Page:The story of the comets.djvu/72



far as their movements are concerned comets may be divided into two classes: (1) those which may be regarded as permanent members of the solar system; and (2) those which have once, and once only so far as is known, visited the solar system. The comets belonging to Class 1 must be further subdivided into two great sub-classes: (a) those which have been ascertained by calculation and observation to be regular visitors to the neighbourhood of the Earth, and therefore of course permanently attached to the Sun; and (b) comets which are believed, so far as calculation goes, to belong to the solar system, but which as yet have only been once seen by us on the Earth. Later on we shall find it expedient to classify in more detail both types of comet, but these broad general divisions will suffice for the present.

The comets which have just been alluded to as permanent members of the solar system are called "Periodical Comets", but their periods vary between the extremes of 3 or 4 years and thousands of years. Comets with periods between 3 years and about 80 years are numerous, and may be regarded as familiar and well-recognised friends, but those whose periods run into hundreds or thousands of years are those which astronomers have found reason to believe are periodic but