Page:The Solar System - Six Lectures - Lowell.djvu/119



comets were not denizens of the Sun's domain, so he considered only parabolic comets.

We now know that all comets probably that man has ever seen are part and parcel of the Sun's retinue. They do not come to us from outer space, but are stable, if erratic, members of the solar system. In the light of this fact, we may profitably reconsider the subject.

Picture a comet, coming in to the Sun from space, to pass close to the planet in its journey, Within a certain distance of Jupiter, the planet's pull becomes so great that it is mechanically more exact to regard the comet as obeying Jupiter and perturbed by the Sun; and if the approach be very close, we may neglect in a first approximation the Sun's effect during the passage. This region is called Jupiter's sphere of influence, and is of the general shape of an ellipsoid, whose longest diameter follows the planet's path. The mean radius of the ellipsoid is three tenths of the Earth's orbit, no inconsiderable distance, and the extreme radii differ as 1 to 1.19.

As the comet is traveling, when it enters the planet's sphere or influence, with Sun-imposed velocity, its speed, even if the orbit be elliptic of small major axis, will exceed what Jupiter could cause. It will, in general, approach Jupiter with