Page:The story of the comets.djvu/216

162 those of any comet which has been previously observed: if so (especially if the parabolic elements indicate a path through the heavens which is evidently not the path being followed by the comet), calculations for an elliptic orbit are undertaken and a period (usually stated in years and decimals of a year) is deduced. To compute an elliptic orbit for a comet or planet will take even an experienced calculator many hours of very hard work. An approximation may, however, be obtained by a graphical process.

It will be desirable to give the reader a little further insight into the nature of cometary orbits without going more deeply than can be helped into mathematical matters. The orbits of all comets, planets, and binary stars are in form sections of cones, whose size, exact form, and position in space are defined by numerical quantities technically termed "elements", which for brevity's sake are usually designated by symbols as follows:—

= Mean Longitude at an Epoch given. Instead of this some computers give the "Mean Anomaly at Epoch", and often it is not explicitly stated which is intended. But in neither case is it an independent element, since it is deducible from the time of the Perihelion Passage (T) and the Rate of Motion (μ). In fact, where this element is given (namely, in elliptic orbits), the time