Page:The Solar System - Six Lectures - Lowell.djvu/128



Therefore, of the comets of Jupiter's comet-family, not only is none now retrograde, but none can ever become so unless some other body interfere with it.

A singular coincidence characterizes the values $$\scriptstyle \omega$$ and $$\scriptstyle \omega'$$ In all but two cases,$$\scriptstyle \omega$$nearly equals $$\scriptstyle \omega'$$, as if for some reason $$\scriptstyle \omega$$ were always trying to attain this maximum as a condition of stable equilibrium. In ten cases out of twenty, or in one half of the whole, the approach is within less than ½°.

It is to be noticed that in orbits potentially retrograde, the potential direct velocity is also greatest; so that both on the score of retrogradation and of greater direct velocity, comets pursuing such orbits are more subject to expulsion.

In course of time, comets possessing a high potential velocity must be weeded out of the sysem sooner or later, they must meet the planet under conditions of approach which convert their high potential velocity into an actual one. This will happen the sooner for comets in proportion to their velocity possibilities. It therefore will occur more speedily for originally parabolic comets than for elliptic ones of short period; but it will require some time even for them.