Magnitude of eclipse

Magnitude of eclipse is the fraction of the diameter of the eclipsed body which is in eclipse. This applies to all celestial eclipses. During a partial or annular eclipse the magnitude of the eclipse is always between 0.0 and 1.0, while during a total eclipse the magnitude is always at least 1.0.

This measure should not be confused with the obscuration of the eclipse, which is the area fraction of the eclipsed body occulted by the eclipsing body, whereas the magnitude of an eclipse is strictly a ratio of diameters. Neither should it be confused with the astronomical magnitude scale of apparent brightness.

The Moon's and Sun's apparent sizes are approximately the same, but both vary because the distance between Earth and Moon varies. (The distance between Earth and Sun also varies, but the effect is slight in comparison.)

In an annular solar eclipse, the magnitude of the eclipse is the ratio between the apparent angular diameters of the Moon and that of the Sun during the maximum eclipse, yielding a ratio less than 1.0. As the magnitude of eclipse is less than one, the disk of the Moon cannot completely cover the Sun. When the centers of the two disks are sufficiently aligned, a ring of sunlight remains visible around the Moon. This is called an annular eclipse, from Latin annulus, meaning "ring".

For a total solar eclipse to happen, the ratio of the apparent diameters of the Moon and of the Sun must be 1.0 or bigger and the three celestial bodies (Sun, Earth and Moon) must be aligned centrally enough. When that is the case, the Moon's disk covers the Sun's disk in the sky completely. The path of totality (i.e. of the travelling shadow of the Moon cutting off all direct sunlight from reaching the Earth's surface) is a relatively narrow strip, at most a few hundreds of kilometers across.

In a partial solar eclipse, the magnitude of the eclipse is the fraction of the Sun's diameter occulted by the Moon at the time of maximum eclipse. As seen from one location, the momentary eclipse magnitude varies, being exactly 0.0 at the start of the eclipse, rising to some maximum value, and then decreasing to 0.0 at the end of the eclipse. When one says "the magnitude of the eclipse" without further specification, one usually means the maximum value of the magnitude of the eclipse.

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