Page:The story of the comets.djvu/288

230 The Perihelion points are not distributed at all evenly in longitude all round the Sun, for more than 60 per cent. are concentrated within 45º of longitude on either side of what has been called "the Sun's Way", i. e. the line in space along which the Sun with its attendant planets is supposed to be travelling. I do not exactly know who was the first to point out this concentration, but Denning has reminded me in a private letter that "the circumstance is so marked that there is certainly some significance in it".

The Ascending Nodes are distributed all round the ecliptic, with however a decided tendency to concentrate around two points having respectively the longitudes of about 80º and 270º.

The Inclinations of the orbits of comets vary from 0º to 90º: in other words, comets may be found moving almost in the ecliptic, or at any angle between that and a perpendicular to the ecliptic. When a new comet is found to have a small inclination (anything under 15º) it is not unlikely that the orbit is an ellipse and the comet a periodical one.

The Perihelion Distances of comets vary greatly. 11 comets have a perihelion distance less than 5 millions of miles: about 64 per cent. of all that have been calculated lie within the Earth's orbit; about 30 per cent. lie outside, but within twice the Earth's distance from the Sun; and 16 comets have been observed with a perihelion distance exceeding that limit. A single one, the Comet of 1729. had a perihelion distance exceeding 4 radii of the Earth's orbit—as great a distance as the remoter asteroids. Young well remarks:—"it must have been an enormous comet to be visible at such a distance."

The figures which have yielded the foregoing results are the following:—

The Direction of Motion of a comet is a matter of some interest. With 2 exceptions all the periodical comets of less