Page:The Solar System - Six Lectures - Lowell.djvu/146



XIV. Orbital inclinations in the satellite systems increase with distance from the primary.

XV. The outer planets show a greater tilt of axis to orbit-plane with increased distance from the Sun (so far as detectable).

XVI. The inner planets show a similar relation.

Tidal friction explains xi. and xii. ; xiii., xiv., xv.,explanation. and xvi. are as yet unexplained.

Tidal friction would account for xiv., but only on the supposition that the outer satellites were given off first. This is contrary to Faye's theory, largely so to Ligondés's, and is not championed by any other, for Laplace's supposition with regard to this point cannot stand.

Not only must the outer satellite have been given off the first, but very long before the next inner one, and so on for all; for tidal friction is potent as the inverse sixth power of the distance.

A similar objection holds against the attempt to explain the increased tilt of rotation axis to orbit planes, as distance from the Sun increases both for the outer and the inner planets. This increased tilt with increased distance is well worth particular notice. It may be seen in the following table.