THE SYSTEM OF MARS AND ITS MOONS CONTRASTED WITH THAT OF THE EARTH AND MOON
In this diagram the markings on the earth and Mars are to scale, the orbits of the planets are seen in perspective and the measurements are according to Prof. Percival Lowell.
The Moon’s Orbit. Her path is approximately an ellipse with the earth in one focus. Its apparent motion in the sky is from west to east, but she moves much faster than the sun, taking about twenty-seven days eight hours to travel all round the earth. The time between two successive new moons (synodic period or lunation) is twenty-nine and one-half days. The reason of the difference is that the sun moves slowly in his annual course through the stars in the same direction as the moon, which therefore in its revolution round the earth has to overtake him when it returns. The moon rotates on its axis in the same time as it performs a revolution in its orbit; hence the same half is always turned toward us.
When the moon in her orbit lies between the sun and the earth, she is said to be in conjunction with the sun; when the earth is between the moon and the sun, the moon is said to be in opposition to the sun. At either of the two points midway from conjunction and opposition, i. e. 90° from conjunction or opposition, the moon is said to be in quadrature.
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.
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.