revolutionise both amateur and professional astronomy.

What is more, Denning informs us that Foucault developed lab-based methods of testing the accuracy of the parabolic surface in such a way as to render traditional testing methods – which involved time consuming and labour intensive trials under the stars – unnecessary. The customer could be assured of the quality of the mirror without it ever having being tested under the stars.

Denning writes:

Silver on glass mirrors immediately came into great request. The latter undoubtedly possess a great superiority over metal, especially as regards light gathering power, the relative capacity according to Sir John Herschel being as .824 to .436. Glass mirrors have also the advantage in being less heavy than those of metal. It is true that silver film is not very durable, but it can be renewed at any time with little trouble or expense.

pp 15.

Mr. Denning gives high praise to two British silver-on-glass mirror makers; George Henry With (1827-1904) of Hereford and George Calver (1834-1927) of Chelmsford, whose reflecting telescopes, ” were found nearly comparable to refractors of the same size.” pp 15.

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Author’s note: Modern scholarship seems to have converged on the name “Lippershey” as one of the earliest constructors of the telescope. Denning refers to the same man as “Lipperheim”. I was once reliably informed that the same chap should correctly be referred to as “Lipperhey”. After attempting to introduce this new nomenclature for a book I wrote, my editor returned it to “Lippershey” lol.

It is amusing that Denning referred to Galileo as “Galilei” to conform with the use of the surname in reference to individuals. Evidently, he thought it odd, which it most certainly is in retrospect.

Some memes are hard to shake.

Denning also points out that the great American refractors had recently employed powers of 3300 diameters in the resolution of the tightest double stars.

Denning was himself a convert to reflectors, after enjoying a fine 4.5 achromatic ( probably of f/15 relative aperture)  for a few years with which he carried out extensive solar work – a job ideally suited to the smaller refractor. In the end though, he sold that telescope in order to purchase a 10-inch With-Browning reflector in 1871 (when he was 23 years old) pictured on page 77 of the book. This telescope, so Denning will inform us, proved far more powerful than his former instrument. Indeed, Telescopic Work for Starlight Evenings is a distillation of twenty years of observations conducted with this same telescope.

The image below, kindly provided me by Denis Buczynski, a prominent member of the BAA, shows a 9.25 inch With-Browning (used by T.W. Webb) on a more sophisticated With-Berthon equatorial mount. But it serves our purposes well in illustrating the working dimensions of the telescope.

Denis Buczynski inspects the With/Berthon reflector ( BAA# 83) at his home in Lancaster.

Denis Buczynski inspects the With/Berthon reflector ( BAA# 83) at his home in Lancaster.

 

 

 

 

 

 

 

 

 

 

The image below pictures Denning beside his 10-inch.

William Denning ( 1848-1931) pictured with his With-Browning reflector on its simple altazimuth mount.

William Denning ( 1848-1931) pictured with his With-Browning reflector on its simple altazimuth mount.

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Chapter II: Relative Merits of Large and Small Telescopes

Covering pages 20 through 37.

Were it not for the vast sea of air that hugs our planet’s crust, the principles of telescopic astronomy would be clear and unequivocal; aperture rules, period. This is the reason for the spectacular success of the Hubble Space Telescope (HST), which, owing to its 2.4 metre primary mirror, has sent back the sharpest images of the heavens ever taken. It is also the reason why the widely anticipated replacement for HST, the James Webb Space Telescope, with its 6.4m segmented beryllium mirror, is expected to completely outclass it when it begins operations in 2018.

Down here on terra firma, the situation is rather more complicated. While there is no substitute for aperture if one wishes to pursue faint fuzzies, there is a great deal of anecdotal evidence that there exist practical limits on aperture in the pursuit of the finest lunar and planetary images. In a nutshell, although larger apertures offer the potential to see finer details, the atmosphere through which the amateur observes, often limits or even negates those advantages. Denning was arguably the first astronomer to raise awareness about this important topic and it was based upon his exceptional experience with instruments of all sizes, as well as his voluminous correspondences with the most active astronomers around the world.

Denning begins the chapter by discussing the rise in the number of large observatory- class instruments that had come to the fore during his lifetime and in past generations. Yet all the while he says:

There are some who doubt that such enormous instruments are really necessary, and question whether the results obtained with them are sufficient return for the great expense in their erection.

pp20

After discussing the realities of large telescopes, including their housing in an observatory, their mounting and maintenance, Denning extols the virtues of smaller instruments and alludes to a quality this author has previously referred to in the past as ‘mileage’:

…..small instruments involve little outlay, they are very portable, and require little space. They may be employed in or out of doors, according to the inclination and convenience of the observer. They are controlled with the greatest ease, and seldom get out of adjustment. They are less susceptible to atmospheric influences than larger instruments, and hence may be used more frequently with success and at places by no means favourably situated in this respect. Finally, their defining powers are of such excellent character as to compensate in a measure for feeble illumination.

pp 20-21

Denning begins with the telescopes of Sir William Herschel. Concerning his 4-foot reflector erected at Slough in 1789, he states that although Herschel discovered two of the inner satellites of Saturn shortly after the instrument was constructed, little else was achieved with it. Denning claims that Herschel much preferred the convenience of a smaller instrument – a 18.5 inch speculum of 20 foot focus in performing his famous sweeps for nebulae. Indeed the 4 foot telescope quickly fell into comparative disuse and his son, Sir John Herschel, had it sealed up for good on New Year’s Day, 1840. For defining power, Denning asserts that the great astronomer allegedly preferred instruments of much smaller size:

He found that his small specula of 7 foot focus and 6.3-in, aperture he had “light sufficient  to see the belts of Saturn completely well, and that here the maximum of distinctness might be much easier obtained than where large apertures are concerned.”

pp 21.

Following on from this, Denning discusses the Great 6-foot aperture telescope erected by the Third Earl of Rosse in Parsonstown (now Birr, Co. Offaly), Ireland. By 1891, this telescope had already been in service for 46 years and thus might provide insights into its relative utility. Denning concedes that it had done important work on elucidating the spiral morphology of many nebulae, M51 being perhaps the finest example. What follows is a fascinating overview of how it behaved. The satellites of Mars had eluded its grasp for three decades, until finally, in 1877, the outer moon, Deimos, was glimpsed twice, yet even then there was so much glare from the planet that no accurate measurements of its orbit were forthcoming. With Jupiter too, its enormous aperture was apparently of little advantage. This seems to be confirmed by a series of drawings made by William Parson’s son, Laurence (1840-1908), in the year 1873, and reproduced on page 128 in Thomas Hockey’s book, Galileo’s Planet: Observing Jupiter Before Photography. They reveal no more detail than could be obtained in a telescope ten times smaller.

Further insight into the efficacy of the Leviathan is gleaned from comments made by the Irish physicist, G.J. Stoney (1826-1911), who regularly used the instrument and who described his impression of γ2 Andromedae in a note made in 1878:

“The usual appearance [ of γ2 Andromedae ] with the best mirrors was a single bright mass of blue light some seconds in diameter and boiling violently.” On the best nights however, “the disturbance of the air would seem now and then suddenly to cease for perhaps half a second, and the star would then instantly become two very minute round specks of white light, with an interval between which, from recollection, I would estimate as equal to the diameter of either of them or perhaps slightly less. The instrument would have furnished this appearance uninterruptedly if the state of the air had permitted.

pp 23

Self evidently, it was not the optical quality of the mirror that was at issue but the environment in which it was placed. This was corroborated by a later observer in charge of the Leviathan, a one Dr. Boeddicker, active during the 1880s, who claimed that on a first class night, the amount of lunar detail seen with the giant mirror was “simply astounding.” We also learn that powers no higher than 600 diameters could be pressed into service, with occasional references to higher powers (1000x).

Denning then considers the work of William Lassell, who fashioned a number of large specula with which he discovered the two large satellites of Uranus; Umbriel and Ariel, independently co-discovered Hyperion, a faint satellite of Saturn and, just 17 days after the discovery of Neptune, its brightest moon, Triton (this name was not referred to by Denning as it was not formerly bestowed upon until a second Neptunian satellite, Nereid, was discovered in 1949).

Though Lassell, together with his assistant Mr. Marth, discovered a large number of nebulae from the sun drenched Mediterranean Island of Malta with his largest telescope of 4 foot aperture, Denning points out that it was with his 2 foot instrument that Mr. Lassell made all his planetary discoveries. Indeed, in 1871, Lassell wrote:

“There are formidable, and, I fear, insurmountable difficulties attending the construction of telescopes of large size…..These are primarily the errors and disturbances of the atmosphere and the flexure of the object-glasses or specula. The visible errors of the aperture are, I believe, generally in proportion to the aperture of the telescope…..Up to the size [referring to an 8in. O-G] in question, seasons of tranquil sky may be found where its errors are scarcely appreciable; but when go much beyond this limit (say to 2 feet and upwards),both these difficulties become truly formidable.”

pp 24.

That being said, Lassell also concluded that when conditions were fine, the advantages of aperture were clear for all to see. Concerning his largest telescopes he said:

“Nothwithstanding these disadvantages, they will, on some heavenly objects, reveal more than any small ones can.”

pp 24.

The chapter continues with Denning providing still more anecdotal evidence for the relative merits of large and small telescopes. Next in line, we hear about the 24.8 inch Cooke refractor erected by a well-to-do gentleman at Gateshead, England, which, despite its intimidating size, proved to have a ”singularly barren record”;

The owner of this fine and costly instrument wrote the author in 1885: “Atmosphere has an immense deal to do with definition. I have only had one fine night since 1870! I saw then what I have never seen since.”

