The Frequency Of Solar And Lunar Eclipses

A solar eclipse, especially a total one, can be seen from only a limited part of Earth, whereas the eclipsed Moon can be seen at the time of the eclipse wherever the Moon is above the horizon.

In most calendar years there are two lunar eclipses; in some years one or three or none occur. Solar eclipses occur two to five times a year, five being exceptional; there last were five in 1935, and there will not be five again until 2206. The average number of total solar eclipses in a century is 66 for Earth as a whole.

Numbers of solar eclipses that have taken place or are predicted to take place during the 20th to 25th centuries are:

1901-2000: 228 eclipses, of which 145 were central (i.e., total or annular);

2001-2100: 224 eclipses, 144 central;

2101-2200: 235 eclipses, 151 central;

2201-2300: 248 eclipses, 156 central;

2301-2400: 248 eclipses, 160 central;

2401-2500: 237 eclipses, 153 central.

Any point on Earth may on the average experience no more than one total solar eclipse in three to four centuries. The situation is quite different for lunar eclipses. An observer remaining at the same place (and granted cloudless skies) could see 19 or 20 lunar eclipses in 18 years. Over that period three or four total eclipses and six or seven partial eclipses may be visible from beginning to end, and five total eclipses and four or five partial eclipses may be at least partially visible. All these numbers can be worked out from the geometry of the eclipses. A total lunar eclipse can last as long as an hour and three-quarters, but for a solar total eclipse maximum duration of totality is only 7% minutes. This difference results from the fact that the Moon's diameter is much smaller than the extension of Earth's shadow at the Moon's distance from Earth, but the Moon can be only a little greater in apparent size than the Sun.

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