What Is a Solar Eclipse?

During a solar eclipse, the Moon disk blocks part or all of the Sun light for observers in its shadow. A total solar eclipse — the most dramatic type — happens when the Moon completely covers the Sun disk, revealing the ghostly solar corona. This is possible because of a remarkable cosmic coincidence: the Sun is about 400 times wider than the Moon, but also about 400 times farther away, making them appear nearly the same size in the sky. Total eclipses occur roughly every 18 months somewhere on Earth, but any given location may wait centuries between events.