Page:The Solar System - Six Lectures - Lowell.djvu/129



Either, then, Jupiter's present comet family has been of very slow growth, and each comet remains for a long time in the family, or it is made up only of short-period comets drawn from the immediate neighborhood.

Now, comets appear to be ephemeral things, being easily disintegrated into meteor swarms, and never abiding long in one stay. Thus the latter supposition seems on the face of it the more likely.

We may conclude provisionally that Jupiter's comet family came from the neighborhood.

It is certain that Jupiter has swept his neighborhood of such comets as do not fulfill the criterion of the angle $$\scriptstyle \chi$$; that is, of all the comets actually or potentially retrograde. If we consider the comet aphelia of short-period comets, we shall notice that they are clustered about the path of Jupiter and the path of Saturn, thinning out to a neutral ground between, where there are none. Two thirds way from Jupiter's orbit to Saturn's, space is clear of them, the centre of the gap falling at 8.4 astronomical units from the Sun.

Let us consider the mean comet; that is, a comet having the mean inclination of parabolic comets, the mean perihelion distance of the comets of Jupiter's family, such being the distance most likely to disclose them to us, and let this