Let us now turn our thoughts beyond the earth and its atmosphere to the phenomena which may properly be described as astronomical. We see a procession of objects moving ceaselessly across the sky—the sun by day, the moon and stars by night. These all appear to cross the sky from east to west, because the rotation of the earth, from which we view the spectacle, causes us to move continually from west to east.

The most conspicuous phenomenon is of course the daily motion of the sun across the heavens, producing the alternations of light and darkness, heat and cold, which we describe as day and night. The rising and setting of the moon and its passage across the sky are only one degree less conspicuous, and must have been not only noticed, but also familiar, since the days when human beings first appeared on the earth.

The sun shews no changes either of shape or brightness, except when our own atmosphere dims its light, but the moon continually varies in both respects. Every month it goes through the complete cycle of changes, which we call its “phases”. It begins as a thin crescent of light, which we describe as the new moon. This increases in size until after about a week we have the semicircle of light we call half moon, and then a week later the complete circle we call full moon.