Page:Hector Macpherson - Herschel (1919).djvu/27

Rh The last-named paper gave an account of the discovery which marked the turning-point in Herschel's career. On 17th August, 1779, he commenced his second review of the heavens. This review was made with his 7-foot telescope of 6·2 inches aperture, and included stars down to the eighth magnitude; its main purpose was to register double stars. On Tuesday, 13th March, 1781, in the course of this review, he jotted down in his journal the following note, in somewhat doubtful English: "In the quartile near Zeta Tauri, the lowest of two is a curious either nebulous star or perhaps a comet. A small star follows the comet at two-thirds of the field's distance." In the paper afterwards communicated to the Royal Society, he explained that he perceived a star which appeared "visibly larger than the rest: being struck with its uncommon magnitude, I compared it to H Geminorum and the small star in the quartile between Auriga and Gemini, and finding it so much larger than either of them, suspected it to be a comet". On Saturday, 17th March, he wrote, "I looked for the comet or nebulous star, and found that it is a comet, for it has changed its place". By Monday, the 19th, he found that the supposed comet "moves according to the order of the signs, and its orbit declines but little from the ecliptic".

The discovery was soon communicated to the Observatories of Greenwich and Oxford. Maskelyne, the Astronomer-Royal, wrote to Dr. Watson on the 4th April that he had observed the strange object, "very different from any comet I ever read any description of or saw". On 23rd April he wrote to Herschel: "It is as likely to be a regular planet moving in an orbit nearly circular round the Sun as a comet moving in a very eccentric ellipsis"; and he immediately notified the French astronomers of the discovery. Messier, the most famous observer of comets, commenced observations on 16th April, and his example was followed by Lalande,