Abstract
In this paper I examine the observational and theoretical researches of William Herschel on 21 comets that he observed over the period 1781–1812. His sister Caroline’s comet discoveries, especially during the period 1786–1797, are also reviewed. Herschel’s focus, unlike most contemporaries studying comets, was on their physical structure, not their orbits. His observing procedures and analysis led him to forge a strong connection between comets and his investigations of nebulae. His overall scheme of cometary “maturation” (1812) involved a comet traveling from star to star after its central “planetary body” was born from gravitational collapse of a nebula. During any subsequent chance close passage of a star, the comet brightened and lost mass from its atmosphere, but it might also, when between stars, encounter another nebula and be rejuvenated by picking up more mass. Laplace soon adopted these ideas to improve his nebula hypothesis for Solar System formation. Finally, Herschel’s five overall cosmological principles are discussed, especially in the light of his ideas about comets.
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Notes
- 1.
The comet’s return in 1759, just as Halley had predicted in 1705, was universally viewed as a triumph of Newtonian theory. At that time 20-year-old Herschel had just arrived in England and was scratching out a musician’s living with his brother Jacob in London. We have no recollection from him of having seen the comet, even though in May it would have been a notable sight in the evening southern sky. We need to remember that, although London then had no light pollution, it did have heavy smoke; furthermore, Herschel was then a musician, not at all an astronomer.
- 2.
Comet Halley’s following apparition in 1835–1836 was well after William Herschel’s death; his sister Caroline, however, saw it at age 85 in October 1835 from Hanover, Germany, and even ~180 years ago she was hampered by light pollution. From her day book:
Oct 14–15, 1835. I saw the Comet, weather hazy. Gas lights all around me in the Street where I was obliged to go, none of my windows allowing me a prospect of that part of the heavens where the comet was visible. I was however gratified by seeing an object which has for many years been an object of conversation.
Oct 17. Saw the Comet again, very Bright, at Mrs. Beckedorff’s Country residence, but very near the horizon.
Caroline’s day book is at the Harry Ransom Center, University of Texas (Austin) Herschel Archives (hereafter Texas), 36.12/p5.
In addition, William’s son John made detailed observations of the comet from South Africa in January–May 1836.
- 3.
Edmond Halley also predicted that the Great Comet of 1661 would return in the winter of 1788–1789. Caroline and William searched for this, as did many others, but to no avail. While searching, however, Caroline discovered her second comet.
- 4.
- 5.
The span of dates for Caroline’s comet discoveries was driven by personal circumstances. As Hoskin has emphasized, seven of her eight discoveries took place between (a) William’s marriage in May 1788, which meant that his observing time greatly shrunk and thus her labor as assistant was less needed, and (b) her moving to a separate apartment in late 1797, making sweeping for comets far less convenient.
- 6.
- 7.
Maskelyne:W. Herschel, 25 Oct 1786, Royal Astronomical Society (London) Herschel Archives (hereafter RAS) W.1/13.1/m30 = i891-2. (A notation such as “p22” refers to page 22 of the original Ms. (where numbered). Often more convenient is the “image number” indicated here by “i24”, referring to PDF image number 24 on the set of three available DVD’s containing the entire RAS Herschel Archives.) Maskelyne is referring here to the Herschels’ technique of usually observing in complete darkness in the open air, resorting only rarely to dim light for jotting notes. The Greenwich refractors were apparently kept inside a partially lit room.
- 8.
C. Herschel:Joseph Banks, 17 Aug 1797, RAS C.1/3.8/i3.
- 9.
The listing of all Phil. Trans. papers on comets (and meteors) is given in Appendix 1 of Olson and Pasachoff (1998). The number of papers on comets peaks in the 1750–1800 period.
- 10.
In addition, we should not forget Herschel’s most prominent “cometary” episode, early in his career. On the evening of 13 Mar 1781, he discovered a non-stellar object and tracked it for months, arguing in many publications and letters that it was a comet. Eventually others worked out a circular orbit and disagreed – the Solar System had acquired a new planet (Uranus) well outside Saturn. The discovery paper of 1781 is entitled “Account of a comet.”
- 11.
- 12.
My rough estimate of ~70% of William’s 2300 nebulae observed only once comes from a perusal of Herschel’s published listings (excluding his clusters). For his Class III (“faint nebulae”), the fraction is much higher at ~90%. But despite this, Steinicke’s (2010:32) exhaustive analysis of Herschel’s catalogs concludes that only five of his nebulae cannot be found today at their reported positions. Might one of these five have been a comet? These results confirm that Herschel made a wise decision not to spend time checking all of his once-only nebulae to see if they might have been in fact comets.
- 13.
