Solar and Lunar Eclipses

An eclipse happens when the Sun, Earth and Moon line up so that one body's shadow falls on another.

Solar eclipse — the Moon's shadow falls on the Earth.

• Order: Sun — Moon — Earth.
• Only possible at New Moon.
• Happens in the daytime, and only for people standing inside the small shadow patch.
• The Sun is hidden, the sky darkens and the temperature drops for a few minutes.

Lunar eclipse — the Earth's shadow falls on the Moon.

• Order: Sun — Earth — Moon.
• Only possible at Full Moon.
• Happens at night, and everyone on the night side of Earth can see it at once.
• The Moon usually turns a deep coppery red rather than going black, because some sunlight bends through Earth's atmosphere and reaches it.

A remarkable coincidence makes total solar eclipses possible at all: the Sun is about 400 times wider than the Moon, but it is also about 400 times further away, so from Earth the two discs look almost exactly the same size.

Worked example — why is there not an eclipse every month?

There is a New Moon and a Full Moon every month, so at first it seems there should be two eclipses a month. There are not, and the reason is a tilt.

The Moon's orbit around the Earth is tilted by about $5^\circ$ compared with the Earth's orbit around the Sun. Five degrees sounds tiny, but across 384 400 km it is enough that at most New Moons the Moon's shadow passes above or below the Earth entirely, and at most Full Moons the Moon passes above or below the Earth's shadow.

Only when a New or Full Moon happens to fall at one of the two points where the tilted orbits cross do the three bodies line up properly — and that is why eclipses happen a few times a year, not twenty-four.