Lunar eclipse and Blood Moon

A lunar eclipse happens when the Earth passes directly between the Sun and the Moon, so the Earth's shadow falls across the Moon. It can only occur at full moon. When the Moon is fully inside the Earth's umbra it doesn't turn black but copper-red — the famous Blood Moon. The colour comes from the only sunlight that reaches the Moon being filtered through Earth's atmosphere, just like at a red sunset. Below are the upcoming lunar eclipses and how well they can be seen from the UK.

The three kinds of lunar eclipse

• Total eclipse (Blood Moon) — the whole Moon passes through the umbra and turns red.
• Partial eclipse — only part of the Moon is covered by the umbra, leaving a dark bite out of the lunar disc.
• Penumbral eclipse — the Moon passes only through Earth's outer penumbra and dims slightly. Hard to see with the naked eye — so these are not in the list above. In 2027 all three of the year's lunar eclipses are of this type.

How to watch a Blood Moon

A lunar eclipse is completely safe to look at — unlike a solar eclipse it needs no protection. You just need to be able to see the Moon. A Blood Moon looks best with the naked eye or binoculars from a spot with a clear view toward where the Moon sits. See the moon today for moonrise and moonset for your location, or the moon calendar to plan ahead.

Why there isn't an eclipse every month

The Moon passes between the Earth and the Sun every single month at new moon, and to the opposite side of the Earth at full moon. Yet eclipses happen only a few times a year. The reason is that the Moon's orbit is tilted about 5° relative to the Earth's orbit around the Sun, so the Moon usually passes a little above or below the shadow.

Eclipses therefore only occur when a full or new moon falls close to one of the two points where the two orbits intersect. Those windows are called eclipse seasons, and they come round roughly every six months.

Why the moon turns red

During a total lunar eclipse the Moon does not disappear — it turns copper red. The light reaching it has passed through the Earth's atmosphere, which scatters the blue away and bends the red into the shadow. In effect you are seeing every sunrise and sunset on Earth at once, projected onto the lunar surface.

How dark and how red an eclipse gets depends on the state of the atmosphere. After major volcanic eruptions, when there is a lot of dust in the stratosphere, the Moon can go almost invisibly dark.

Why a lunar eclipse lasts hours, not minutes

Anyone who has seen a total solar eclipse knows it is over in a couple of minutes. A total lunar eclipse can hold totality for over an hour and a half, with the partial phases stretching the whole event past five hours. The difference is a matter of scale.

The Earth is about four times the moon's diameter, and at the moon's distance the Earth's shadow is still roughly 2.5 times wider than the moon itself. So the moon has a large target to cross, and it does so at its orbital pace. In a solar eclipse the geometry is reversed: the moon's shadow arrives at Earth as a spot at most a couple of hundred kilometres across, racing over the surface at well over 1,000 km/h.

The same asymmetry explains who gets to see one. A solar eclipse is visible only from the narrow track the shadow traces, which is why people travel across the world for them. A lunar eclipse is visible from the entire night side of the Earth at once — every person for whom the moon is above the horizon sees exactly the same event at exactly the same moment. Roughly half the planet, with no travel required.

How often, and why they cluster

Between two and five lunar eclipses occur each year counting the faint penumbral ones, though total eclipses are rarer — typically one every year or two, and any given location sees a total lunar eclipse on average every two to three years.

They do not arrive evenly. Because the moon's orbit is tilted about 5°, alignments are only possible near the two points where its orbital plane crosses Earth's, and the Earth passes through that geometry roughly twice a year. The result is eclipse seasons: windows of about a month, twice a year, in which eclipses cluster — usually a solar and a lunar eclipse a fortnight apart, since the required new moon and full moon are half a cycle from each other.

The whole pattern repeats on an 18-year, 11-day cycle known as the saros, a relationship the Babylonians worked out well over two thousand years ago and used to predict eclipses without any model of what was physically happening.

How red is red

Not every total lunar eclipse looks the same, and the variation is genuinely informative rather than atmospheric decoration. The light reaching the moon during totality has been refracted through the entire ring of Earth's atmosphere, so it carries the state of that atmosphere with it.

Astronomers score it on the Danjon scale, from L=0 for an eclipse so dark the moon nearly disappears, through the typical L=2 brick-red, to L=4 for a bright coppery orange with a bluish rim. What pushes an eclipse towards the dark end is dust and aerosol high in the stratosphere — which in practice means major volcanic eruptions. The eclipses following Pinatubo in 1991 were exceptionally dark, and the same was recorded after Krakatoa. Watching a lunar eclipse is, in a small way, taking a reading of the world's air.

An occasional bonus: because the moon is passing through the Earth's shadow rather than the sun's glare, a total lunar eclipse briefly restores a dark sky in the middle of a full-moon night. Stars that were washed out an hour earlier come back — one of the few times you can watch the moon and the Milky Way in the same frame.

Frequently asked questions about lunar eclipses

What is a Blood Moon?

A Blood Moon is a total lunar eclipse where the whole Moon is inside Earth's umbra and glows copper-red, because the sunlight reaching it is filtered through Earth's atmosphere.

When is the next lunar eclipse?

See the list above of upcoming lunar eclipses and how well each one can be seen from the UK.

Is it dangerous to look at a lunar eclipse?

No. Unlike a solar eclipse, a lunar eclipse is completely safe to watch without any protection.