ECLIPSE (Gr. e leXegliiic from ciaciirew to forsake the accustomed place, to fail to ap pear), an obscuration of the light of a heavenly body by another, effected either, directly, by its intervention between the body and the observer, or, indirectly, by its interception of the source of light which illumines the body. The earth, by means of its shadow, may indirectly cause an eclipse of the moon. Similarly, eclipses of the Jovian satellites are caused by immersion in the shadow of Jupiter, and for the system of Saturn a more complicated result arises on account of its ring. When the shadow of a satellite falls upon either of these planets it is called a transit of the shadow. Eclipses are not observable in the other planetary systems, since, in the case of Mars, the satellites dis appear in the light of the planet, and with the more distant planets the shadow cone extends too slight a distance beyond the disc. Again, our moon can by its direct interposition between the sun and the earth produce eclipses of the sun. The moon may interpose itself between the stars or planets and occult them (see OCCULTATION ) , and the other bodies of the solar system may act similarly. The most important of these phenomena are the eclipses of the sun and moon.
Eclipses of the A lunar eclipse is caused by the interposition of the earth between the sun and the moon and the moon's consequent immersion in the shadow of the earth. This can occur only when the moon is in opposition to the sun in the heavens, that is, at the time of full moon, and is only possible when both the sun and the moon are near the moon's nodes, i.e., the points where her orbit crosses the plane of the ecliptic. Let the sun, earth and moon be regarded as spherical bodies and their centres ranged in a right line. Then the sun will project beyond the earth a conical shadow called the umbra, in which the moon is centrally immersed. The surface of this cone will be described by a line tangent to the same side of both sun and earth and moving about the line joining their centres. The apex of the shadow cone, or of the umbra, is on the average about 857,000 miles beyond the earth's centre, and the diameter of the umbra' cone at the point where the moon crosses it on the average about two and two-thirds the diameter of the moon. If, again, two lines tangent on opposite
sides of the sun, and crossing between sun and earth, move about the line joining the centres of these bodies, these lines will describe the surface of two cones of which the one extend ing beyond the earth is that of the penumbra. Unless the moon be near one of her nodes at the time of full moon, she will not strike the shadow, but pass below or above it. If the sun is less than eight days from a node of the moon's orbit, an eclipse must occur ; if more than 15 days from a lunar node, an eclipse is impossible. These are called the lunar ecliptic limits. Since the moon must be quite near her node to produce a lunar eclipse, it may happen that an entire year may elapse without an eclipse of the moon. The lunar eclipses may be either partial or total according as the moon is immersed either partly or entirely in the earth's shadow cone.
The phenomena of a total lunar eclipse are of a simple character. The immersion of the moon in the penumbra can be observed only with difficulty. The edge of the shadow itself presents a hazy outline when seen in a telescope and the times of contact cannot be observed with accuracy. When completely within the earth's shadow, the moon is usually still plainly visible; it is then usually of a dull copper color, although it may be greenish or even of a deep red; usually the moon's disc appears mottled by patches of different colors. The cause of this coloration is the sunlight shining through the earth's atmosphere, suffering absorption of its higher tints and experiencing deflection into the shadow cone through refraction. Lunar eclipses have, however, occurred in which the moon was entirely invisible during the total phase. The obscuration of the earth's atmos phere by clouds accounts for these dark eclipses. The most interesting physical observations effected during its eclipse concern the luminous and heat radiations of the moon. Valuable astronomical observations then possible are the determinations of the times of occultation of faint stars whence precise values of lunar position, diameter and parallax may be derived.