An eclipse is caused by the movement of a moon or planet into the shadow of another. The two types of eclipses that occur on Earth (solar and lunar) involve the interplay of the intersecting rotations of Earth and the moon around the sun.
A solar eclipse occurs when, as the moon orbits Earth, its path crosses between Earth and the sun, obscuring the sun from view from Earth. This can result in a total, partial or annular eclipse of the sun. A solar eclipse occurs every 18 months.
A lunar eclipse occurs when Earth's rotation around the sun causes it to come between the sun and the moon. This results in a partial or total lunar eclipse, which can only be observed from Earth at night and with a full moon. Two lunar eclipses occur each year.