Page:The story of the comets.djvu/147

IX. The Comet of 1682 seems to have been very generally observed by all the principal astronomers of the time, and amongst those who have left behind them observations we find the familiar names of Kirch of Leipzig, and Montanari of Padua; and the less familiar names of Zimmermann of Nuremberg and Baërt of Toulon.

Halley, making use of Flamsteed's observations, calculated parabolic elements of the comet in accordance with the rules laid down by Newton; and having also determined by the same methods the orbits of the Comets of 1531 and 1607 he was immediately struck by their similarity, and suspected from "the like situation of their planes and perihelions that the comets which appeared in the years 1531, 1607, and 1682



were one and the same comet that had made three revolutions in its elliptical orbit". This supposition implied that the comet's period was somewhere about 75 years. There were nevertheless 2 circumstances which might be supposed to offer some difficulty, inasmuch as it appeared that the intervals between the successive returns were not precisely equal; and that the inclination of the orbit was not exactly the same in each case. Halley, however, "with a degree of sagacity which, considering the state of knowledge at the time, cannot fail to excite unqualified admiration, observed that it was natural to suppose that the same causes which disturbed the planetary motions would likewise act on comets"; in other words, that the attraction of the planets might be expected to