Magnitude of eclipse
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In a 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 eclipse. The two 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.)
This measure should not be confused with the astronomical magnitude logarithmic scale of brightness.
[edit] Effect of the magnitude
When the eclipse magnitude is larger than one, the Moon disk may completely cover the Sun disk in the sky, and so the eclipse may be total. This is not guaranteed, however, unless the two disks are aligned, which depends on the position of the observer. Otherwise the eclipse is only partial. The path of totality, that is, of the travelling shadow of the Moon cutting off all sunlight from reaching the Earth's surface, is in any case a relatively narrow strip, at most a few hundreds of kilometers across.
When the eclipse magnitude is less than one, the disk of the Moon can not completely cover the Sun. When the centres of the two disks are aligned, a ring of sunlight remains visible around the Moon. This is called an annular eclipse, from Latin annulus, meaning "ring".
The eclipse magnitude varies not only between eclipses, but also during a given eclipse. It may happen that an eclipse starts out as annular, and then becomes total. The reverse is also possible. In very rare cases, the eclipse may proceed from annular, via total, back to annular. These mixed-type eclipses are called hybrid.
