Page:The story of the comets.djvu/44



of the more usual and prominent features connected with the tails of comets from the standpoint of recorded facts will now be dealt with, leaving more or less on one side the vast mass of theory and speculation which surrounds the subject.

It was observed by Peter Apian that the trains of 5 comets seen by him between the years 1531 and 1539 were turned from the Sun, forming more or less a prolongation of the radius vector, which is the name given to an imaginary line joining the centre of the Sun and the centre of the head of a comet. This may be regarded as a general rule, although exceptions do occur. Thus the tail of the Comet of 1577 deviated 21° from the line of the radius vector. Valz stated that the tails of the Comets of 1863 (iv. and v.) deviated from the planes of the orbits, and that only 2 other comets are known the tails of which did the same. In some few instances, where a comet has had more than one tail, the second