As the Moon travels around the Earth, it sometimes passes between the Earth and the Sun causing a shadow of the Moon to wind its way across the surface of the Earth. To see a total solar eclipse, you have to be in just the right place on the Earth. There is a strange coincidence in the apparent sizes of the Sun and Moon as seen from Earth - both look the same size in the sky. They are not really the same size. The Sun's diameter is 400 times larger than the Moon's diameter, but the Sun is also 400 times further away from the Earth so to us it looks the same size as the Moon! It is an amazing co-incidence that the Moon is exactly the right size and distance from us to just block out all the light from the solar surface.
When you are standing on the Earth looking up at the two of them (with a special filter!), you must be in a very special place to see the Moon cover the entire face of the Sun. If you move a little to the North, the Sun would peek out over the top of the Moon; a little to the South, and the Sun shines past the southern limb of the Moon. The match is so good that the "path of totality" is never more than 268 km wide, and is usually less. This means that very few people have seen a total eclipse because the shadow only covers a very small area on the Earth.
This picture shows how the Sun, Moon and Earth line up during a total Solar eclipse. From anywhere in the penumbra, you will see (remember - with a special filter!) some part of the Sun shining from behind the Moon while people within the umbra will witness a total eclipse.
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