The Heavens Above: A Popular Handbook of Astronomy

by J. A. Gillet; W. J. Rolfe

207. The Lunar Ecliptic Limits.—In Fig. 236 the line AB represents the plane of the ecliptic, and the line CD the moon's orbit. The large black circles on the line AB represent sections of the umbra of the earth's shadow, and the smaller circles on CD represent the moon at full. It will be seen, that, if the moon is full at E, she will just graze the umbra of the earth's shadow. In this case she will suffer no eclipse. Were the moon full at any point nearer her node, as at F, she would pass into the umbra of the earth's shadow, and would be partially eclipsed. Were the moon full at G, she would pass through the centre of the earth's shadow, and be totally eclipsed.

It will be seen from the figure that full moon must occur when the moon is within a certain distance from her node, in order that there may be a lunar eclipse; and this space is called the lunar ecliptic limits.

The farther the earth is from the sun, the less rapidly does its shadow taper, and therefore the greater its diameter at the distance of the moon; and, the nearer the moon to the earth, the greater the diameter of the earth's shadow at the distance of the moon. Of course, the greater the diameter of the earth's shadow, the greater the ecliptic limits: hence the lunar ecliptic limits vary somewhat from time to time, according to the distance from the earth to the sun and from the earth to the moon. The limits within which an eclipse is inevitable under all circumstances are called the minor ecliptic limits; and those within which an eclipse is possible under some circumstances, the major ecliptic limits.

Fig. 237.

208. Lunar Eclipses.—Fig. 237 shows the path of the moon through the earth's shadow in the case of a partial eclipse. The magnitude of such an eclipse depends upon the nearness of the moon to her nodes. The magnitude of an eclipse is usually denoted in digits, a digit being one-twelfth of the diameter of the moon.

Fig. 238.

Fig. 238 shows the path of the moon through the earth's shadow in the case of a total eclipse. It will be seen from the figure that it is not necessary for the moon to pass through the centre of the earth's shadow in order to have a total eclipse. When the moon passes through the centre of the earth's shadow, the eclipse is both total and central.

At the time of a total eclipse, the moon is not entirely invisible, but shines with a faint copper-colored light. This light is refracted into the shadow by the earth's atmosphere, and its amount varies with the quantity of clouds and vapor in that portion of the atmosphere which the sunlight must graze in order to reach the moon.