Page:The Solar System - Six Lectures - Lowell.djvu/123



whence it came. Practically, Jupiter's bulk does get in its way, and the limit of the planet's power lies below such direct reversal; nevertheless, it is sufficient in many positions to cause the comet to sweep round and dart away from the Sun with a speed such as to carry it beyond the Sun's control.

The planet's greatest effect in turning the comet is shown in three different conditions of approach. The comet enters along the unbroken lines and leaves by the broken ones.

You will notice that Jupiter's power is solely one of deflection. He cannot vie with the Sun directly in a tug of war; but he can deflect the comet and thus use the very speed imparted by the Sun against the Sun's attraction. It is like the Japanese jiu-jitsu, or scientific wrestling, of which the art consists in so adroitly turning another's strength against himself as to make the man's own momentum cause his fall.

Considering the case in this wise, we shall have the key to all of Jupiter's control. Form a triangle of velocities, of which the one side shall represent Jupiter's motion in amount and direction, a second the comet's, and the third the relative motion of the one body about the other; then draw a circle with the last for radius from the