Positions of the sun, moon and Earth during a solar eclipse. Umbra and penumbra are regions of the moon's shadow.

Photo courtesy NASA

What Is a Solar Eclipse?

A solar eclipse occurs when the moon passes in a direct line between the Earth and the sun. The moon's shadow travels over the Earth's surface and blocks out the sun's light as seen from Earth.

Because the moon orbits the Earth at an angle, approximately 5 degrees relative to the Earth-sun plane, the moon crosses the Earth's orbital plane only twice a year. These times are called eclipse seasons, because they are the only times when eclipses can occur. For an eclipse to take place, the moon must be in the correct phase during an eclipse season; for a solar eclipse, it must be a new moon. This condition makes solar eclipses relatively rare.