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Author’s note: The Gateshead debacle is a particularly poignant story that has value for contemporary amateurs. Showmanship has no place in astronomy! The chap who installed the telescope obviously gave paltry attention to the environment in which the instrument was erected. The same gentleman seems to have had only a casual interest in astronomy, with little or no real experience of how such instruments would likely perform. The local seeing rendered the great telescope still born. One cannot help but wonder how many amateurs have done likewise over the years. Before spending lavish amounts of capital on a telescope, field testing the site on which it is to be constructed or used is mandatory. This accounts for the relative success of the large American refractors atop Mount Hamilton, for example, and the Great Meudon Refractor outside Paris, the sites of which were thoroughly field tested prior to their erection.

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The chapter continues with Denning relating other reports carried out by astronomers located at various observatories throughout the world. At the Paris Observatory, Dr. M. Wolf gained intimate acquaintance with various instruments including a 47.2 inch silver-on glass reflector, and a variety of smaller instruments, including a refractor of 15 inch aperture and 15.7 inch silver-on-glass reflector. Wolf wrote Denning concerning his visual experiences with these instruments:

“I have observed a great deal with the two instruments (both reflectors) of 15.7 and 47.2 inches. I have rarely found any advantage in using the larger one when the object was sufficiently luminous.” M. Wolf also avers that a refractor of 15 inches and a reflector of 15.7 inches will show everything  in the heavens  that can be discovered by instruments of very large aperture. He always found a telescope of 15.7-inch aperture surpass one of 7.9 inches, but expresses himself confidently that beyond about 15 inches increased aperture is no gain.

pp 26

Denning then relates the findings of Professor Young, who was assigned to a number of refractors, the largest being of 23 inch aperture, at Princeton, who related the following:

“The greater susceptibility of large instruments to atmospheric disturbances is most sadly true; and yet, on the whole, I find also true what Mr. Clark told me would be the case on first mounting our 23-inch instrument, that I can  almost always  see with the 23-inch everything I see with the 91/2 inch under the same atmospheric conditions, and see it better- if the seeing is bad, only a little better, if good immensely better.”

pp 27.

Another notable report comes from Mr. Keeler, who gained extensive experience with a number of instruments of various aperture atop Mount Hamilton:

Mr. Keeler adds: “According to my experience, there is a direct gain in power with increase in aperture. The 12-inch equatoreal brings to view objects entirely beyond the reach of the 61/2 inch telescope, and details almost beyond the perception with the 12-inch are visible at a glance with the 36-inch equatoreal.”

pp 28.

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Author’s note: These testimonies help to establish the veracity of a certain notion, that, from a visual perspective at least, greater aperture is only advantageous when atmospheric conditions cooperate. The relative efficacy of a given instrument is strongly dependent on the environment in which it is housed. Thus, no contradictions are found between theory and experiment.

I am mindful that this discussion focuses primarily on instruments generally larger than those found in amateur hands, but in recent years there has been an attempt by some amateurs (salesmen?), zealous to promote premium refractors over other models, to cultivate the erroneous view that the former can ” punch through the seeing” better on account of their “higher optical quality.” This arose from a deliberate twisting of some theoretical work conducted by this author in conjunction with theorist, Vladimir Sacek (which dealt mostly with the defocus aberration and its effects in long and short focal length systems). Although it was conceded that a slight advantage may be conferred on such higher quality instruments, in general, the seeing error completely overwhelms any small gains conferred in this way. As a further note of proof, many modern reflecting telescopes have Strehl ratios at or above those exhibited by ED refractors** (as measured by their polychromatic Strehl ratios) and so, by implication, ought to “punch through the seeing” even more effectively. That this is not commonly reported (either historically or in the ‘legitimate’ contemporary literature) demonstrates the effect is largely fictitious and irrelevant to any serious discussion of this interesting topic.

** The reader will note that, of the mirrors tested, it was the mass produced ones – read “least expensive” – that exhibited the highest Strehl. This is just one of many emerging test results found by inquisitive customers. This author can personally vouch for the quality of these mass produced mirrors, having regularly employed a 203mm and 130mm Newtonian(made by SkyWatcher) in field work.

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We shall not dwell further with the ideas conveyed by Mr. Denning in this engaging chapter, save to say that he concluded that there must exist some optimised aperture combining the best of both worlds for work on average nights:

There is undoubtedly a certain aperture which combines in itself sufficient light-gathering power with excellent definition. It takes a position midway between great illuminating power and sharp definition on the other. Such an aperture must form the best working instrument  in an average situation upon ordinary nights and ordinary objects. M. Wolf fixes this aperture at about 15 inches, and he is probably near the truth.

pp 35.

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Author’s note: This author is in agreement with Denning’s general conclusion. Indeed, this topic was explored in relation to the efficacy of resolving double stars, where a 8-inch aperture was found to be optimal in one interesting analysis.

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Chapter III: Notes on Telescopes and Their Accessories

Covering Pages 38 through 65.

In this chapter, the Last Master discusses the best choices of telescope and accessories needed by the amateur who wishes to pursue a serious, long-term study  of the firmament and begins with some sage advice:

The subject of the choice of telescope has exercised  every astronomer more or less, and the question as to the best form of instrument is one which has occasioned  endless controversy. The decision is an important one to amateurs, who at the outset of their observing careers require the most efficient instruments obtainable at reasonable cost. It is useless applying to scientific friends who, influenced by different tastes, will give an amount of contradictory advice that will be very perplexing. Some invariably recommend a small refractor and unjustly disparage reflectors, as not only unfitted for very delicate work, but as constantly needing re-adjustment and re-silvering.*

Others will advise a moderate-sized reflector as affording wonderfully fine views of the Moon and planets. The question of cost is greatly in favour of the latter construction, and, all things considered, it may claim an unquestionable advantage. A man who has decided to spend a small sum for the purpose not merely of gratifying his curiosity but of doing really serviceable work, must adopt the reflector, because refractors of, say, 5 inches and upwards are far too costly, and become enormously expensive as the diameter increases. This is not the case with reflectors; which come within the reach of all, and may indeed be constructed by the observer himself with a little patience and ingenuity.

*My 10-inch reflector by With-Browning was persistently used for four years without being resilvered  or once getting out of adjustment.

pp 38-39

Denning emphasises the convenience of reflectors over equivalent aperture refractors and mentions the innovations of the new silver-on-glass telescope makers, who managed to decrease the focal ratio, allowing decent aperture and viewing comfort to be maximised. George Calver had already begun to make telescopes with focal ratios as short as 5 or 6, which are now ubiquitous and deservedly popular. Denning estimates that a 8-inch reflector is equivalent to a 7-inch refractor (referring to a long focus instrument) in relative light gathering power, but in terms of defining power, especially in relation to planetary observing, Denning considers them equally good at equal aperture.

Having had the pleasure of observing through some of the finest telescopes in England, Denning was in a unique position to offer sensible advice to his readers:

An amateur who really wants a competent instrument, and has to consider cost, will do well to purchase a Newtonian reflector. A 41/2-inch refractor will cost about as much as a 10-inch reflector, but, as a working tool, the latter will possess a great advantage. A small refractor, if a good one, will do wonders, and is a very handy appliance, but it will not have sufficient grasp of light for it to be thoroughly serviceable on faint objects. Anyone hesitating in his choice should look at the cluster about χ Persei through instruments such as alluded to, and he will be astonished at the vast difference in favour of the reflector….. When high magnifications are employed on a refractor of small aperture, the images of planets become very faint and dusky, so that details are lost.

pp 41-42

Later he elaborates on the relative effectiveness of reflectors and refractors:

To grasp details there must be a fair amount of light. I have seen more with 252 on my 10-inch reflector than with 350 on a 51/4 inch refractor, because of the advantage of the brighter image in the former case.

pp 49

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Author’s note: How refreshingly honest and insightful Mr. Denning is! Having owned and enjoyed a number of smaller refractors of apochromatic and long-focus achromatic pedigree over the years (of 5- and 6-inch aperture), they have all paled in comparison to a 8-inch f/6 Newtonian on virtually all objects (the Sun being a memorable exception), and yet cost many times less. Vanity formed a large part in this author’s recalcitrance to embrace the genius of Newtonian optics, but when given a fair chance (proper acclimation and accurate alignment of the optical train), the refractors left little to be desired. For some, it remains an inconvenient truth that a well executed, mass market 20cm f/6 reflector would wipe the floor with the finest 5-inch glass on Earth, but it is undoubtedly true.

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Mr. Denning is however, sympathetic to the casual observer and acknowledges the role a small refractor might play in the pursuit of happy adventures:

Out of door observing is inconvenient in many respects, and those who procure a telescope merely to find a little recreation will soon acknowledge a small refractor to be eminently adapted to their purposes and conveniences.

pp42

That being said, Denning is careful to qualify this statement with the following:

Those who meditate going farther afield, and taking up observations habitually as a means of acquiring practical knowledge, and possibly of doing original work, will essentially need different means. They will require reflectors of about 8 or 10 inches aperture; and if mounted in the open on solid ground, so much the better, as there will be a more expansive view, and a freedom from heated currents, which renders an apartment unsuited to observations, unless with small apertures where the effects are scarcely appreciable. A reflector of the diameter mentioned will command sufficient light grasp to exhibit the more delicate features of planetary markings, and will show many other difficult objects in which the sky abounds. If the observer is especially interested in the surface configuration of Mars and Jupiter he will find a reflector a remarkably efficient instrument. On the Moon and planets it is admitted that its performance is, if not superior, equal to that of refractors.  If however, the inclination of the observer leads him in the direction of double stars, their discovery and measurement, he will perhaps find a refractor more to be depended upon, though there is no reason to why a well mounted reflector should not be successfully employed in this branch.

pp 42-43.

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Author’s note: Denning’s commentary here resonates very strongly with this author’s field experience. In respect of double stars, the 8-inch Newtonian was found to be a more effective instrument than a custom-made 5″ f/12 classical refractor, though historically, and inch for inch, there is overwhelming evidence to show that the classical refractor is better suited to resolving binary systems to the limits imposed by their aperture. Indeed, for this exacting task, they remain primus inter pares.

