It is an amazing coincidence that the size of the sun in the sky is the same as the size of the moon. When the moon happens to be in the same place in the sky as the sun, it exactly covers the sun, and the stars come out. If the moon were smaller, the sun would be bright, and we would not see the stars. If the moon were larger, we would not see the fiery ring around the sun called the corona.

 

What happens is that the moon exactly covers the sun, and it gets dark as night. This is a solar eclipse.