obtained; and of course the longer the interval of time on which they are based the more trustworthy the figures will become. The reason why the exact character of a comet's orbit is often very uncertain is that we on the Earth only see a comet when it is more or less near the Sun, near, that is, the focus of the orbit: and the shape of an orbit, whether really elliptic, parabolic or hyperbolic, differs very little at, and immediately on either side of, the perihelion point or point of
Fig. 29.
DIAGRAM SHOWING THAT WHEN A COMET IS NEAR PERIHELION AN ELLIPSE, PARABOLA AND HYPERBOLA DIFFER VERY LITTLE IN PLAN.
nearest approach to the Sun, as may be inferred by an inspection of Fig. 29.
Recently photography has come to play some part in the work of comet-hunting,[1] and it may here be mentioned that the Comet of 1892 (v.), discovered by Barnard on Oct. 12, was the first discovery which was due to a comet having recorded itself on a photographic plate arranged for stellar survey. He exposed a plate for a certain time, directed to the neighbourhood of α Aquilæ. On developing the plate a trail,
- ↑ Special photographic lenses may be had which permit of a large field of view being embraced; but a good driving clock for the telescope is essential as, owing to their faintness, comets need a long exposure before they will reveal themselves.
