How do solar and lunar eclipses happen?

Eclipses occur when the Sun, Earth and Moon line up almost in a straight line in space. One body casts a shadow on another or passes in front of it, blocking light.

Solar eclipse

At new Moon, the Moon moves between Earth and the Sun. Its shadow falls on a small area of Earth's surface, and observers there see the Sun partly or fully blocked.

• Total eclipse: The Moon covers the Sun completely; only observers in a narrow path see it.
• Partial eclipse: Only part of the Sun is covered.
• Annular eclipse: The Moon is far from Earth and looks smaller, leaving a bright ring around the Sun.

Lunar eclipse

At full Moon, Earth lies between the Sun and the Moon, and the Moon passes through Earth's shadow. A lunar eclipse can be seen from anywhere the Moon is above the horizon. In a total eclipse the Moon often looks reddish, because red light bent by Earth's atmosphere still reaches it.

Why don't eclipses happen every month?

The Moon's orbit is tilted by about five degrees relative to the plane of Earth's orbit around the Sun. So at most new and full Moons, the Moon passes a little above or below the shadow line. An eclipse occurs only when a new or full Moon falls near the points where the two planes cross.

1. 1AlignmentThe Sun, Earth and Moon line up.
2. 2Shadow formsLight is blocked and a cone-shaped shadow extends into space.
3. 3Shadow moves onAs Earth and the Moon move, the shadow shifts and the eclipse ends.