Page:The Solar System - Six Lectures - Lowell.djvu/64

 46 of Jupiter and Saturn will be found to behave similarly.

Consummation of tidal effect marks the last stage in the planetary career. So soon as identity of rotation and revolution is effected, the planet is placed in a changeless, or largely changeless, state, which, so far as we can conceive, means as a world its death. It now turns the same face, except for libration, in perpetuity to the Sun, Day and night, summer and winter, have ceased to exist. One half of it is forever being baked, the other half forever frozen; and from this condition there is no escape. The planet must remain so until the Sun itself goes out.

Mercury, therefore, represents planetary decrepitude; and the symptoms of this old age are: loss of air; isochronous rotation and revolution; rotundity.