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Mr. Denning feels the images of stars in refractors are better than reflectors:

As far as my own experience goes, the refractor gives decidedly the best image of a star. In the reflector, a bright star under moderately high power is  seen with rays extending right across the field, and these appear to be caused by the supports of the flat.

pp 43.

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Author’s note: The stellar images in refractors are indeed very pretty, the Fraunhofer diffraction rings being very subdued and sometimes quite invisible on stars of lesser glory. Newtonians show diffraction spikes around bright stellar luminaries, and brilliant planets like Venus present with a singularly peculiar aspect in a moderately large Newtonian.

A Cruciform Venus

A Cruciform Venus, as seen with a 8-inch f/6 Newtonian on the evening of Saturday April 25 2015.

While this is certainly the case, it is a subjective point. Having accustomed myself to viewing through Newtonians, I must confess to finding these difrraction spikes to be rather beautiful. And while they may bother some individuals, they do not degrade the image in any significant way and can be ignored or unlearned.

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The chapter continues with a brief discussion on telescope testing. Denning’s approach is very down to Earth in this regard. He recommends that one should always try before you buy, especially if the instrument is second hand. As for the tests themselves, Denning does not recommend the Moon, as it is “too easy,” there being too much wonderful detail on view to side track the observer. Instead he recommends turning the telescope on bright planets, especially Jupiter and Venus to assess its defining power. Elaborating on Venus, he recommends  viewing at dusk or dawn, preferably when the planet has reached a decent altitude. As the magnification is cranked up, the disk of the planet should remain ” beautifully sharp and white.” A good telescope ought to hold its definition as the power is increased, with only an enfeebling of light as the image is spread over a larger area. A lesser telescope will show a deterioration of definition under the same conditions, producing a “mistiness” which confuses the definition in a palpable manner. Nor can these errors be ‘focused out’.

Denning also recommends star testing on a second or third magnitude star, the high power image of which ought to be tiny, circular and free from other distortions. If colour is seen in a reflector, it is probably the eyepiece and not the telescope that is at fault, though he does not mention the effects of atmospheric refraction that can manifest itself if the object under scrutiny is at a low altitude. He is also careful to distinguish between atmospheric distortion and a bona fide optical fault. Testing even a first rate telescope on a bad night of seeing is sure to produce iffy results and so these tests ought to be carried out over several nights to be certain of where the problem lies. Denning also  mentions the intra- and extra-focal colours of the diffraction rings seen in well corrected achromatic refractors.

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Author’s note: It is interesting that Denning does not suggest double stars as a test of telescope optics, in sharp contradistinction to many of his contemporaries. In reality though, the resolution of double stars is not a particularly stringent test of optics, as even so-so telescopes will manage some tricky pairs. Such tests are more a measure of atmospheric seeing and transparency than anything else. The best tests are on bright planets, especially Jupiter, which can display a rich variety of low contrast details that may prove elusive in a lesser instrument and become beautifully manifest in a higher quality telescope.

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No matter how wonderful or impressive the telescope being employed, without a sturdy mount, its powers will be greatly compromised. Denning considers both alt-azimuth and equatorial mounting systems, favouring the latter for high resolution projects, although stressing that high quality work can be done with simple non-driven mounts. Mr. Denning estimates that with an undriven, altazimuth mount, roughly 50 per cent of the observer’s time has to be expended adjusting the telescope in order to keep the object centred in the field, particularly if one is examining an object at high magnifications. In the end though, he cautions that a determined individual can make do with very simple equipment, and, in time, the observer “will gain patience and perseverance which will prove a useful experience in the future.”

pp 55.

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Author’s note: Denning actually opted for simplicity over technical sophistication with his own telescope, a 10-inch With-Browning Newtonian. It was mounted on a good but sturdy alt-azimuth mount, equipped with slow motion controls. By all accounts the instrument was permanently exposed to the elements (as evidenced by comments he made on page 76), the optics and tube assembly covered over when not in use. Denning’s telescope was thus in a permanent state of acclimation with its environment. No cooling fans were used with the telescope, as they were not available at the time, and indeed, were never really necessary.

While some modern amateurs would balk at this modest setup, it pays to remind the reader that Denning established himself as a world authority on planetary observing – particularly Jupiter and Saturn and their satellite systems – contributing a great body of knowledge in the form of drawings and written descriptions of his observations.

That Denning chose this setup over something more sophisticated reinforces an old maxim, that the quality of the observer is far more important than the type of equipment employed, a maxim that resonates strongly with this author’s ethos. This is especially true today when the amateur can enjoy high-quality mass-market optics at very reasonable prices. Denning’s estimate of the time lost in active observing must be tempered by the fact that the oculars he employed had very much smaller fields than those enjoyed by amateurs today, many of which can cover several times the area of sky he would have routinely encountered, thus reducing the time needed for object centering and adjustment.

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It is in this chapter also that Denning advances a brief but most engaging commentary on eyepieces;

Good eyepieces are absolutely essential. Many object-glasses and specula have been deprecated by errors really originated by the eyepiece. Again, telescopes have not infrequently been blamed  for failures through want of discrimination  in applying suitable powers. A consistent application of powers, according to the aperture of the telescope, the character of the object, the nature of the observation, and the atmospheric conditions prevailing at the time, is necessary to obtain the best results.

pp 46.

Denning describes the three most common oculars available to amateurs in his day: the negative, or Huygenian, the positive, or Ramsden, both of which had narrow fields of view and worked best at large relative apertures. he also mentions the Kelner, which afforded much wider fields of view (typically 40 or 50 degrees) for deep sky viewing and decent definition at relative apertures at f/6 and higher. Mr. Denning is sceptical of the claims of some telescope makers and users who have stated that their telescopes can bear powers of up to 100 per inch of aperture:

Telescopes are sometimes stated to bear 100 to the inch on planets, but this is far beyond their capacities even in the best condition of air. Amateurs soon find from experience that it is best to employ those powers that afford the clearest and most comprehensive views of the particular objects under scrutiny. Of course, when abnormally high powers are mentioned in connection with an observation, they have an impressive sound, but this is all, for they are practically useless for ordinary work. I find that 40, or at most 50 to the inch, is ample, and generally beyond the capacity of my 10-inch reflector.

pp 47

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Author’s note: Better oculars were invented in the mid to late 19th century, particularly the Plossl and orthoscopic, but owing to the greater number of un-coated elements, they were not commonly employed by amateur astronomers in Denning’s day. In respect to his comments regarding the 100x per inch claims by some observers and telescope sellers, this was a reasonable conclusion to draw, as one finds from experience that such high powers are indeed disadvantageous to delivering the best planetary images, especially in moderate and large aperture telescopes. Denning finds that 40-50x per inch of aperture to be the maximum upper limit for the vast majority of applications, and this remains true to this day. Indeed, we find that Denning commonly employed a power of 252 diameters on his 10-inch Newtonian in pursuing his studies of the bright planets, corresponding to ~ 25x per inch of aperture, in agreement with the recommendations of the majority of planetary observers even today. Indeed, it is only in the pursuit of the most difficult double stars and small planetary nebulae that higher powers are found to be useful, and only on the best nights.

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After these comments, Denning shares with us the details of a curious practice, apparently popular with dedicated observers for at least a century; using a single lens as an eyepiece:

A great advantage, both in light and in definition, results in the employment of a single lens as eyepiece. True, the field is very limited, and, owing to the spherical aberration, the objects so greatly distorted near the edges that it must be kept near the centre, but, on the whole, the superiority is much evident.

pp 47

Mr. Denning informs us that some distinguished observers, such as the Reverend William Rutter Dawes and Sir William Herschel had also noted an improvement in light grasp and distinctness employing the same technique.

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Author’s note:

Though we take so much for granted today, with our high quality optical glass, free of striations and other artifacts, broadband multi-coatings and the like, the glass out of which these early oculars were constructed would surely have been inferior to even the ‘budget’ oculars we enjoy today. The complete lack of anti-reflection coatings would have generated ghost-images due to internal reflections, especially on bright objects, cutting down on contrast and definition of low contrast details. Adopting a singlet would have greatly reduced these effects at the expense of introducing horrid off-axis aberrations.

This author once experimented with a modern ‘ball eyepiece’, that is, a single, spherical eye lens, and while the definition at the centre of the field was very nice, off axis images were very badly distorted. In the end, it was considered more a novelty than a useful tool and has not been used since.

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Denning recommends that the observer acquire three eyepieces, corresponding to low, medium and high power, appropriately chosen to match the aperture of the telescope, and cautions that the magnifications they profess to deliver may not in fact be the values they generate. He offers a means of experimentally determining actual magnifications described on pages 49-50. Denning continues by discussing the curious custom adopted by some telescope makers of using magnifying power as a ‘sales pitch’. Surprisingly, Denning identifies the famous maker, James Short (1710-68), as the individual who originated this dubious consuetude, who made his fortune selling small Gregorian-type reflectors, and which has sadly endured at least for so-called ‘department store’ telescopes right up to the present day.

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An ornate, table top Cassegrain reflector by James Short, dating from the mid-18th century.

An ornate, table top Gregorian reflector by James Short, dating from the mid-18th century.

A curious aside: Denning owned a 4-inch Gregorian telescope by Watson, similar to the instrument shown above, which, although over a century old at the time, had speculum metal mirrors that were still in good condition.

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The importance of observing in comfort (pages 53-54) is a subject very close to Denning’s heart and, accordingly, he stresses the importance of using a chair while observing and even mentions some innovations made by amateurs published in the English Mechanic. He also reveals that for objects located high overhead, a small step ladder was found to be very useful  with his 10-inch Newtonian. Comfort is of paramount importance in gaining the maximum enjoyment from an observing experience and can even make the difference between seeing something and not at all.

