JOHN HERSCHEL and HALLEY'S COMET
1835 – 1836
Preparations for the 1835 return of Halley's Comet began early. In 1817, the French astronomers, the Baron Damoiseau and the Count de Pontecoulant, calculated the movements of the comet, including the perturbations imposed by the planet Uranus, which had been discovered since the last return in 1759. (Neptune was not discovered until 1846). Damoiseau calculated that the comet's perihelion would be on November 4, 1835. Pontecoulant said it would be November 12.
On August 6, Father Dumouchel and Francisco di Vico, at an Italian observatory, were the first to see the barely visible comet, in the constellation of Taurus. Wilhelm Struve in Estonia saw the comet with the naked eye on September 23, and on the following night it had developed a visible tail. By October 14, Halley's Comet was bright, with a tail over 20 degrees long. Perihelion was on November 16.
John F. W. Herschel, grandson of William Herschel, the discoverer of Uranus, was studying the southern sky from an observatory at Feldhausen, near Cape Town, South Africa. He began his search for Halley's Comet on January 29, 1835, but it was not until October 28 that he “obtained a view of this long expected and anxiously looked for member of our system.” The sky at the Feldhausen observatory was often obscured by the 'vast pile of clouds' which frequently formed over the Table Mountain.
28 October 1835. 7-feet achromatic telescope
Several persons in Cape Town saw the comet with the naked eye on October 24, but the 'tablecloth' of Table Mountain continued to block Herschel's view. On the 28th, he gave up, and transported a 7-feet achromatic telescope to the sandhills of the Cape Flats, several miles away. As the twilight faded, he obtained an excellent view of the comet. It looked to the naked eye like a hazy star of the 3rd magnitude, with, as darkness deepened, a barely perceptible tail. No 'remarkable peculiarity about the head, nucleus, or tail' was visible through the telescope.
29 October 1835. 20-feet reflector telescope
The following evening (29th) was clear at Feldhausen. Herschel cut down several trees which were blocking his view of the region where the comet would set, and turned a 20-feet reflector, with its mirror recently polished for the event, to the twilight heavens. The appearance of the comet “was most singular, and such as I had never observed...in any previous comet.” The small bright nucleus “was shielded...on the side next the sun by a vivid but narrow crescent of nebulous light”. As the twilight faded, “the exterior strata of the crescent – the coma and the tail...became visible'. The coma appeared as a fan of pale light behind the crescent, but the tail 'was never very apparent.' On October 31st and November 1st, the crescent became 'less remarkable', although still visible.
28 January 1836. 20-feet reflector telescope
Observations of the comet were becoming difficult as it approached the Sun on its way to perihelion. On the 10th November, Herschel could still glimpse it by climbing above nearby trees, as a 3rd-magnitude star in strong twilight. A tail of 2½ degrees was visible through a night-glass as the head of the comet vanished behind Table Mountain.
Herschel searched for the comet after perihelion on November 16, but failed to see it until January 26, 1836, when it was seen in Scorpio as a dim hazy star. No tail was visible, even in a night-glass. However, with the 20-feet reflector, “the comet was indeed a most singular and remarkable object.” Within the well-defined 'head' of the comet was “a phenomenon, I believe, quite unique in the history of comets...a vividly luminous...object which I know no better way to describe than by calling it a miniature comet having a nucleus, head and tail of its own...considerably exceeding in intensity of light, the luminous disc or envelope`'. As the comet rose higher, “a minute bright point, like a very small star” became visible within this object, which Herschel called the nucleus.
On January 28, with the 20-feet reflector, the curve of Halley's envelope showed 'delicate and regular'. The nucleus was no longer a misty speck, but looked like “one of Jupiter's satellites in a thick fog of hazy light.” Herschel believed that he could almost resolve it into a disc. Herschel considered that the comet had been more or less evaporated into invisible vapour by the Sun's heat at perihelion, and was now in the process of “rapid condensation and re-precipitation on the nucleus.”
31 January 1836. 20-feet reflector telescope
On January 31, the head of the comet filled the viewfield of the 20-feet telescope. The head, 'a charming object', formed a clear parabolic outline, and resembled an 'alabaster vase illuminated from within.' On this night a tail began to develop, as a prolongation of the parabolic envelope., whilst the 'miniature comet' within also extended its tail. From the 1st to the 11th of February, the Moon hindered observations of the comet. On the 11th, Herschel described it as “a superb object, but grown much too large for the grasp of this telescope with its sweeping power.” The parabolic envelope had elongated into a tail.
On the 17th, the comet was even larger, and the nucleus brighter, in fact “much brighter than ever seen before”. The 'miniature comet', was now almost lost in the main tail. On February 19th, the nucleus was still bright, but “the parabolic envelope hardly to be made out, and evidently distorted.” The comet's envelope continued to expand and become fainter, until on the 18th of March, only the nucleus was visible amid a dim nebulous light. After May 3 it grew fainter. Herschel himself was the last person to see the comet, about May 20.
19 February 1836. 20-feet reflector telescope
What the nucleus of the comet actually was remained a mystery. Herschel's observations could not confirm or deny that it was a solid body, although “no phase was perceptible on that central mass”. He concluded that the expansion of the comet after January 26 could only be partly accounted for by its approach to the Earth, and, that, on January 21, 83 days after perihelion, Halley's Comet consisted only of the nucleus. On January 22, Professor M. Boguslawski of Breslau, observed the comet “as a star of the 6th magnitude, a bright, concentrated point, which showed no disc with a magnifying power of 140”. After this, the formation of the envelope began, as did the 'miniature comet', or 'Ray'. Then the material of the envelope which had been expelled by the nucleus was swept away into the tail by the action of the Sun.