What Is a Blood Moon? Why the Eclipsed Moon Turns Red

On the night of a total lunar eclipse, the full Moon does something quietly spectacular. It slides into Earth's shadow — and instead of going dark, it catches fire in slow motion, dimming through grey and orange into a deep, sombre red that hangs in the sky for as long as an hour and a half. People have called it a blood moon for centuries. The name is dramatic; the physics is better.

The shadow with a glowing edge

A lunar eclipse happens when the Sun, Earth and Moon line up — Earth in the middle — and the full Moon passes through Earth's shadow. That shadow has two parts: a pale outer fringe (the penumbra, where Earth blocks only part of the Sun) and a dark central core (the umbra, where Earth blocks the Sun completely).

If Earth had no atmosphere, the umbra would be truly black and the eclipsed Moon would simply vanish. But Earth does have an atmosphere, and it does two things to the sunlight grazing our planet's edge:

1. It bends the light. Refraction curves sunlight around Earth's rim and funnels some of it into the shadow — right onto the Moon.
2. It filters the light. Air molecules scatter blue wavelengths far more strongly than red (this is Rayleigh scattering — the reason our daytime sky is blue and our sunsets are red). By the time sunlight has skimmed sideways through hundreds of kilometres of atmosphere, mostly the reds survive.

So the light that reaches the eclipsed Moon is, quite literally, the combined light of every sunrise and sunset on Earth at that moment, projected onto the lunar surface. Stand on the Moon during totality and you'd see why: Earth would hang in the sky as a black disk, ringed by a thin, blazing red-orange circle of atmosphere.

Why some blood moons are darker than others

No two total eclipses look the same. The Moon's colour and brightness during totality depend on:

• How deep it goes. An eclipse where the Moon skims the edge of the umbra stays brighter and more orange; a central eclipse — where the Moon crosses the shadow's very axis — turns darker and redder. The 26 June 2029 eclipse is a central one, with over 100 minutes of totality, and should be strikingly dark.
• The state of Earth's atmosphere. After major volcanic eruptions, stratospheric ash and aerosols choke off the light path and the Moon can go almost black — the eclipses following Pinatubo's 1991 eruption were famously dark. Astronomers grade the colour on the five-step Danjon scale, from L=0 (nearly invisible) to L=4 (bright coppery orange).
• Dust, wildfire smoke, even weather along Earth's day–night edge at that hour. The blood moon is a live readout of our own atmosphere.

Partial eclipses can blush too

You don't need full totality to see the colour. In a deep partial eclipse — like the one on the night of 27–28 August 2026, when 93% of the Moon's diameter sinks into the umbra — the shadowed portion often glows the same dusky red while a brilliant sliver of the Moon stays lit. The contrast between the bright rim and the red shadow is one of the most photogenic sights in the sky.

Shallow partials and penumbral eclipses are subtler. A penumbral eclipse dims the Moon so slightly that most people looking straight at it notice nothing at all — which is why our eclipse calendar says so honestly, rather than hyping every entry.

How to watch one

This is the easiest astronomical event there is:

• No equipment, no filters. A lunar eclipse can never be brighter than a full Moon. Look up. (The safety rules that make a solar eclipse dangerous do not apply here.)
• No travelling. Everyone on the night side of Earth sees the same phase at the same moment; the only question is whether the Moon is above your horizon during the eclipse. Our per-city eclipse pages compute exactly that for 132 cities.
• Give it time. The drama builds slowly — the umbra's curved bite grows for over an hour before totality. That curved edge, incidentally, is Earth's own shadow, and its shape is how Greek astronomers knew 2,300 years ago that the Earth is round.
• Binoculars, if you have them, turn the colour from "nice" to "unforgettable."

The name

"Blood moon" is a folk name, not an astronomical term — popularised in recent decades, especially around the 2014–2015 series of four consecutive total eclipses (a "tetrad"). Astronomers just say total lunar eclipse. But the folk name earned its keep: it describes exactly what you see.