Beginning on page 55 and ending at the top of page 57, Denning remarks on the ‘character’ of the observer. Variations in visual acuity account for some of the discrepancies reported by observers, as well as their level of experience. Some individuals will see more than others. Historically, these differences have sometimes led to controversy:

….as a rule, amateurs should avoid controversy, because it rarely clears up a contested point. There is argument and reiteration, but no mutual understanding or settlement of the question at issue. It wastes time, and often destroys that good feeling which should subsist amongst astronomers of ever class and nationality…. paltry quibblings, fault finding, or the constant expression of negative views, peculiar to sceptics, should be abandoned, as hindering rather than accelerating the progress of science….. There are some men whose reputations do not rest upon good or original work performed by themselves, but rather upon the alacrity with which they discover grievances and upon the care they bestow in exposing trifling errors in the writings of their non-infallible contemporaries.

pp 56

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Author’s note: There is nothing new under the Sun, and Denning’s comments are as true today as the day they were written. We are all fallen, all of us wretched, and in need of redemption. And yet, we can rise above it all and do great things. Men vainly look to the heavens in search of people among the stars, yet the only ‘aliens’ we will ever encounter are our neighbours. We need to get on with each other.

Denning himself was the subject of controversy concerning some of his ideas on meteor radiants. He held some erroneous views but was bitterly attacked by some of his contemporaries, just to ‘prove’ that they were ‘right’ and he was ‘wrong.’ They wounded him deeply. This is likely one of the reasons why he withdrew from public life at the height of his career.

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On page 58, Denning discusses the practice of stopping down, i.e. the act of deliberately reducing the effective aperture by means of a ‘stop.’ The practice was sometimes done to increase the defining power of the telescope, which, for clarity, we shall equate with image sharpness, but the underlying reason for this was thought by some to be caused by blocking off a defective part of the object glass or speculum. Denning however, offers us another explanation; the atmosphere and its effects on the fielded aperture. While it is true that stopping down could mask a figuring error (more likely to occur at the edges of the objective), Denning’s own experiments seemed to favour the idea that large apertures can often benefit from stopping down on nights of poor or average seeing because smaller apertures are less affected by atmospheric turbulence than larger ones. He suggests that, for visual use, apertures of 18 inches and over can quite often benefit from an aperture stop of 16- or 14-inch stop. But he cautions that the practice is of little value in the case of moderate aperture;

With my 10-inch reflector, I rarely, if ever, apply stops, for by reducing the aperture to 8 inches the gain in definition does not sufficiently repay for the serious loss of light. But in the case of large telescopes, the conservation of light is not so important, and a 14-inch or 16-inch stop may be frequently employed on an 18-inch with striking advantage.

pp 58

In a curious note under the subtitle, Cleaning Lenses, Denning tangentially discusses some of the properties of silvered mirrors, in particular, the factors that may prolong the life of the thin silver layer. He notes that keeping the mirror dry is of benefit, as well as placing a protective cap over the optics when not in use. He claims that Calver was aware that some silvered glass mirrors held their reflectance longer than others and was related to the frequency with which the instrument was used and the environment in which it was fielded. Some mirrors held their reflectivity well for a decade or more, but this was apparently the exception rather than the rule. Intriguingly, he also states that the tarnish accumulated on silvered mirrors can work surprisingly well on lunar and planetary targets:

A mirror that looks badly tarnished and fit for nothing will often perform wonderfully well. With my 10-inch in a sadly deteriorated state I have obtained views of the Moon, Venus and Jupiter that could hardly be surpassed. The moderate reflection from a tarnished mirror evidently improves the image of a bright object by eliminating the glare and allowing the fainter details to be readily seen.

pp 60

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Author’s note: When silver tarnishes it generally leaves a tan coloured film owing to the formation of silver sulphide, which can indeed reduce the relectivity of the mirror, but the moderate deterioration Denning speaks of seemed to enhance his views of the Moon and bright planets. I believe that this can be attributed to a filter-like or ‘apodising’ effect. As this author has commented on elsewhere, filters work superbly well on moderate and large aperture telescopes owing to their ability to suppress glare and enhance the visual appearance of  subtle details that would otherwise be ‘washed out’ in the unfiltered image. This author has previously alerted readers to the benefits of employing a simple and inexpensive neutral density filter to improve the planetary images in large reflectors. More sophisticated filters, such as a polariser, also work very well in this regard. The Televue bandmate planetary filter was also found to work brilliantly on the author’s 8-inch Newtonian, which he employs routinely  to observe Jupiter. Filters are capable of adding a whole new dimension to the art of visual observing; an effect serendipitously ‘discovered’ by Denning.

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The remaining pages of this chapter are devoted to miscellaneous topics, including dewing up and cooling down of telescope optics, the celestial globe, presumably a fore-runner of the modern planisphere, the utility of opera glasses and finally a brief description of a new type of observatory showing up the length and breadth of the country; the Romsey. Unlike the all-brick, monolithic, cathedral-like domes housing the great refractors of the day, the Romsey offered a much more economical means of housing one’s telescope and keeping all one’s ancillary equipment in a single place.

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Chapter IV Notes on Telescopic Work

Covering pages 66 through 86

In this invaluable chapter, Mr. Denning provides a distillation of his practical experience in the field. He begins by suggesting that the would-be astronomy enthusiast gain some background knowledge of the objects he/she wishes to devote time to. This can be achieved by reading up on the general descriptions provided by trusted authorities in the field. But theory ought to be a guide and not an absolute means to an end, for Denning seems to value practical knowledge over that learned in books:

An observer should take the direction of his labours from previous workers, but be prepared to diverge from acknowledged rules should he feel justified in doing so from his new experiences.

pp 68

Denning feels that the observer ought to prepared for a night of observing, by making up a suitable list of objects he/she wishes to study. It need not be long and over-elaborate, nor should such a list be over ambitious.  A few objects studied well is far better than several dozen casually visited.

When no such preparation is made much confusion and loss of time is the result. On a cloudy, wet day, observers often consider it unnecessary to make such provision and they are taken at a great disadvantage when the sky suddenly clears. A good observer, like a good general, ought to provide, by proper disposition of his means, against any emergency. In stormy weather, valuable observations are often permissible if the observer is prompt, for the definition is occasionally suitable under such circumstances.

pp69

Denning estimates that the British climate offers about 100 hours of exceptional seeing per year, considerably more than is commonly believed today, but these are not confined to just a few nights, but occur sporadically over the course of the weeks and months, for he says that a night might start out with decidedly mediocre seeing only to be found to be considerably improved just a few hours later.

Denning claims that an east wind is often detrimental to viewing high resolution targets, but does not consider this to be an absolute. He differs from the general opinion expressed by contemporary astronomers in claiming that windy weather can often bring very good seeing:

I have sometimes found in windy weather after storms from the west quarter, when the air has become very transparent, that exceptionally sharp views may be obtained; but unfortunately, they are not without drawbacks, for the telescope vibrates violently with every gust of wind and the images cannot be held long enough for anything satisfactory to be seen.

pp 69

Denning mentions the favourable conditions that often attend hazy skies:

Calm nights when there is a little haze and fog, making the stars look somewhat dim, frequently afford wonderfully good seeing….. The tenuous patches of white cirrus cloud, which float at high altitudes will often improve definition in a surprising manner, especially on the Moon and planets.

pp 69.

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Author’s note: Denning’s knowledge was gained actively, in the field, more so than any of his contemporaries, for how else could he provide such extraordinary (and mostly correct) insight? Denning’s telescope was in a constant state of preparedness, as it was permanently fielded in the open air. He was thus ready to take advantage of any change in the weather that may have come about and use it to his advantage. Such knowledge cannot be learned from a book. That Denning entrusted experience over theory resonates well with this author’s own findings, especially in relation to double stars, where striking discrepancies between field observations and the prognostications of individuals posing as ‘theorists’ have been uncovered. Indeed, in some of these cases, the differences between theory and experiment have been totally irreconcilable.

Iustitia, iustitia, iustitia!

Beware of theorists posing as observers!

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Mr. Denning next discusses vision and its relation to telescopic observation. He concedes that there is much inter-individual variation among the visual acuity of individuals, with some displaying remarkable vision and others seeing much less. He mentions a one Dr. Kitchiner, who claimed that the eye of a dedicated telescopist aged 47 is as much impaired as an ordinary individual aged 60! Denning is somewhat sceptical of that claim stating that, “the Doctor’s opinion is not generally confirmed by other testimony, the fact being that the eye is usually strengthened by special service of his character.”

Further, he states:

Before the observer may hope to excel as a telescopist it is clear that a certain degree of training is requisite. Many men exhibit very keen eyesight under ordinary circumstances, but when they come to the telescope are hopelessly beaten by a man who has a practised eye. On several occasions the writer was most impressed with evidences of extraordinary sight in certain individuals, but upon being tested at the telescope they were found very deficient, both as regards planetary detail and faint satellites.Objects which were quite conspicuous to an experienced eye were totally invisible to them.

pp 71

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Author’s note: Denning’s comments agree with the experiences of this author. On many occasions over the years, I have attempted to show my students some of the delights of the second heaven, only to discover that, although they have decidedly better eyesight than my own, were unable to ‘see’ the duplicity of test double stars, plainly seen with my own eyes. Only after pointing it out and after prolonged scrutiny did they come to ‘see’ what was plainly visible. The same is true of the Great Red Spot.  Experience is a far better tool than raw visual acuity. Seeing is most definitely an art that must be learned.

I believe this also has implications for those that have derided the classical achromat, despite an enormous body of evidence demonstrating that their users (who’s eyes were trained) saw far more than what is commonly reported in the contemporary ‘literature.’ Indeed they appeared to have seen things that still largely elude the majority of self-proclaimed veterans.

