Of the other periodical comets it will be unnecessary to give details. Some of them are still without full corroboration, only one return to perihelion having been observed. The reappearance of Pons’s Comet (1812) in 1883-4, and of Olbers’s Comet (1815) in 1887 furnished two excellent examples of well-determined comets belonging to the same class as that of Halley. Tempel’s discoveries in 1867, 1869, and 1873 afforded some interesting additions to the family of short-period comets, and the list of these is continually extending owing to the assiduity of observers, though the lost comets of Biela and Brorsen will have to be removed from it. Peters’s Comet of 1846 is also doubtful, as it escaped rediscovery in about 1859, 1872, and 1884; but this object may yet be captured at one of its succeeding apparitions. These bodies often evade redetection when their periods and paths are not accurately known. This has been fully exemplified in the case of the comets of Pons-Winnecke and Tuttle, which were unseen at several consecutive returns. It has been supposed, and not without reason, that the periodical comets are in process of wearing away. They apparently grow fainter at each return. Halley’s Comet in 1835 was only moderately bright, whereas in ancient times its appearance was magnificent.
Pons’s Comet (1812). Telescopic view. 1884, January 6, 5h 50m.
(10-inch reflector, power 60. W. F. Denning.)
Pons’s Comet (1812). Telescopic view. 1884, January 21, 13h 18m.
(5-inch refractor; comet-eyepiece, field 1-1/4°. E. E. Barnard.)
Grouping of Periodical Comets.—It is a curious circumstance that these bodies are assorted into groups having their aphelia near the orbits of major planets. The short-period comets comprised within the orbits of Encke’s (3·29 years) and Denning’s (8·69 years) have aphelia in the region of the path of Jupiter, hence they are occasionally referred to as Jovian comets. The next group is represented by the comets of Peters and Tuttle, with aphelia near Saturn. The third group includes the comets of Tempel and Stephan, with aphelia just outside the orbit of Uranus. The fourth group is shown by the comets of Halley, Pons, and Olbers, with three others less certainly ascertained, with aphelia exterior to Neptune. There are unmistakable indications of other groups far outside the known boundaries of the solar system, but these are not so well defined. This clustering of cometary orbits has been ascribed to the attractive influences of the large superior planets, which are so capable of disturbing the paths of comets passing near that the orbits become transformed, and the aphelia henceforth lie near the points of extreme perturbation. This has been called the “capture theory;” and there is also an “ejection theory,” which supposes the periodical comets to have had their birth in planetary ejections.
M. Hoek of Utrecht has found cases in which the orbits of two or more comets exhibit a common point of intersection in distant space, and infers their derivation from the same origin.
Further Observations required.—One of the chief and essential features in cometary work is the accurate determination of positions. But this entails the possession of expensive instruments, and a knowledge which amateurs have not always acquired. This department of labour can well be left to the trained hands at large observatories, where, fortunately, it meets with every attention. Ordinary observers will merely require to know the approximate place, and this is to be found by estimating the difference in R.A. and Dec. between a comet and a known star. The position of the latter may be found in a good catalogue and corrected for precession; then, allowing for the observed differences, the comet’s place may be assigned to within very small limits of error. A low power, embracing a field of 1° or more, is best adapted for these observations, as it is more likely to include a catalogued star, and will exhibit the comet, especially if a large one, to the best effect.