Direction of the tail opposite to the sun; discovered by Apian; the Chinese astronomers were acquainted with this law–Deviations in some comets–Variable aspect of the tail according to the relative positions of the comet, the earth, and the sun.

In respect to the direction of cometary tails let us call attention to an important point–to a general phenomenon which was remarked by the ancients, in the very earliest times. Seneca refers to it in the following line:–

The comæ of comets fly the ray a of the sun. According to Edward Biot the Chinese astronomers had observed, since the year 837. this constant direction of cometary tails from the sun. ‘ In Europe, ’ says Lalande, ‘ Apian was the first to perceive that the tails of comets were always opposite to the sun; this rule was afterwards confirmed by Gemma Frisius, Cornelius Gemma, Fracastoro. and Cardan. Nevertheless, Tycho Brahe did not believe it to be very general or well demonstrated; but the fact itself is beyond a doubt. ’

Pingré observes with truth that the direction of the tail is not always strictly opposite to the sun. He instances the comet of 1577, whose tail was deflected as much as 21° towards the south, and the great comet of 1680, when the deflection was about 4 ½°. On both these occasions, however, the comet and the earth occupied the same relative positions in the heavens.