About 1% of Herschel’s nebulae were classified “cometic.”
- 14.
Herschel’s confusion over whether some of his new nebulous objects might be comets shows how difficult it was to tell the two categories apart without multi-night observations. This in fact was precisely the reason that Messier had published his listing of nebulous objects; he wanted to make life easier for comet hunters by establishing a reliable “nuisance list” of potentially misleading objects bright enough to be visible in small telescopes.
- 15.
W. Steinicke, Historical Catalogue of William Herschel Nebulae and Star Clusters. www.klima-luft.de/steinicke (accessed Nov 2016).
- 16.
In the RAS Herschel archives is an undated document (no source given) in which Herschel lays out the many complicated steps to determine the parameters of a comet’s orbit, given a few observed positions. But there is no extant evidence that he ever carried out such a calculation. (RAS W.3/39.2/pp48-52 = i28-32).
- 17.
Herschel’s support for the term asteroid, which first appeared in print in Herschel (1802a:228), led to its eventual adoption by the astronomical community. The story of the naming is given in detail in Cunningham (2016a).
Searching for new asteroids himself, Herschel made a few sweeps within the ecliptic plane, but came up empty-handed.
- 18.
In 1785 Herschel had also considered the possibility that his nebulae of Class IV (coined “planetary nebulae”) might actually be comets far from the sun. But he decided not, based on their large inferred brightness and size if at that distance compared to comets near the Earth and sun, when one would expect the opposite effect (Herschel 1785:265).
- 19.
Herschel:J. Banks, 19 Nov 1781, RAS W.1/7.
- 20.
Significant figures apparently were a concept unknown to Herschel and his contemporaries.
- 21.
His calculated phases had the object’s illuminated diameter ~20–25% less in one direction than the other. His observations to look for this, as well as his uncertainties, are discussed in the following section</InternalRef>”.
- 22.
Herschel also reported colors, but never used them in his interpretations. For example: “The colour of [the nucleus] was nearly white inclining to red, resembling the brilliancy of a coal in the fire when it is nearly as white as it can be, but not so white as Iron when it is in a welding heat.” (RAS W.3/1.12/p24 = i14).
- 23.
Newton, at the start of his Principia, had listed as one of the four “Rules of Reasoning in Natural Philosophy”: “To the same natural effects we must, as far as possible, assign the same causes.”
- 24.
Book 8, end of Chap. 22, where it is confusingly referred to as the “comet of 1812.” I have not been able to find any source reliably reporting that Napoleon himself viewed it as “his” comet, but certainly other persons did, portending good or bad depending on their nationality. In 1808 Napoleon was undoubtedly pleased when Messier pointed out that the Emperor’s birth coincided with the appearance of a bright comet in 1769 (Schechner Genuth 1987:54).
- 25.
If Herschel had published such a drawing as Fig. 2.7, in subsequent decades it would have become the standard to illustrate what a bright comet looks like; not until mid-nineteenth century did such detailed drawings finally appear. It is surprising that he did not seize this opportunity.
Fig. 2.6 
William Herschel’s observing log (in his hand) for 3 Nov 1811. This is his only drawing of the Great Comet of 1811 . The circle defines the field of view of 29′ on his 7-ft telescope at magnifying power of 118. The labels and line indicate orientation of the comet on the sky. RAS W.2/2.8/p8v = i11
Fig. 2.7 
Author’s drawing of the Great Comet of 1811 , based on the detailed description by William Herschel (1812a). The drawing is a “negative,” meaning that dark areas here were bright against the night sky, and light areas were dark. The small dot (slightly off-center) is the nucleus or central “planetary body.” It is surrounded by the “head” of size 4′. The head is surrounded by an empty gap, which Herschel called the “transparent cometic atmosphere”; this gap is bounded by the thin bright “envelope,” which wraps around the head and defines the outer edges of the tail. The “transparent cometic atmosphere” is presumed to extend indefinitely (and invisibly) outside the envelope (indicated in the drawing by the dashed circle). The tail extends far off the edge of the drawing (Drawing by Woodruff Sullivan)
- 26.
- 27.
Herschel (1785:218) had coined the term “globular cluster” much earlier.
- 28.
However, Herschel had also observed (p. 121) a slight sunward shift of the center of the comet’s head and atmosphere relative to the planetary body (see Fig. 2.7). This he ascribed to a preferential heating and dilation of the atmosphere on the sun side.
- 29.
Although Herschel had seen evidence for a nucleus in only 4 of 18 observed comets, he apparently felt that the visible nuclei of the much larger 1807 and 1811 comets inferred that all comets had central planetary bodies, but were often too small to discern.
- 30.