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On page 72-73, Mr. Denning stresses the importance of note making. The content of these notes need not be elaborate, just a few salient points about the date, time and seeing conditions, the instrument fielded and some brief written details on the objects observed. Denning recommends that such notes be made as close to the time of observation as possible. “If the duty is relegated to a subsequent occasion,” he says, “it is either not done at all or done very imperfectly.”  Something ‘trivial’ recorded on an earlier date may turn out to be very important at a later date.

Denning also recommends sketching what one sees at the telescope, even if the would be observer is not skilled in such activities. They need not be works of art but simply show the ‘definite’ features of the object under scrutiny. With time, the note maker/sketcher becomes a “draughtsman:”

My own plan in sketching at the telescope  is to first roughly delineate  the features bit by bit  as I successively glimpse them, assuring myself, as I proceed, as to the general correctness in outline and position, then, on completion, I go indoors to a better light and make copies while the details are still freshly impressed on the mind.

pp 74

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Author’s note: It is a sad state of affairs that the noble art of note making is in decline; perhaps terminally. Many amateurs do not make any notes of any description. More’s the pity, for notes provide a means of assessing progress over time, and form the bedrock of an amateur’s experience under the starry firmament. They are an integral part of the culture of amateur astronomy and can prove invaluable in resolving issues that sometimes appear contradictory, especially if one is viewing through different instruments at different times. An observer without notes is liable to make the same mistakes over and over again.

An observer without notes has no past.

Sketching is also an enjoyable and invaluable way of preserving information and when conducted over a long period of time, can prove to be of vital importance, especially if one records something novel. There is no ‘right’ or ‘wrong’ way to sketch. Denning preferred to sketch the basic features of his subjects at the telescope, while refining them indoors a short time afterwards. Others choose to scrutinise the object intently, committing to memory all the detail one can capture before returning indoors to perform the sketch. Find the method that works best for you.

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In a section called “Friendly Indulgences,” Denning recognises the need for outreach and to be gracious to friends and the curious passerby, who express an interest in viewing an astronomical object. But in the end, he feels that there is a fine line between getting on with one’s observing and being a “showman.” One comes away with the feeling that he was, for the most part, a solitary observer, who was happy in his own company and would rather get on with things than engage in some lengthy discussion with someone else while the sky remained clear:

Of course it is the duty of us all to encourage a laudable interest in the science, especially when evinced by neighbours or acquaintances; but the utility of an observer constituting himself a showman, and sacrificing many valuable hours which might  be spent in useful observations, may be seriously questioned. The weather is so bad in this country that we can ill spare an hour from our scanty store.

pp 74.

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Author’s note: One cannot help but wonder what Denning would have thought of the star parties amateurs attend these days. I suspect he would have kept well away from them. While star parties can be fun and provide a means of looking through various kinds of telescope to assist making a decision on a future purchase, our hobby is still, by and large,  a solitary passtime. We must remember that for every lion among us there is also a leopard.

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Denning discusses the realities of open air observing, wittily commenting that a shiny new telescope tube exposed to the elements of nature will soon lose its “smart and bright appearance”, although the views will remain as good as they always were. It is here also that the forgotten Bristolian reveals his great love affair with the heavens, providing vivid descriptions of the physical conditions he had to endure night after night. Because he remained a bachelor all his life and thus, had no dependants, his life could be best be likened to that of a monk, wedded as it were to his astronomical investigations. As a dedicated meteor observer, he spent endless hours on every clear night recording their brilliant tracks across the sky. This kind of work is not for the faint hearted, especially in the cold of winter. Denning describes his lot vividly:

Night air is generally thought to be pernicious to health; but the longevity of astronomers is certainly opposed to this idea. Those observers who are unusually susceptible to affections of the respiratory organs must of course exercise extreme care, and will hardly be wise in pursuing astronomical work out of doors on keen, wintry nights. But others, less liable to climatic conditions, may conduct operations with impunity and safety during the most severe weather. Precautions should always be taken to maintain a convenient degree of warmth; and for the rest, the observer’s enthusiasm must sustain him. A “wadded dressing gown” has been mentioned as an effective protection from cold.  I have found that a long, thick overcoat, substantially lined with flannel, and under this a stout cardigan jacket, will resist the inroads of cold for a long time. On very trying nights, a rug may also be thrown over the shoulders, and strapped round the body.  During intense frosts, however, the cold will penetrate (as I have found during prolonged watches for shooting stars) through almost any covering. As soon as the observer becomes uncomfortably chilly, he should go indoors and warm his things before a fire.

pp 75.

After relating many humorous stories about finding wee beasties taking up residence inside his telescope tube, Denning returns to more pressing matters, emphasising that an observer eager to discover something of importance must necessarily be a person of method and perseverance and not to divest too much importance to his instruments;

A skilled workman will do good work with indifferent tools; for after all it is the character of the man that is evident in his work; and not so much the resources which art places in his hand.

pp 80

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Author’s note: The old adage is true: a bad workman always blames his tools. Today, we are blessed to have vastly superior ‘tools’ to anything Mr. Denning could have dreamed of. And yet, all the while, we seem to want  more and more. Fortunate indeed is the man who is happy with his tools!

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In the final pages of this interesting chapter, Denning discusses the potential of photography to revolutionise the science of astronomy. His opinions are brief but he was essentially correct about its growing prowess, but since very few amateurs had the means of conducting photographic studies of the heavens at that time, he does not dwell on the topic.

The author encourages his readers to follow the astronomical literature, and recommends all the greats of the age including the Reverend T.W Webb’s,Celestial Objects for Common Telescopes, Chamber’s Descriptive Astronomy and Noble’s, Hours with a 3-Inch Telescope, as well as more specialised texts. He also encourages his readers to consult practical periodicals of the day, especially the English Mechanic, Nature and Knowledge. 

Denning encourages those who live in towns and cities to get out and do some observing, explaining that the conditions can be quite good, especially for viewing the Moon and the planets. He mentions that smoggy conditions may actually aid in bringing out detail on the planetary bodies:

I have frequently found planetary markings very sharp and steady through the smoke and smog of Bristol. The interposing vapours having the effect of moderating the bright images and improving their quality.

pp 81.

He ends this chapter on an enthusiastic note:

A telescope may either be employed as an instrument of scientific discovery and critical work, or it may be made a source of recreation and instruction. By its means the powers of the eye are so far assisted and expanded that we are able to conceive of the wonderful works of the Creator than could be obtained in any other way.

pp 86.

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Author’s note: The population of Bristol, where Denning lived for most of his life, quintupled during the 19th century, reaching 330,000 by 1901. The burning of coal would have been the main source of energy driving commerce and heating households, and so smog would have been a common phenomenon especially during still winter nights in the city.

Denning yet again mentions the beneficial effects of dimming the image as regard gleaning more defining power from planetary bodies. I cannot help but think that were he alive today, he would have been an enthusiastic proponent of filters in the planetary astronomy.

Unbridled enthusiasm distinguished Denning from many of the classic authorities of his day. His tone was approachable, unpretentious and reassuringly upbeat. What he saw through his telescope was his reaction to the natural wonders created by the Living God, who placed these things in heaven so that we might marvel at them and be reminded of His omnipotence. In this capacity, he was a kindred spirit, who saw no conflict between scientific investigation and his personal faith.

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Chapter V: The Sun

Covering pages 87 through 112

We now begin to explore the late 19th century knowledge of the heavenly bodies, one by one, beginning with the Sun. As a guide, this author will bring to bear his experiences with a telescope not too dissimilar to Denning’s own; a 8″ f/6 Newtonian on a modern alt-azimuth mount, which has given him wondrous views of the firmament.

Jupiter as it appeared at 10:00pm local time on the evening of May 5, 2016. Magnification 200 diameters.

Jupiter as it appeared at 10:00pm local time through a 8″ f/6 Newtonian on the evening of May 5, 2016. Magnification 200 diameters.

 

 

 

 

 

 

 

 

 

 

The opening pages of this chapter discuss basic solar facts, as true today as they were in Denning’s time. The Sun, we learn, has a mean distance from the Earth of about 92,900,000 miles, computed from a solar parallax of 8.8″, and a diameter of 866,000 miles. Interestingly, Denning provides a series of micrometer measures (pp 88) of the solar disk diameter, showing that it varies from 32 minutes 66 seconds at the end of December, to 31 minutes 32 seconds at the end of June. This reflects the slight ellipticity of the Earth’s orbit, carrying our planet slightly closer to the Sun in mid-winter in the Northern Hemisphere  and a little further away in mid-summer.

Denning relates the fact that the most conspicuous feature of the solar disk – sunspots – were likely seen throughout antiquity, and among observers from a number of civilizations. The earliest account offered by the author dates to 188AD. These spots were seen by the naked eye through dense fog, most commonly at sunrise and sunset. Denning himself speaks of observing four large spots (pp 89) on a foggy autumnal evening in 1870, just as the Sun was setting. He claims that if these spots are bigger than about 50″, they should be picked up by the average eye.

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Author’s note: Although a curious visual phenomenon, this author strongly advises that the reader not look at the Sun even in the very foggy conditions described above. Many a tyro has damaged his/her eyes in doing so.

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Denning is reluctant to attribute the telescopic ‘discovery’ of sunspots to any one individual but mentions, in particular, various early telescopists including, Fabricius, Galilei, Harriot and Scheiner, as claiming the limelight. He also corroborates later accounts by historians of astronomy, who claim that England’s Thomas Harriot saw these spots telescopically as early as December 8, 1610:

“The altitude of the Sonne being 7 or 8 degrees, and it being a frost and a mist, I saw the Sonne in this manner.”

pp 90.

