Page:The Solar System - Six Lectures - Lowell.djvu/117



of producing such effect is a layer of atmosphere surrounding the disk of considerable depth. Jupiter's atmosphere is dense, and the absorption to which a ray of light would be subjected in passing in from the Sun and then out to us would increase from centre to circumference, and thus dim the edges of the disk.

Jupiter has two families of bodies connected with him; one an own one of satellites, the other with an adopted one of comets. With his satellites we made acquaintance in the last chapter; we must now be introduced to his comets.

Thirty-two comets circle near the planet and agree in the following distinguishing characteristics:—

1. Their aphelia hug Jupiter's orbit.

2. Their ascending nodes occur close to it.

3. Their motion is direct.

At some time in the past, therefore, each of these comets must have passed close to Jupiter, the comet and the planet chancing to arrive together at the node. At that epoch the comet must have suffered great disturbance at the hands of the planet, and its previous orbit have been radically changed.

D'Alembert, accordingly, suggested that Jupiter had captured these comets, and Laplace