On and on round us moves the moon, showing more light at every step. Now she passes quite outside the earth’s orbit, away from the sun. Not the slightest chance here of an eclipse of the sun, though an eclipse of the moon herself is quite possible. But more of that presently. As she reaches a point in a line with earth and sun--only generally a little higher or lower than the plane of the earth’s orbit--her round, bright face, shining in the sun’s rays, is turned exactly towards us. Then we have “full moon.” Still she goes on. Once more her light narrows and wanes, as part of her bright half turns away. Again at the “last quarter,” as at the first, she occupies a “sideways” position, turning towards us half her bright side and half her dark side. Then she journeys on, with lessening rim of light, till it vanishes, and once more we have the dark, invisible “new moon.” It was these phases and aspects of the moon which formerly gave birth to the custom of measuring time by months, and by weeks of seven days, on account of the return of the moon’s phases in a month, and because the moon appears about every seven days, so to say, under a new form. Such was the first measure of time; there was not in the sky any signal of which the differences, the alternations, and the epochs were more remarkable. Families met together at a time fixed by some lunar phase. The new moons served to regulate assemblies, sacrifices, and public functions. The ancients counted the moon from the day they first perceived it. In order to discover it easily, they assembled at evening upon the heights. The first appearance of the lunar crescent was watched with care, reported by the high priest, and announced to the people by the sound of trumpets. The new moons which correspond with the renewal of the four seasons were the most solemn; we find here the origin of the “ember weeks” of the Church, as we find that of most of our festivals in the ceremonies of the ancients. The Orientals, the Chaldeans, Egyptians, and Jews religiously observed this custom. An eclipse of the sun has already been described. An eclipse of the moon is an equally simple matter. An eclipse of the sun is caused by the dark, solid body of the moon passing just between earth and sun, hiding the sun from us, and casting its shadow upon the earth. An eclipse of the moon is also caused by a shadow--the shadow of our own earth--falling upon the moon. Here again, if the plane of the moon’s orbit were the same as ours, eclipses of the moon would be very common. As it is, her orbit carries her often just a little too high or too low to be eclipsed, and it is only now and then, at regular intervals, that she passes through the shadow of the earth.