The earth, however, is turning on an axis, carrying the observer with it. And so it is continually presenting him to a new part of the sky. At any moment he sees but a single half-sphere; yet the very next instant it is no longer the same; a small portion has passed out of sight on one side by going down behind the turning earth, while a corresponding new section has come into view on the opposite side. It is this coming into view that we call the rising of a star; and the corresponding disappearance on the other side is called setting. Thus rising and setting are, of course, due entirely to a turning of the earth, and not at all to actual motions of the stars; and for this reason, all objects in the sky, without exception, must rise and set again. But the moon really has a motion of its own in addition to this apparent one caused by the earth's rotation. Somewhere in the dawn of time early watchers of the stars thought out those fancied constellations that survive even down to our own day. They placed the mighty lion, king of beasts, upon the face of night, and the great hunter, too, armed with club and dagger, to pursue him. Among these constellations the moon threads her destined way, night after night, so rapidly that the unaided eye can see that she is moving. It needs but little power of fancy's magic to recall from the dim past a picture of some aged astronomer graving upon his tablets the Records of the Moon. "To-night she is near the bright star in the eye of the Bull." And again: "To-night she rides full, and near to the heart of the Virgin." And why does the moon ride thus through the stars of night? Modern science has succeeded in disentangling the intricacies of her motion, until to-day only one or two of the very minutest details of that motion remain unexplained. But it has been a hard problem. Someone has well said that lunar theory should be likened to a lofty cliff upon whose side the intellectual giants among men can mark off their mental stature, but whose height no one of them may ever hope to scale. But for our present purpose it is unnecessary to pursue the subject of lunar motion into its abstruser details. To understand why the moon moves rapidly among the stars, it is sufficient to remember that she is whirling quickly round the earth, so as to complete her circuit in a little less than a month. We see her at all times projected upon the distant background of the sky on which are set the stellar points of light, as though intended for beacons to mark the course pursued by moon and planets. The stars themselves have no such motions as the moon; situated at a distance almost inconceivably great, they may, indeed, be travellers through empty space, yet their journeys shrink into insignificance as seen from distant earth. It requires the most delicate instruments of the astronomer to so magnify the tiny displacements of the stars on the celestial vault that they may be measured by human eyes.