Page:The story of the comets.djvu/146

104 study of cometary orbits which ended, as we shall soon see, in a famous and remarkable prediction. He undertook to investigate the movements of a large number of the comets previously recorded, with the view of ascertaining whether any, and if so which, of them had appeared to follow the same path. Careful investigation soon showed that the orbits of the Comets of 1531 and 1607 were similar to each other, and similar in fact to that of the Comet of 1682 seen by himself.

On Aug. 15, 1682, Flamsteed's assistant at the Royal Observatory, Greenwich, discovered a comet. A few days later the diameter of the head was about 2' of arc, and it had a tail 5° long. On Aug. 21 the tail had become 10° long. Flamsteed's observations seem not to have extended beyond Sept. 9, when the head had become enfeebled and was scarcely visible in the twilight. Halley himself, however, saw it a day later. Picard at Paris found the comet on Aug. 26, the head shining as a star of mag. 2. On Aug. 29 the tail was curved, the concavity being on the E. side. On Sept. 11 the head was so confused that it was only with difficulty that a luminous point could be perceived. Picard's last observation was on Sept. 12. Hevelius at Dantzig says that the comet was bright at the end of Aug. and could be seen all night with a tail from 12° to 16° long. In large telescopes a nucleus of an oval or gibbous form was constantly noticed. It was also remarked that on many occasions the direction of the tail was not exactly from the Sun, as P. Apian's observations of earlier comets suggested. The most remarkable of the matters mentioned by Hevelius was the existence of a luminous ray, or sector, thrown out from the nucleus into the tail. He has left behind a picture of this which is reproduced in the opposite woodcut (Fig. 41).

This ray was first noted about Sept. 8, and even making every allowance for the vagaries of the astronomical artists of the 17th century it is impossible to doubt that some sort of ray of light was thrown out from the head of the comet, and we shall presently see that the same thing happened in 1835.