He also mentions a drawing made by Harriot, showing three large sunspots. Thus, Denning was probably aware of Harriot’s early telescopic observations; possibly predating those made by Galileo Galilei.

The most common way in which observers in Denning’s day observed the Sun was to employ deeply coloured glass of various ‘depths’; either red or green, placed at the focal plane, together with a Herschel wedge, invented in 1830 by Sir John Herschel. Denning claims that red tinted glass is inferior to its green tinted counterpart:

The diagonal, by preserving a part only of the solar rays, which are transmitted by the object glass. This little instrument is comparatively cheap, and no telescope is complete without one.

pp 92.

Denning suggests that a small telescope, a refractor of 3- or 4-inch aperture, or a reflector of no more than 4-inches, are best employed in solar studies and recommends that larger instruments be stopped down to improve definition. He also mentions, owing to the great natural brilliance of the Sun, that unsilvered mirrors are perfectly adequate for obtaining good solar images.

With comfort and safety never being far from the mind of the author, Denning stresses that the solar observer be shaded from the Sun’s burning rays as much as is practical. He also recommends keeping the ‘solar telescope’ in the shade to ensure it does not induce the annoying thermals that can destroy high-definition features. As regard suitable magnification, he suggests that a power of about 60 and a field of view of just over half an angular degree is desirable to get a good ‘whole disk’ perspective. Higher powers can prove usual to gain better images of smaller features, though he does not recommend magnifications higher than about 150x. Once again, Denning mentions using a singlet eyepiece (presumably the field lens of a Huygenian ocular) in obtaining high power views yielding the highest definition.

Oddly enough, Denning gives scant mention to other methods of observing the solar disk, particularly by projecting the image onto a smooth, white surface. There is one reference made to this technique, appearing on page 93-94:

At Stonyhurst Observatory excellent delineations of solar phenomena are made; and the late Father Perry, who lost his life in the cause of science, thus described the method:- “On every fine day the image of the Sun is projected on a thin board attached to the telescope, and a drawing of the Sun is made, 101/2 inches in diameter, showing the position and outline of the spots visible.

pp 93-94

Solar projection techniques were used by the very earliest telescopists, including Galileo Galilei.

In a most curious account related on page 95, Denning describes the use of a primitive reticle scale, just a graduated piece of plane glass, mounted at the focal plane of a 4-inch Cooke refractor, borrowed from a friend, with which he was able to estimate the size of a large sunspot, observed on June 19, 1889. Using this technique he calculated that the real size of the spot was 27,000 miles! This technique could also be done using projection methods.

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Author’s note: Man and his numbers! How wonderful it is to be able to measure anything! Among other reasons, our heavenly Father gave us these powers so that we could project our imagination into realms hopelessly beyond the ability of our frail bodies to experience directly. Denning was no mathematician, of course, but he did have an excellent command of numbers, as we shall see in many other references explored later in the book.

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The chapter continues with discussions on various solar phenomena, beginning with the majesty of a solar eclipse, briefly describing their prediction (saros cycles) and rarity at any arbitrarily chosen location. On page 98, the aspects of a series of 12 partial solar eclipses as seen (or imagined) from England through the years 1891 to 1922, are reproduced. This is followed by an equally brief discussion on the sunspot cycle and how it may be followed by the amateur equipped with modest equipment.

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Another curious aside: In his younger years, Denning was a keen hunter of the hypothetical planet Vulcan, explaining why he had a particular interest in all things solar. Indeed, he helped organise coordinated searches for the planet among a number of English solar astronomers. Moreover, in Chapter IV, page 85, he made two lists of (I) ‘suspected objects to be erased’, and (II) ‘objects that in the future will add to our store’. Vulcan appears in the former list, suggesting that, at the time of writing of the book, he had firmly given up on the prospect of finding an intra-mercurial planet.

The quest for Vulcan reached fever pitch in Europe and across the United States during the late 19th century, bolstered by the work of mathematical (but myopic) astronomers of the ilk of (the arrogant) Urban Leverrier(1811-77), who uncovered a small, residual perihelion shift in the position of the planet Mercury, amounting to 43 arc seconds per century. Indeed a hitherto obscure physician and amateur astronomer, Edmond Modeste Lescarbault (1814-94), claimed to have observed such a planet in March 1869 at his private observatory in the picturesque village of Orgères-en-Beauce, in Northern France. Leverrier was happy to accept him as the discoverer and formally named the planet Vulcan – after the Roman god of fire – in March 1860, which circled the Sun every 19.7 days, at a distance of about 21 million kilometres from the solar surface. But soon, the astronomical community grew sceptical of Lescarbault’s sensational ‘discovery,’ claiming that such a world, even though as small as the Moon, would have been easily visible to many astronomers who had watched the Sun for many years.

By the time Denning penned his tome, most astronomers had dismissed the notion that a ninth planet, Vulcan, really existed, even though the reason for the measurable 43” per century perihelion shift of Mercury was not yet accounted for. The explanation had to wait until Albert Einstein formulated his epochal theory of general relativity in 1915, which perfectly accounted for the Mercury anomaly. Indeed, Einstein was to later write that his heart raced when his calculations exactly explained the planet’s sojourn through the curved space near the Sun. “For a few days,” he wrote, “I was beside myself in joyous excitement.”

All the while, I cannot help but think that Denning, in the exuberance of his youth, also searched for Vulcan with “joyous excitement.”

More on Vulcan here.

From pages 100-112, Denning goes on to describe, in considerable detail, the telescopic morphology of sunspots as well as their distribution on the solar surface. He provides an accurate an essentially modern value for the solar rotation period of ~25 days and 8 hours. It was also known to him that the rotation period varies with solar latitude, thus providing good evidence (like Jupiter, discussed later) for its essentially gaseous nature. On page 105, Denning presents a list of historically interesting astronomers and their estimates of the solar rotation rate from Cassini (1678) to Wilsing (1888), showing that such knowledge was known for nearly two centuries.

Denning displays his voluminous knowledge of solar phenomena in these closing pages of Chapter V, including the work of many astronomers – both contemporary and historical – as well as his some of his own detailed observations carried out with a 4-inch glass. This includes a discussion on solar faculae, prominences and historically significant eruptions, as well as some observational anomalies including spots noted at unusually high solar latitudes;

Mechain saw a spot in 1780 having a latitude of 401/3 degrees; in April 1826 Cappoci recorded one having 49 degrees of S. latitude Schwabe and Peters observed  spots 50 degrees from the equator. Lahire, in the last century, described a spot as visible of 70 degrees; but the accuracy of this observation has been questioned.

pp111.

Finally, on page 112, Denning provides a curious reference to a quantitative brightness differential between the solar limb and its centre, a measure previously unknown to this author:

In observing the Sun with a telescope the amateur will soon notice that the surface is far more brilliant in the central parts than round the margin of the disk. Vogel has estimated that immediately inside the edges the brightness does not amount to one seventh that of the centre.The difference is entirely due to the solar atmosphere, which is probably very shallow relatively to the great diameter of the Sun.

pp 112

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Author’s note: The Sun is, by and large, composed of a fourth form of matter called plasma. At temperatures in excess of a few thousand Kelvin, atoms break up to form a ‘soup’ of charged particles consisting of electrons, protons and an assortment of atomic nuclei. It is this moving plasma that generates the Sun’s prodigious magnetic field and all its associated phenomena.

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Chapter VI The Moon

Covering pages 113-136

Early in autumn, when the evenings are frequently clear, many persons are led with more force than usual to evince an interest in our satellite, and to desire information which may not be conveniently obtained at the time. The aspect of the Moon at her rising, near the time of the full, during the months of August, September, and October, is more conspicuously noticeable than at any other season of the year, on account of the position she then assumes on successive nights, enabling her to rise at closely identical times for several evenings together. The appearance of her large, ruddy globe at near the same hour, and her increasing brilliancy as her horizontal rays give way under a more vertical position, originated the title of “Harvest Moon,” to commemorate the facility afforded by her light for the ingathering of the corn preceeding the time of the autumnal equinox.

pp 113

It is with such wonderful prose that Denning opens his chapter on observing our closest neighbour in space. Denning was a man happy to be in the open air, either with telescope or with his unaided eyes, observing the grand spectacles of the heavens. In the proceeding paragraphs, he clearly outlines why the Moon is of such critical importance to life on Earth, in issuing the tides, for example, and stabilising the Earth’s climate. But he also notes its importance, since time immemorial, in human time keeping, as well as how its welcome light assisted the plight of navigators of the seven seas.

What follows thereafter are some basic physical facts about the Moon. For example, he states the apparent size of the Moon at apogee and perigee (29’21” and 33’5”, respectively), though he appears to have mistakenly stated these the wrong way round on page 114. The lunar diameter he quotes – 2160 miles – and its mean distance from the earth – 237,000 miles – are essentially those of the modern value.

Denning then launches into a general overview of the lunar regolith as seen through a good telescope;

When we critically survey the face of the Moon with a good telescope, we see at once that her surface is broken up into a series of craters of various sizes, and that some irregular formations are scattered here and there, which present a similar appearance to mountain ranges. The crateriform aspect of the Moon is perhaps the more striking feature, from its greater extent; and we recognise in the individual forms a simile to the circular cavities formed in slag or some other hard substances under the action of intense heat. In certain regions of the Moon, especially near the south pole, the disk is one mass of abutting craters, and were it not for the obvious want of symmetry in form and uniformity in size, the appearance would be analogous to that of a giant honeycomb. These craters are commonly surrounded by high walls or ramparts, and often include conical hills rising from their centres to great heights. While the eye examines these singular structures, and lingers amongst the mass of intricate detail in which the whole surface abounds, we cannot but feel impressed at the marvellous sharpness of definition with which the different features are presented to our view. It matters not to what district we direct our gaze, there is the same perfect serenity and clearness of outline. Not the slightest indication can be discerned anywhere of mist or other obscuring vapours hanging over the lunar landscape.

pp 114-115

Denning correctly states that the Moon is devoid of an atmosphere and probably doesn’t have water, in spite of the many ‘seas’ that adorn lunar maps. On page 115 through 116, Denning presents an explanation for why the Moon shows the same face toward the earth throughout its cycle (it is almost completely tidally locked) and presents the interesting phenomenon of libration, where the lunar countenance can show up to 59 per cent of its surface over the course of its earth orbit.  Denning also mentions the wonderful phenomenon of earthshine, the “ new Moon in the old Moon’s arms,”  and how the observers of old remarked that a waning Moon showed this earthlight more strongly than the new Moon.

