Page:The Solar System - Six Lectures - Lowell.djvu/131



It is worthy of notice how near the two are, showing that the parabolic comets come to us, practically, indifferently from all parts of space.

Calculating $$\scriptstyle\omega$$ and $$\scriptstyle\chi$$ for the successive aphelia, we find that, on the first supposition, $$\scriptstyle\omega$$ passes $$\scriptstyle\chi$$ at 8.4 astro, units; on the second, at 8.75 ditto. It is Jupiter, then, that has swept this space of comets.

Only a small fraction of Jupiter's comet family can ever come within our ken; for any comet whose perihelion lay outside of two astronomical units must, perforce, escape recognition. Invisibility would be caused both by the comet's distance from us and by its distance from the Sun, for the commotion set up in these bodies, as they near the Sun, is chiefly responsible for the display they make.

The family undoubtedly consists of many more comets with greater perihelion distance.

Jupiter is not the only planet that has a comet-family. All the large planets have the like. Saturn has a family of two, Uranus also of two, Neptune of six; and the spaces between these planets are clear of comet aphelia; the gaps prove the action.

Nor does the action, apparently, stop there. Plotting the aphelia of all the comets that have