Page:The story of the comets.djvu/75

IV. Although suggestions have been thrown out that several short-period comets discovered during recent years might possibly be returns of Lexell's Comet, yet the evidence is inadequate and unsatisfactory. Arago has a remark on this subject which deserves quotation. He says:—"Du Séjour has proved that a comet whose mass is equal to that of the Earth which would pass at a distance of 37,500 miles only, would extend the length of the year to 367d 16h 5m and could alter the obliquity of the ecliptic to the extent of 2°. Notwithstanding its enormous mass and the smallness of its distance, such a body would then produce upon our globe only one kind of revolution—that of the Calendar ."

The influence of the larger planets on comets is now so thoroughly recognised that it has become customary to speak of such planets having "families" of comets belonging to them.

The influence of Jupiter on certain periodical comets which constitute its "family" may be inferred from Fig. 25, without the necessity of any detailed statement. Many other comets besides those included in the engraving may be regarded as subject to Jupiter's influence. The following are the names of some of these arranged in the order of their aphelion distances, but the list is by no means complete, because it is limited to comets which have been observed more than once, whilst there are a number of comets which are believed to be Jupiter comets, but which have only at present been seen once. A full list of these will be found elsewhere.