Collier's New Encyclopedia (1921)/Sun, Eclipses of the

SUN, ECLIPSES OF THE, caused by the moon coming between the earth and the sun, may be either partial, total, or annular. In a partial eclipse the observer is situated in the penumbra of the moon's shadow, and only a part of the sun's light is cut off on one side. In a total eclipse the observer is in the umbra of the moon's shadow, and all the light of the sun is cut off except that from the prominences and corona surrounding the sun. In an annular eclipse the disk of the moon is wholly projected on that of the sun, but is not large enough to cover it completely, so that a ring of sunlight is left all round the moon. The apparent diameters of the sun and moon are so nearly equal that their variations with their varying distances make a central eclipse sometimes total and sometimes annular. In fact, the same eclipse may be total for some parts of the earth where the sun and moon are near the meridian at the time, and annular for other parts where they are low down in the E. or the W. at the time of central phase. In the annular eclipse the observer, instead of being in the cone of the moon's shadow, is in the prolongation of that cone, beyond its apex, in the other nap of the cone. At present the only scientific importance of partial and annular eclipses is the use that may be made of them for determining the relative positions of the sun and moon, and thus correcting the elements of the terrestrial and lunar orbits. But the fleeting minutes of every total eclipse are now utilized so far as possible to study the sun's surroundings, especially the mysterious corona, which is so faint that it is only visible when the bright light of the photosphere is cut off (see ). Geometrically considered, a solar eclipse is only an occultation of the sun by the moon, like that of any other heavenly body by the moon in its motion across the heavens.