A lunar eclipse happens when Earth passes directly between the sun and the moon, so the moon moves into Earth’s shadow. It can only occur at full moon, and only when the full moon happens near one of the two points where the moon’s tilted orbit crosses Earth’s orbital plane.
That tilt — about 5° — is why we do not get an eclipse every month. Most full moons pass above or below the shadow entirely.
The red
A fully eclipsed moon does not go dark. It turns a deep coppery red, and the reason is genuinely lovely: the only sunlight reaching it has passed through Earth’s atmosphere first.
Our atmosphere scatters short blue wavelengths and lets long red ones through — the same physics that makes sunsets red. It also refracts that remaining light inward, bending it into the shadow cone where the moon sits. The light falling on an eclipsed moon is the combined light of every sunrise and every sunset happening on Earth at that moment, focused onto one grey rock.
An astronaut standing on the eclipsed moon would look up and see Earth as a black disc ringed by a thin, brilliant band of red light: the whole planet’s terminator, glowing at once.
Why the colour varies
Some eclipses are bright orange, others so dark the moon nearly vanishes. The difference is the state of Earth’s atmosphere. Dust and volcanic aerosols block the refracted light, so eclipses in the year or two after a major eruption are markedly darker. The December 1992 eclipse, following Mount Pinatubo, was so dark the moon was hard to find.
Astronomers grade this on the Danjon scale, from L=0 (almost invisible) to L=4 (bright coppery-orange with a bluish rim).
Three kinds
A total eclipse puts the whole moon inside the umbra, Earth’s full shadow. A partial eclipse puts only part of it there, producing a distinct dark bite. A penumbral eclipse passes only through the outer partial shadow and is subtle enough that many people looking straight at it fail to notice anything at all.
Unlike solar eclipses, lunar eclipses are completely safe to look at, need no equipment, and are visible simultaneously from the entire night side of Earth — which is why you will see far more of them in your life than total solar eclipses.