Page:The Moon (Pickering).djvu/31

Rh is slight, if the Moon at the same time happens to rise in the early evening, which it does when it is full, the phenomenon is quite noticeable and is called the Harvest Moon. This is the full moon which occurs nearest to the autumnal equinox. The next full moon following is called the Hunter's Moon.

The time required by the Moon to travel around the heavens until it arrives at the same point or star again is, as we have just seen, twenty-seven and one-third days. This period is called one sidereal revolution, and is the true period of revolution of the Moon. The time required by the Moon to again reach the same phase—as, for instance, from full moon to full moon—is twenty-nine and one-half days. This period is called its synodic revolution, and is what we ordinarily speak of as the lunar month. Both periods are liable to a slight variation; in the latter case it sometimes amounts to a little over half a day one way or the other. This period of twenty-nine and one-half days also represents the time required by the Sun to again reach the same altitude as seen from any particular lunar formation, and corresponds in the case of the Earth to what we ordinarily speak of as the solar day of twenty-four hours. The variation in length of this lunar day never amounts to more than a few minutes, so that, although it has the same average length as the synodic revolution, the two do not always exactly coincide. Thus, for instance, if the Moon is on the first quarter the same formation may sometimes be in full sunlight and sometimes in shadow. Since the lunar day is twenty-nine and one-half terrestrial ones, we always have to wait two months, or fifty-nine days, before we can repeat an observation with the terminator in the same position—or what comes to the same thing, with the Sun at the same altitude as seen from any given point upon the Moon.

The phases of the Moon are due to the fact that the Moon is a dark spherical body shining only by reflected light. Usually we can see but a portion of the hemisphere that is illuminated by the Sun. If we see less than half the illuminated hemisphere, the Moon is said to be a crescent; if more than half, it is spoken of as gibbous. When we see exactly half, the Moon is said to be on the quarter. Astronomically speaking, it is new moon only when the Moon is just between us and the Sun, and is therefore invisible. In ordinary parlance, however, we speak of it as new moon two or three days later, when we first see it in the west after sunset. Under these circumstances, when the sky is sufficiently dark, we can always see the complete outline of the disk, faintly showing against the background of the sky. This faint illumination is due to light coming