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Total eclipse of the Sun
Total eclipse of the sun. This is a heavily processed picture of the total solar eclispe of August 1999 visible from Europe to India.

Lunar and Solar Eclipses

Perhaps the most spectacular astronomical events that one can observe without a telescope, lunar and solar eclipses were considered omens of great fortune or complete disaster in ancient times. We now know that the occurrence of eclipses is a consequence of the orbits of the Earth and Moon with respect to the Sun.

The Earth orbits the Sun once a year, and the Moon orbits the Earth once a month; it turns out that the planes of the Moon's and Earth's orbits are almost, but not quite, aligned (the offset is about 5 degrees). This means that every once in awhile (a few times a year), the Moon passes through the Earth's shadow at night, blocking our view of the Moon: we call this a lunar eclipse. Similarly, the Moon can come between the Sun and the Earth during the day, temporarily blocking the Sun from view: these events are called solar eclipses. Thus contrary to the ancients' beliefs, these alignments are not conjured up by the divine but are predictable consequences of the Moon's and Earth's orbits in the solar system. It is quite fortuitous, however, that despite their vastly different physical extents, the Sun and the Moon appear to be about the same size in the sky (the Sun is much, much bigger than the Moon, but also much farther away from the Earth). So, when a solar or lunar eclipse occurs, it is possible for one object to completely obscure the other from view: this is called a total eclipse. It is more common, however, that only part of the Moon or Sun is obscured, in which case we call it a partial eclipse.

Though a total solar eclipse may be seen more than once a year on Earth, from a given spot on the planet these events are almost as rare as they are spectacular. The relative motions of the Earth and the Moon cause solar eclipses to be visible only within a strip of a few degrees in latitude, and total obscuration lasts no more than about seven minutes. Thus, at any single location on Earth, a total solar eclipse occurs only once every 300 years or so. Because the shadow cast by the Earth is quite a bit larger than the Moon, lunar eclipses are more common than solar eclipses, and totality can last for about an hour. Nonetheless, the beauty of such events entices both professional and amateur astronomers alike to chase them all around the globe!

The Ask an Astronomer team's favorite links about Lunar and Solar Eclipses:

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