THE PHASES OF THE MOON. Except at opposition--i. e. when the earth is between the moon and sun--the whole of the moon’s disc does not appear bright to us, and the amount of the bright surface seen by us is found to depend on the relative positions of moon and sun. Half of the moon is always illuminated by the sun; but when it is in conjunction between the earth and sun the whole of the bright surface is on the side away from us; so that the moon is invisible. As it moves farther from the line joining earth and sun, a small portion of the bright side comes into view as a narrow crescent. This increases till half the disc is illuminated, when the lines joining earth and moon and earth and sun are at right angles. From this time the moon loses its crescent shape and becomes convex on both sides, or gibbous (Lat. _gibbus_, a hump)--the maximum brightness, or full moon, occurring when sun and moon are on opposite sides of the earth. After this the moon becomes gibbous, then crescent, and vanishes before the time of new moon.
It is worthy of note that the moon is higher in the heavens and longer above the horizon in the winter than in summer. This is owing to the plane of its orbit being at night high towards the south in winter and low in summer, as is the ecliptic. The moon’s orbit, like that of other planets, is elliptical, but irregular. When nearest to the earth, she is said to be in _perigee_; when at the greatest distance, in _apogee_.
[Illustration: =DIAGRAM SHOWING HOW THE MOON’S PHASES ARE CAUSED=
In the above diagram, the earth is in the center, and the circle ACFH the orbit of the moon. Since the inclination of the plane of the moon’s orbit to the plane of the ecliptic is only a few degrees, we may neglect it in this case, and suppose the two planes to coincide. Let the sun lie in the direction ES. Since the distance of the sun from the earth is about three hundred and eighty-seven times the distance of the moon from the earth, the lines ES, HS, BS, etc., drawn to the sun from different points of the moon’s orbit, may be considered to be sensibly parallel. Let us first suppose the moon to be in conjunction with the sun at the point A. Here only the dark portion of the moon is turned towards the earth, and the moon is therefore invisible. This is called new moon. As the moon moves on towards B, the enlightened part begins to be visible, and when it reaches C, half the enlightened part is visible, and the moon is at its first quarter. When the moon is at F, in opposition to the sun, all the illuminated part is turned towards the earth, and the moon is full. The moon wanes after leaving F, passes through its last quarter at H, and finally becomes again invisible at A.]