An eclipse of the Sun takes place when the Moon comes between Earth and the Sun so that the Moon's shadow sweeps over the face of Earth. This shadow consists of two parts: the umbra, a cone into which no direct sunlight penetrates; and the penumbra, which is reached by light from only a part of the Sun's disk.
To an observer within the umbra, the Sun's disk appears completely covered by the disk of the Moon; such an eclipse is called total. To an observer within the penumbra, the Moon's disk appears projected against the Sun's disk so as to overlap it partly; the eclipse is then called partial for that observer. The umbral cone is narrow at the distance of Earth, and a total eclipse is observable only within the narrow strip of land or sea over which the umbra passes. A partial eclipse may be seen from places within the large area covered by the penumbra. Sometimes Earth intercepts the penumbra of the Moon but is missed by its umbra; only a partial eclipse of the Sun is then observed anywhere on Earth.
By a remarkable coincidence, the sizes and distances of the Sun and Moon are such that they appear as very nearly the same angular size (about 0.5°) at Earth, but their apparent sizes depend on their distances from Earth. Earth revolves around the Sun in an elliptical orbit, so that the distance of the Sun changes slightly during a year, with a correspondingly small change in the apparent size, the angular diameter, of the solar disk. In a similar way, the apparent size of the Moon's disk changes somewhat during the month because the Moon's orbit is also elliptical. When the Sun is nearest to Earth and the Moon is at its greatest distance, the apparent disk of the Moon is smaller than that of the Sun. If an eclipse of the Sun occurs at this time, the Moon's disk passing over the Sun's disk cannot cover it completely but will leave the rim of the Sun visible all around it. Such an eclipse is said to be annular. Total and annular eclipses are called central.
In a partial eclipse the centre of the Moon's disk does not pass across the centre of the Sun's. After the first contact, the visible crescent of the Sun decreases in width until the centres of the two disks reach their closest approach. This is the moment of maximum phase, and the extent is measured by the ratio between the smallest width of the crescent and the diameter of the Sun. After maximum phase, the crescent of the Sun widens again until the Moon passes out of the Sun's disk at the last contact.
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