Herschel (1795:60–1) had earlier suggested that comets might well collide with our sun and thus restore its ever-decreasing mass due to emission of light particles. But in the present 1812 paper he did not mention this idea, nor specify the final resting place of the comet’s stripped atmospheric material.
- 31.
Herschel never provided estimates of how long it would take before an encounter occurred (but certainly he was thinking of a very long time), or what the odds were that such an encounter would even take place.
- 32.
The term unperihelioned does not appear in the Oxford English Dictionary, but many other Herschelian neologisms do: planetary nebula, globular cluster, asteroid, binary system, star gauge, penetration (power of a telescope), invisible ray (infrared radiation).
- 33.
Herschel believed that all planets, moons and stars were inhabited. However, the evidence for whether or not he extended the presence of intelligent life to comets (as did many) is ambiguous.
- 34.
John Herschel:William Whewell, 20 Aug 1837, Royal Society HS 21.228. Cited by M. Bolt on p. 289 of John Herschel entry in New Dictionary of Scientific Biography (Vol. 3, 2007), ed. Noretta Koertge (New York: Charles Scribner’s Sons).
- 35.
To be more exact, William’s logs are a “fair copy” (a neat copy, edited to varying degrees) made by Caroline of either William’s original written notes or Caroline’s notes as dictated to her by William with his eye to the telescope. Sometimes they are a Caroline copy of a copy of the originals. These copies even include reproductions by Caroline of William’s sketches of star patterns, planetary features, sunspots, comets, etc. Once copied, the originals were sometimes unfortunately discarded, but the evidence of the archives is that Caroline was fastidious in her copying and made very few mistakes. It is much the same story for almost all of William’s manuscripts submitted for publication – few drafts of any kind survive.
- 36.
These log book quotations for both comets are from RAS W.3/1.12, at cited page number.
- 37.
RAS W.3/37.1/p4.
- 38.
Using Voyager planetarium software, I have verified Herschel’s calculated phases and distances for these two comets.
- 39.
Today we know that each comet indeed does have a solid, icy body at its center, but its size is only ~10 km, meaning that Herschel had no chance of discerning it; only spacecraft passing close by comets have been able to see such nuclei. We do not know what apparent feature Herschel observed and measured.
- 40.
Although during Herschel’s era citations were far less frequent than today, my impression is that Herschel, even for his time, was below average in citing other’s work.
- 41.
- 42.
Bibliographie Astronomique (Paris), 850 (1803); cited by Schaffer (1987:67).
- 43.
Schechner Genuth (1997:209).
- 44.
The idea also shows up in the philosopher David Hume’s work Dialogues concerning Natural Religion (1779); cited by Schechner Genuth (1997:213–4). As an aspiring 22-year-old musician, Herschel had dinner with Hume in Edinburgh in 1761.
- 45.
RAS W.7/2.1.
- 46.
Englefield, H. (1788). p. 9 in Tables of the Apparent Places of the Comet of 1661, Whose Return is Expected in 1789. London: P. Elmsly. Also see footnote 3.
- 47.
Laplace also argued that the mass of at least one comet was less than 1/5000 the Earth’s mass, based on the fact that he could find no perturbations on the Earth’s orbit (specifically the length of the year) arising from the close passage (0.015 AU) of the comet of 1770 (Heidarzadeh 2008:196–9). This of course made it unlikely that comets could turn into planets.
- 48.
Herschel:J. Banks, 19 Nov 1781, RAS W.1/7.
- 49.
See Crowe (2011) for a full historical account of the notion of an inhabited sun, an idea that started long before Herschel and, abetted by his authority and arguments, lasted well past his time. Crowe (1986) exhaustively covers the larger question of extraterrestrial life during the eighteenth and nineteenth centuries. Sullivan (2013) examines aspects of Herschel’s views on extraterrestrial life, in particular his use of analogy.
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Acknowledgments
I give many thanks first to Michael Hoskin for his friendship and the insights and facts provided by his publications on the Herschels over the past half-century. In particular, his creation of purchasable DVD’s containing the entire Herschel archives of the Royal Astronomical Society has been of immense use to me and many others. Michael Crowe has also been extremely helpful in my Herschel researches. For assistance with aspects of this paper I thank Peter Abrahams, Don Brownlee, and Don Yeomans. The excellent service of the Interlibrary Loan section of the University of Washington Libraries has been indispensable, as have many librarians and archivists, in particular at the Harry Ransom Center of the University of Texas, and the Royal Society in London.
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Sullivan, W.T. (2018). William Herschel and Comets. In: Cunningham, C. (eds) The Scientific Legacy of William Herschel. Historical & Cultural Astronomy. Springer, Cham. https://doi.org/10.1007/978-3-319-32826-3_2
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