Page:The story of the comets.djvu/40

18 common origin in some giant comet, centuries previously. The comet discovered by Coggia and Winnecke, in November 1873, as it has an orbit resembling Biela's, may have sprung from a common stock, whilst its orbit is very similar to that of 1818 (i.). Similar relationships may be traced between Barnard's Comet of 1884 (ii.), Wolf's Comet of 1884 (iii.), Wolf's Comet of 1875, and Coggia's Comet of 1874 (iii.); also between the comets of 1807, 1880 (v.), 1881 (iii.), 1888 (i.), 1889 (iv.), and 1892 (i.). The periods of the first 5 range from 1700 years to 5130 years. The period of the last named is 20,200 years, its greater length being due to the influence of Saturn. I give all these details on the authority of Berberich, but do not hold myself responsible for them.

A less ambitious and more justifiable scheme of grouping than any of those just mentioned is that which puts together the comets (all of them "great" ones) of 1668, 1843 (i.), 1880 (i.), 1882 (iii.), and 1887 (i.). The members of this group all have orbits remarkable for their small perihelion distances, and also have elements almost identical, yet they cannot possibly be different appearances of one and the same comet. Their elements are:—

What these figures mean is this: that we have been visited by 5 comets pursuing nearly the same orbits, and following one another round the Sun at varying intervals as if they