Eclipse (Gr. to fade away, vanish), the obscuration of one celestial body by another, whether by intercepting the light coming from it or the light illuminating it. Eclipses of the former kind are sometimes called eclipses, sometimes occultations, and sometimes transits. Thus when the moon hides the sun's face wholly or in part, the phenomenon is called an eclipse of the sun or a solar eclipse; but when the moon conceals a star, the phenomenon is called an occultation of the star. Again, when Jupiter or Saturn hides one of his satellites by intervening between the earth and that satellite, the event is called an occultation of the satellite; but when a satellite of Jupiter hides a portion of the planet, a transit of the satellite is said to be in progress; and when Venus or Mercury hides a portion of the sun's face, a transit of Venus or Mercury is said to be in progress. Events of the second kind are called eclipses in nearly all cases, but occasionally the transit of a shadow.
Thus if the earth's shadow falls upon the moon, so as to obscure her face either wholly or partially, the phenomenon is called an eclipse of the moon; so also if the shadow of Jupiter or Saturn obscures a satellite of either planet, the phenomenon is called an eclipse of the satellite; but when the shadow of a satellite of either planet falls upon the planet, a transit of the shadow is said to be in progress. - For the principal phenomena presented during a solar eclipse, see Sun. It is only necessary to mention here that, owing to the various distances of the sun and moon, a solar eclipse may be total or annular when central; that is, either the whole face of the sun may be concealed, or a ring of the sun's light may remain visible all round the disk of the moon. When the eclipse is not central, so that at the moment of greatest obscuration a crescent-shaped portion of the sun remains visible, the eclipse is said to be partial. - In lunar eclipses the shadow of the earth seldom falls very nearly centrally upon the moon; yet owing to its great breadth at the moon's distance, the moon is often totally immersed in the shadow. When this happens the phenomenon is called a total lunar eclipse.
When the moon passes only in part into the earth's true shadow the phenomenon is called a partial lunar eclipse. As the earth's shadow is surrounded by a penumbra, it occasionally happens that the moon passes partly within the penumbra without reaching the true shadow. Such phenomena have been called penumbral lunar eclipses. It might be supposed that when the moon is totally immersed in the true shadow of the earth she is totally obliterated from view; but this seldom happens. Usually the moon in the heart of the earth's shadow presents a dark red or copper-colored disk. It is believed that the light which thus illuminates her is sunlight which has undergone refraction by the earth's atmosphere. Indeed, it is demonstrable that if the moon is centrally eclipsed, even when she is nearest to the earth, a visual line from any point of the moon tangential to the earth's globe would be so bent by refraction in its passage through the earth's atmosphere as to reach the sun's disk; so that an inhabitant of the moon would see a ring of reflected sunlight all round the earth's disk during one of these total eclipses.
But this would not happen if all those parts of the terrestrial atmosphere which bordered on the earth's visible half were cloud-laden; and this is believed to be the reason why occasionally the moon disappears wholly from view when totally eclipsed.