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From Earth, a lunar eclipse occurs when Sun, Earth and Moon are in a single line with Earth in the middle. If this occurs, the Moon (or part of it) does not receive light from the Sun because it is in the shadow of the Earth, and thus the Moon becomes invisible even though there would normally have been a full moon. Most of the light that is refracted through the Earth's atmosphere is red, and thus an eclipsed Moon will glow in reddish hues.
A penumbral eclipse occurs when the Moon passes through the Earth's penumbra only. In such a case the Moon still looks full but appears dimmer. A total lunar eclipse occurs when the Moon travels completely into the Earth's umbra. If only part of the Moon enters then umbra, then there is a partial lunar eclipse.
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Lunar eclipses are only possible during a full moon, when the Moon is directly opposite the Sun. When it is full, the Moon usually passes above or below the ecliptic, which is why lunar eclipses are rare. However, on the occasions when it does align with the ecliptic, an eclipse may occur!
Unlike a solar eclipse, which can only be viewed in a certain relatively small area of the world, a lunar eclipse may be viewed from anywhere on the night side of the Earth.
If you were on the Moon during a lunar eclipse you would witness a solar eclipse, with the Earth passing in front of the Sun.
The Moon's speed through the Earth's shadow is about 1km per second (3,600 km per hour) and the longest the Moon can remain completely within the Umbra is 102 minutes.
History of Lunar Eclipses
Ancient Greeks noticed that during lunar eclipses the edge of the shadow was always circular. They thus concluded that the Earth was spherical.
See also : eclipse, solar eclipse