A phenomenon in which the light from a celestial body is temporarily cut off by the presence of another. This may be: (1) the passage of a planetary satellite, such as the Moon, into shadow so that the direct illumination from the Sun normally causing it to shine is cut off; (2) the obscuration of all or part of the Sun by the passage of the Moon directly across it (solar eclipse); (3) the passage of a star belonging to a binary system behind its companion so that the total light received from the system is reduced.
There is some confusion of usage between the terms ╘eclipse' and occultation. If ╘eclipse' is reserved for the cutting off of sunlight by shadow, solar ╘eclipses' and the phenomena observed in eclipsing binary stars are, strictly speaking, occultations. However, the use of ╘eclipse' in these contexts is firmly established and normal.
In the description of the motion of the moons of other planets, such as Jupiter, it is usual to distinguish between eclipses and true occultations.
The Moon's orbit around the Earth is inclined at only 5í to the plane of the Earth's orbit around the Sun. From time to time, the three bodies become aligned and an eclipse of the Sun or Moon occurs (see illustration).
A solar eclipse can occur only at or very close to new Moon. Though the Moon is much nearer than the Sun, their apparent diameters are nearly equal at about half a degree. This coincidence makes total solar eclipses possible, with a maximum duration of 71/2 minutes. However, there are small variations in the apparent sizes of the Sun and Moon because the orbits of the Moon and Earth are elliptical rather than circular. The ratio of the diameters of the Moon and Sun is described as the ╘magnitude' of a solar eclipse. If a solar eclipse that would otherwise have been total occurs when the Moon's diameter appears less than the Sun's, an annulus (ring) of the Sun's disc remains visible when the centres of the two bodies are aligned. Such a solar eclipse is described as ╘annular'.
The Moon's shadow on the Earth is only a few hundred kilometres wide. It traces out a curved path as the motion of the three bodies makes the eclipse visible at successive locations. Over a wider region either side of the path of totality, a partial eclipse is seen. Partial eclipses may occur when no part of the Earth witnesses a total eclipse.
During the brief moments of a total solar eclipse, darkness falls, and the outer parts of the Sun, the chromosphere and the corona, whose light is normally swamped by the bright photosphere, become visible.
The relative motions of the Sun, Earth and Moon are such that at least two eclipses of the Sun must occur in any year (though most will be partial). The maximum number of eclipses in any one year is seven, two or three of which must be lunar. It is theoretically possible for solar eclipses to occur at successive new Moons, and for there to be a lunar eclipse in between. However, lunar eclipses at two successive full Moons are not possible.
