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_. [Illustration: 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. [Illustration: 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. The duration of an eclipse varies between very wide limits, being, of course, greatest when the eclipse is central. A total eclipse of the moon may last nearly two hours, or, including the _partial_ portions of the eclipse, three or four hours. Every eclipse of the moon, whether total or partial, is visible at the same time to the whole hemisphere of the earth which is turned towards the moon; and the eclipse will have exactly the same magnitude at every point of observation. 209. _When there will be an Eclipse of the Sun._--There will be an eclipse of the sun _whenever any portion of the moon's shadow is thrown on the earth_. It will be seen from Fig. 235 that this can occur only when the moon is in conjunction, or at _new_. It does not occur every month, because, owing to the inclination of the moon's orbit to the ecliptic, the moon's shadow is usually thrown either above or below the earth at the time of new moon. There can be an eclipse of the sun only when new moon occurs at or near one of the nodes of her orbit.