The chapter continues by discussing the kinds of instruments best suited to lunar work, for the casual as well as the more serious observer:

A small instrument with an object glass of about 2 ½ inches will reveal a large amount of intricate detail on the surface of our satellite, and will afford the young student many evenings of interesting recreation. But for a more advanced survey of the formations, with a view to discover unknown objects or traces of physical change in known features, a telescope of at least 8 or 10 inches is probably necessary, and powers of 300 to 350, and more.

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Author’s note: Yet again, Mr. Denning dispenses sterling advice to the would-be student of the Moon that is entirely in agreement with all subsequent authorities on the subject. You’ll see more with a larger telescope and will be able to use higher powers to ferret out the finer details. Such advice appears to have been lost on a current subsection of amateurs who are willing to squander a veritable fortune for small refractors of very limited aperture. Such are the times we live in!

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On pages 118 and 119, Mr. Denning discusses the fascinating phenomenon of a lunar eclipse, their frequency and appearance, both telescopically and to the naked eye. Here we find some invaluable historical records of how the intensity of a total lunar eclipse varied from apparition to apparition, with references to observations conducted by astronomers dating back nearly nine centuries. While some total lunar eclipses were spectacularly bright, with a beautiful, coppery orb being clearly visible to the naked eye, at other times, the eclipsed Moon completely disappeared:

On May 5 1110, Dec.9, 1620, May 18, 1761, and June 10, 1816, our satellite is said to have become absolutely imperceptible during eclipse. Wargentin, who described the appearance 1761, remarks:- “The Moon’s body disappeared so completely that not the slightest trace of any portion of the lunar disk could be discerned, either with the naked eye or with a telescope.”

pp 119.

 Denning recalls his own observations of a peculiarly dim lunar eclipse:

On Oct. 4 1884, I noticed that the opacity was much greater than usual; at a middle period of the eclipse the Moon’s diameter was apparently so much reduced that she looked like a dull, faint, nebulous mass, without sharply determinate outlines. The effect was similar to that of a star or planet struggling through dense haze.

pp 119

In contrast, Denning describes the eclipse of March 19, 1848 as unusually bright:

The Moon presented a luminosity quite unusual. The light and dark places on the face of our satellite could be almost as well made out as an ordinary dull moonlight night.

pp 119.

In addition to these records, Mr. Denning mentions some explanations for the variability of the intensity of such eclipses. In particular, he describes a theory first suggested by the great German astronomer and mathematician, Johannes Kepler, who attributed this variability to differences in the humidity of the atmosphere, as well as more contemporaneous explanations proffered by a one Dr Burder, who attributed such changes in the activity of the solar corona.

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Author’s note: Mr. Denning did not mention the considerable effects of atmospheric dust, which has a known reddening effect on astronomical bodies, e.g. sunsets, owing to a phenomenon known as Rayleigh scattering. His description of the unusually dim appearance of the lunar eclipse of October 4 1884, could be explained by the Volcanic eruption of Krakatoa, Indonesia, in August 1883, which would have ejected a considerable mass of dust into the Earth’s upper atmosphere, causing freak meteorological conditions well into 1884.

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Our satellite presents such a wealth of intricate detail through a Newtonian of moderate aperture, that it is scarcely possible to describe the impact with which it assaults the eye on a clear and tranquil night. The images of the Moon at moderate and high power through this author’s 8-inch f/6 Newtonian would have been broadly comparable to what Mr. Denning saw and recorded so diligently, that it is possible, at least to some degree, to ‘re-live’ the visual extravaganzas he remarks upon in the subsequent pages of his chapter on our faithful satellite in space.

While there is little doubt in Denning’s mind that the Moon is, to all intents and purposes, geologically dead, he is of the opinion, like so many other dedicated lunar observers before and after him, that changes can and do occasionally occur on its surface. Pages 120 through 123 recount a number of observations carried out by historical figures concerning this perennially interesting subject, beginning with the views of Sir William Herschel, who conducted extensive lunar observations using his “most excellent” 6.3 inch Newtonian reflector of 7 foot focus. On page 120 he reproduces Herschel’s lunar observations dated to April 1787:

“I perceive three volcanoes in different places of the dark part of the New Moon. Two of them are already nearly extinct, or otherwise in state of going to break out, the third shows an eruption of fire or luminous matter.”

pp 120.

But other observers soon offered less far-fetched explanations of these ‘fiery’ structures, particularly Schröter, who in fact used an identical 7 foot reflector to that employed by Herschel, suggesting they were due to reflected light from the Earth falling upon elevated spots of the Moon  having ” the unusual capacity to return it.

Denning’s contemporary, Wilhelm Tempel, of comet fame, reported what he thought was an impact of some sort on the evening of June 10, 1866, near the locus of the great crater Aristarchus;

“Of course,” he wrote, “I am far from surmising a still active chemical outbreak, as such an outbreak supposes water and an atmosphere, both of which are universally not allowed to exist on the Moon, so that the crater-forming process can only be thought of as dry, chemical, although warm one.”

pp 121.

On the same page, Denning recounts the extraordinary tale of the German astronomer Johann Friedrich Julius Schmidt (1824-1885), who claimed that the 5.5 mile diameter crater Linné had completely disappeared in 1866;

He averred that he had been familiar with the object as a deep crater since 1841 but in October 1866 he had found its place occupied by a whitish cloud. This cloud was always visible but the crater itself appeared to have become filled up, and was certainly invisible under its former aspect.

pp 121.

Denning discusses the observations of other observers, who took Schmidt’s report seriously, but in the end, the lack of confirmation led him to think that it was a trick of light. On page 122, he also relates the case of a one Dr. Klein, who, in contrast to Schmidt, reported the actual appearance of a “deep, dark crater” – about 18 miles to the west-northwest of Hyginus! This time, Denning himself had a look at the region with his 10-inch With-Browning Newtonian, but like many of his contemporaries, described it as a “saucer like depression” rather than the “sharply cut, deep crater” described by Klein

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Authors note: Schmidt’s observations caused international controversy for several decades, drawing the attention of many astronomers of repute. But while Schmidt had established himself as a careful and experienced observer, in the end the case was considered unproven. It is now known that the visibility of this crater is highly dependent on seeing conditions, being all but invisible under poor conditions of seeing.

Throughout the 20th century, a sizeable fraction of lunar observers continued to search for so-called transient lunar phenomena, which basically refer to any sudden changes to the lunar surface and which have a scientific basis in meteorite impacts, lunar out-gassings and the like. The lunar enthusiast is encouraged to keep reporting such lunar anomalies, as and when they occur. But you need to get out and look to see them!

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In the next section, Mr. Denning brings to our attention to the importance of timing when it comes to observing high resolution objects on the lunar surface:

As the Sun’s altitude is constantly varying with reference to lunar objects, they assume different aspects from hour to hour. In a short interval the same formations become very dissimilar.

pp 122.

Furthermore, Denning offers the reader some excellent advice, which, sadly, is not at all stressed by contemporary lunar observers:

The lunar landscape must be studied under the same conditions of illumination and libration, with the same instrument and power, and in a similar state of atmosphere, if results are to be strictly comparable. But it is very rarely that observations can be effected under precisely equal conditions; hence discordances are found amongst the records.

pp 123.

What follows on from this is an excellent summary of the most prominent lunar visual spectacles, together with brief notes on what can be observed with a modest telescope. The importance of note taking is once again stressed, especially the local time to the nearest minute. The text is illustrated by some exquisite drawings of T. Gwyn Elger (and reproduced quite well in this inexpensive reprint!).

On page 135 Denning discusses the occultation of stars by the Moon, which, he reminds us, occur several times each month! Here he mentions something rather curious:

The stars do not always disappear instantaneously. On coming up to the edge of the Moon they have not been suddenly blotted out, but have appeared to hang on the Moon’s limb for several seconds. This must arise from an optical illusion, from the action of a lunar atmosphere, or the stars must be observed through fissures on the Moon’s edges.

pp 135

The reader is encouraged to find out how the discussion develops!

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Author’s note: One gets the strong impression that Denning was an advocate of the volcanic origin of the lunar craters, a theory that was supported well into the 20th century. This is despite the fact that the impact theory of crater formation was alive and well ever since the time of Dr. Robert Hooke (1635-1703), who was among the first to suggest the latter as a plausible, alternate theory (discussed at length in this author’s up-and-coming book, Tales from the Golden Age of Astronomy), based on experimental science.

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Chapter VII  Mercury

Covering Pages 137-144.

In the opening paragraphs of this chapter, Mr. Denning identifies Mercury as the closest planet to the Sun, though he still gives mention to the elusive planet Vulcan, discussed previously in connexion with the Sun.  He then presents the basic astronomical information about Mercury, including its orbital period, eccentricity, elongations, true and apparent diameter, which, he informs us, varies from 4.5” to 12.9” at superior and inferior conjunction, respectively. These data are essentially modern. Denning also mentions the curious fact that the great Polish astronomer Nicolaus Copernicus, never once saw Mercury!

Copernicus, amid the fogs of the vistula, looked for Mercury in vain, and complained in his last hours that he had never seen it!

pp 138.

