An eclipse, in astronomy, is when one celestial object blocks all or part of another object from view. Usually when we talk about eclipses we are talking about those related to Earth: solar and lunar eclipses. In a solar eclipse, the moon moves between the Earth and the sun. The shadow cast by the moon falls over a portion of the Earth and blocks the majority of sunlight there.Solar Eclipse In a lunar eclipse, the Earth comes between the moon and the sun and the moon is enveloped in Earth's shadow.

The shadow cast by a celestial body is called the umbra. It is a cone-shaped shadow that falls behind that body relative to the sun. The Earth, being large compared to the moon, casts a considerably large umbra. The moon's umbra, however, is much smaller. During a solar eclipse, the largest area to be covered by the moon's shadow is around 150 miles or so in radius.

This lasts, on average, for about three minutes before the moon moves out from between the sun and the Earth. Because of this, seeing total solar eclipses is not a common occurrence. For example, in the United States, the last total eclipse occurred in 1991 in Hawaii. The last total eclipse in the lower 48 states occurred in 1979 in Washington. The next total eclipse in the U.S. will not occur again until 2017.

If an eclipse occurs when the moon comes between the Earth and the sun, then why don't we have an eclipse every time there is a new moon? The reason is that the moon's orbit is not exactly on the ecliptic, but rather it is tilted slightly. We can only have an eclipse when the moon's orbit falls on the ecliptic during the new moon or full moon phases.