Why does an eclipse happen?

An eclipse happens when one object in space blocks sunlight headed toward another. In a solar eclipse, the Moon crosses our sightline to the Sun, so a moving patch of lunar shade touches Earth and daylight briefly dims or disappears for people inside it.

The Sun and Moon seem nearly matched in size because the Moon is much nearer, even though the Sun is vastly larger. During totality, the bright face is hidden enough for the corona, a pale outer glow of hot gas, to show around the Moon.