FROM the earliest times since intelligent man has been watcher of the sky, men have been puzzled over the curious motions of certain bright objects which, because of their erratic movements in the otherwise orderly firmament, they nicknamed the "wanderers." Today we call them "planets," taking over the Greek label (πλανήτης = wanderer).
While these so-called planets all shared in the daily motion of the stars across the sky, rising in the east and setting in the west, it was early sensed that the planets, together with the sun and moon, had slow movements all their own among the starstrewn background of the sky. This slow individual motion for the moon was west to east and at such a rate as to allow of a complete circuit of the starry sphere once each month of about twenty-eight days. Any one who has watched the young crescent moon for the first few days after new moon recalls how each successive night the crescent is seen higher and higher in the western sky at sundown. When a week has elapsed sunset finds the moon a quarter around the sky, due south, with its half-illumined surface standing upright on the meridian. Another week elapses and the moon, now