Following on from this, Denning discloses the number of sightings of the planet he made at this point in his astronomical career:

I have seen Mercury on about sixty-five occasions with the naked eye. In May 1876, I noticed the planet on thirteen different evenings, and between April 22 and May 11, 1890, I succeeded on ten evenings. I believe that anyone who made it a practice to obtain naked-eye views of this object would succeed from about twelve to fifteen times in a year.

pp 139.

He then follows up with details of particular apparitions of Mercury, as preserved in his voluminous notes, when the planet was particularly bright and striking to the eye, such as in February 1868 and in November 1882.

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Author’s note: It is quite probably true that many an amateur astronomer has never observed Mercury, owing to its very low altitude and proximity to the Sun. Denning was a prodigious observer though and the number of sightings he mentions pays testimony to that precocity.

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With characteristically delightful prose, Denning describes the momentous first sighting of the planet in the telescope and the excitement it induces in the observer:

The first view of Mercury forms quite an event in the experiences of many amateurs. The evasive planet is sought for with the same keen enthusiasm as though an important discovery were involved. For a few evenings efforts are in vain, until at length a clearer sky and a closer watch enables the glittering little stranger to be caught amid the vapours of the horizon. The observer is delighted and, proud of his success, he forthwith calls out the members of his family that they, too, may have a glimpse of the fugitive orb never seen by the eye of Copernicus.

pp 139.

After presenting further historical titbits, he then describes the general appearance of the little planet as it appeared through his telescope;

Mercury has a dingy aspect in comparison with the bright white lustre of Venus. On May 12, 1890, when the two planets were visible as evening stars, and separated from each other by a distance of only 2 degrees, I examined them in a 10-inch reflector, power 145. The disk of Venus looked like newly polished silver, while that of Mercury appeared of a dull leaden blue. A similar observation was made by Mr Nasmyth on September 28, 1878. The explanation appears to be that the atmosphere of Mercury is of great rarity, and incapable of reflection in the same high degree as the dense atmosphere of Venus.

pp 140

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Author’s note: While some observers have reported a pinkish tinge to the planet over the years, this is indeed reminiscent of the appearance of the planet seen in various telescopes over a few decades of time by this author. Regarding Mercury’s lack of an appreciable atmosphere, Denning’s conclusion is absolutely sound. Any primordial air it might have had has long been stripped away by the solar wind. What remains now is an extremely nebulous vapour, consisting mostly of the ions of the alkali and alkaline earth metals.

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Continuing on in this chapter, Mr. Denning discusses the ways in which the enthusiast may derive the maximum amount of information from this small and somewhat elusive world. With his simple, undriven mount, he advises the would-be observer to catch the planet just before dawn and to carefully follow it as it rises higher in the sky.  He refrains from making any detailed studies until a few hours after rising however, when the disk takes on a much steadier appearance. During these better moments, he most likens Mecury to the planet Mars in terms of the dark markings and spots it presents to the trained eye. For this he employed a power of about 212 diameters with his 10-inch silver-on-glass reflector.

On page 141-2, Denning reproduces the details of a correspondence he had with the famous Italian astronomer, G.V. Schiaparelli (of Mars fame) in 1882, who, using a fine 8.5 inch Merz achromatic refractor, agreed wholeheartedly with Denning that Mercury most resembles the Red Planet, at least superficially. Two fine drawings of the planet made by the great Bristolian observer himself are presented on page 143.

Denning further discloses details of Schiaparelli’s belief that the length of Mercury’s day is the same as its orbital period, in the same way as our Moon. He does however stress that these details still required corroboration.

The final pages of the chapter discuss transits of the planet as well as an occultation of Mercury by the Moon, dating to April 25, 1838.

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Author’s note: Schiaparelli’s claims about the length of a Mercurial day were not ultimately borne out. The planet in fact takes twice as long to revolve on its axis (176 days) as it does to complete one orbit of the Sun (88 days). However, this was not determined until 1965 using radar techniques.

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Chapter VIII Venus

Covering pages 145-154

Denning begins this chapter by commenting on the illustrious beauty of Venus at dawn or at dusk, and how the ancient believed the morning and evening stars were not one and the same. As harbinger of the day, Venus was known as Lucifer by the ancient Greeks and Hesperus, when the planet appeared as an evening star. When it appeared as an evening object in the autumn of 1887, Denning recalls that many people thought that the Star of Bethlehem had returned after a 19 century hiatus. He explains that at its greatest brilliancy, the planet is reduced to a slender crescent subtending an angular diameter of 65” at inferior conjunction. And when displaying its full disk, it shrinks in both size and luminous glory, presenting a disk scarcely one seventh as large (9.5”). As anyone who has examined Venus with telescopic aid will attest, the planet can be disappointing:

When the telescope is directed to Venus it must be admitted that the result hardly justifies the anticipation. Observers are led to believe, from the beauty of her aspect as viewed with the unaided eye, that instrumental power will greatly enhance the picture and reveal more striking appearances than are displayed on less conspicuous planets. But the hope is illusive……….. There are no dark spots, of bold outline, such as we may plainly discern on Mars, visible on her surface. There is no arrangement of luminous rings, such as encircle Saturn. There are no signs of dark variegated belts, similar to those that gird Jupiter and Saturn; nor is there any system of attendant satellites, such as accompany each of the superior planets.

pp 146-7.

Nonetheless, Denning concedes that Galileo’s observations of the phases of Venus through his primitive telescopes were enough to put the Copernican principle on a firm footing.

As with observing Mercury, he recommends that Venus is best observed during the day. He then launches into a brief survey of historical observations of the planet by celebrated observers of past centuries including J. D. Cassini (1666), Bianchini ( 1726-7), Schröter (1788) and Sir William Herschel (1777-93), and observations made in his own century including, Mädler (1833) and Di Vico (1840-1). Denning recounts in detail some observations conducted by Schröter, who thought that Venus had enormous mountains, the peaks of which would occasionally penetrate the clouds and reveal their presence in the telescope.

Like Mercury, the rotation period of Venus was unknown in Denning’s day and varied enormously from 23 hours, 21 min (Cassini 1666) to 224.7 days (Schiaparelli 1880).

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Author’s note: Schiaparelli was the closest to getting Venus’ rotation period correct. At 224 days it was less than 20 days short of the modern determined value of 243 days. He deduced this time period by assuming that the planet was tidally locked owing to its closer proximity to the Sun than the Earth. We now know that Venus rotates in a retrograde direction, a result of a possible collision with a large embryonic planet early in the history of the Solar System.

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Beginning on page 150 through 151, Denning discusses the nature of the many faint markings made by observers over the years. He notes that many of these reports were made by astronomers using rather small telescopes and how observers endowed with the visual acuity of the Reverend Dawes, failed to detect any markings with the telescopes he employed. He cautions that small telescopes will often create illusory views:

Perhaps it may be advisable here to add a word of caution to observers not to be hastily drawn to believe the spots are visible in very small glasses. Accounts are sometimes published of very dark and definite markings seen with only 2 or 3 inches aperture. Such assertions are usually unreliable. Could the authors of such statements survey the planet through a good 10- or 12-inch telescope, they would see at once they had been deceived. Some years ago I made a number of observations of Venus with 2-, 3- and 41/2 inch refractors and 4- and 10-inch reflectors, and could readily detect with the small instruments what certainly appeared to be spots of a pronounced nature, but on appealing to the 10-inch reflector, in which the view became immensely improved, the spots quite disappeared, and there remained scarcely more than a suspicion of the faint condensations which usually constitute the only visible markings on the surface.

pp 151

Denning gives mention to one of Venus’ most mysterious and enduring phenomena,  referred to today as the Ashen Light; a faint ‘ashy light’ similar to earthshine seen on the Moon, when the planet is near inferior conjunction and its slender crescent is most prominently displayed. He refers to the kind of illumination as a ‘phosphoresence’. He reports that a one Zanger, based in Prague, observed a ‘coppery ring’ completely encircling the planet on a number of occasions.

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Author’s note: The Ashen Light has a very long history associated with it, dating back to the mid-17th century. One of the finest astronomical artists of the post-war era, Richard Baum, of Chester, England, produced some wonderful renderings of the Ashen Light using his beloved old 4.5 Cooke refractor, which he enjoyed for many years. In more recent times however, some unscrupulous swine stole it from him, the whole affair disturbing him so much that he gave up observing altogether. What a shame!

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The remainder of the chapter discusses alleged observations of a satellite of Venus dating from the 17th and 18th centuries. The putative Cytherean moon, unofficially named ‘Neith’, was never positively identified and the consensus among astronomers of the 19th century was that the earlier sightings were nothing more than an ignis fatuus resulting from ghost reflections from eyepieces and the like. Curiously, Denning mentions the transits of Venus which occurred in 1874 and 1882, which he himself observed and even mentions the ‘future’ transit of 2004, which would thrill a new generation of astronomers.

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Author’s note: It is noteworthy that Denning completely avoids speculating on the nature of the Cytherean environment, particularly in light of the wild speculations that were doing the rounds in the late Victorian period. Back in 1870, his compatriot, Richard A. Proctor (1837-88), embracing Darwinian pseudoscience, thought nothing of considering Venus as the abode of life;

It is clear that, merely in the greater proximity of Venus to the sun, there is little to render at least the large portion of her surface uninhabitable by such beings as exist upon our earth. This undoubtedly would render [the sun’s] heat almost unbearable in the equatorial regions of Venus, but in her temperate and subarctic regions a climate which we should find well suited to our requirements might very well exist … I can find no reason … for denying that she may be considered the abode of creatures as far advanced in the scale of creation as any which exist upon the earth.

Many of Denning’s contemporaries thought it a certainty that life exists on other planets. Today, many of us know better though.

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To be Continued in Part II

 

De Fideli

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