Solar and lunar eclipses

An eclipse occurs when one body in space passes into the shadow of another, or blocks the light reaching it. For observers on Earth, the two familiar kinds are solar and lunar eclipses, both produced by the changing alignment of the Sun, the Earth and the Moon as the Moon orbits our planet. Although the Sun is vastly larger than the Moon, it is also much farther away, so the two appear almost the same size in the sky — a coincidence that makes total solar eclipses possible.

A solar eclipse happens at new moon, when the Moon passes directly between the Sun and the Earth and casts its shadow onto the Earth's surface. Within the narrow path of the darkest part of the shadow, observers may see a total eclipse in which the Moon completely covers the Sun's bright disc, briefly revealing the faint outer atmosphere. Outside that path a partial eclipse is seen, and when the Moon is slightly farther from Earth it cannot quite cover the Sun, leaving a bright ring in what is called an annular eclipse.

A lunar eclipse happens at full moon, when the Earth lies between the Sun and the Moon and the Moon passes through the Earth's shadow. During a total lunar eclipse the Moon is not blacked out but often turns a dim red, because sunlight bent through the Earth's atmosphere still reaches it. Eclipses do not occur every month because the Moon's orbit is tilted relative to the Earth's path around the Sun, so the three bodies line up closely enough only at certain times. Astronomers can predict eclipses precisely from the known motions and orbits of the Sun, Earth and Moon.

Frequently asked questions

What is the difference between a solar and a lunar eclipse?

A solar eclipse happens when the Moon passes between the Sun and the Earth and blocks the Sun; a lunar eclipse happens when the Earth passes between the Sun and the Moon, casting its shadow on the Moon.

Why don't eclipses happen every month?

The Moon's orbit is tilted relative to the Earth's orbit around the Sun, so the three bodies line up closely enough to produce an eclipse only at certain times.

Why does the Moon turn red during a total lunar eclipse?

Sunlight passing through the Earth's atmosphere is bent and filtered, and some of this reddened light reaches the Moon, giving it a dim red colour.