Page:The evolution of worlds - Lowell.djvu/263

Rh For both reasons, nearness and speed, his angular revolution about the Sun varies greatly from point to point according to where he finds himself in his orbit. His rotation, however, is necessarily uniform. For even the Sun has no power at once to change the enormous moment of momentum of his axial spin. In consequence, at times his angular velocity of revolution gains on his rotation, at other times loses, both coming out together at the end of a complete Mercurial year. The result is a superb rhythmic oscillation, a true mercurial pendulum compensated by celestial laws to perfect isochronism of swing.

The outward sign of this shows in the movement of the markings. To observers in space like ourselves, the planet seems to sway his head as he travels along his orbit. For weeks he turns his face, as shown by the markings on it, more and more over to the left; then turns it back again as far over to the right. It is as if he were looking furtively around as he hastens over his planetary path.

Venus, of course, is equally subject to this law of distraction, but owing to the almost perfect circularity of her orbit she is less visibly affected. In fact, it is not possible to detect her lapse from a fixed regard to the Sun. At most it is no more than a glance out of the corner of her eyes—her slight deviation from perfect rectitude of demeanor. Knowledge of the laws