A total eclipse of the Sun occurs when the Moon gets between the Sun and the Earth and covers up the Sun.
It just so happens that the Moon, as seen from Earth, and the Sun, as seen from Earth, are the same size in the sky. So if the two are exactly lined up, the Moon can hide the Sun from our sight. Total eclipses of the Sun are only visible on a small part of the Earth’s surface where the lineup of the Moon and Sun is exact, inside the inner shadow. The Moon always casts a long shadow in space, but most months it completely misses the Earth:
This video from NASA shows the motion of the Moon around the Earth and our shadows to scale.
Notice Earth's large shadow...when the Moon passes through the Earth's shadow, we have a lunar eclipse.
What to expect, stage by stage, from D. McGlaun
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