It is a strange coincidence that the Moon is about 400 times smaller in diameter than the Sun and about 400 times farther away. This creates a fascinating natural phenomenon - eclipses of the Sun. From time to time, as the Moon circles around the Earth, it moves in front of the Sun and completely blots out its light. We call this a total eclipse of the Sun. It happens at a new moon, but not at every new moon. A total eclipse can last up to 71 minutes, but is usually much shorter. It affects only a small part of the Earth: the Moon casts a shadow at most of only about 250 km across. Outside this complete shadow (umbra), a partial eclipse can be seen, with the Moon covering only part of the Sun. Astronomers travel all around the world to witness eclipses. It is only at such times that they can see the eruptions called prominences on the Sun, and the Sun's outer atmosphere, or corona. Eclipses of the Moon also occur. They happen when the Moon moves into the Earth's shadow in space. The Moon does not totally disappear from sight, because it is lit dimly by light refracted (bent) around the Earth by the atmosphere. At most a total eclipse of the Moon can last up to 21 hours.
