When Is the Next Blood Moon?

Why the moon turns red

In a total lunar eclipse, Earth sits exactly between the sun and the full moon, and the moon moves into Earth's shadow. It doesn't go black, because some sunlight still reaches it: the light that passes through the edge of Earth's atmosphere is bent into the shadow, and on the way the blue part of it is scattered out, leaving red and orange. NASA describes it as if all the world's sunrises and sunsets were projected onto the moon; the more dust or clouds in the atmosphere, the redder it looks.^[54]

No two blood moons look the same. Astronomers rate them on the Danjon scale, from L0, so dark the moon almost disappears, to L4, a bright copper-orange. After big volcanic eruptions, which fill the upper atmosphere with fine dust, eclipses can stay very dark for a few years.^[3]

Total, partial and penumbral

Earth's shadow has two parts: a dark core, the umbra, and a pale outer fringe, the penumbra. When the whole moon passes into the umbra, the eclipse is total and the moon turns red. When only part of it does, the eclipse is partial: a dark bite is taken out of the moon. When the moon only grazes the penumbra, the eclipse is penumbral; the moon dims so slightly that most people don't notice anything.

Why there isn't an eclipse every full moon

A lunar eclipse can only happen at full moon, when the moon is opposite the sun. But the moon's orbit is tilted about 5° to Earth's,^[6] so most full moons pass a little above or below Earth's shadow. Only when a full moon falls near the points where the two orbits cross does the moon go through it. That gives two lunar eclipses in most years, sometimes three or four, but many of them are only partial or penumbral.

How to watch

A lunar eclipse is completely safe to look at: unlike a solar eclipse, you don't need glasses or any special equipment, though binoculars or a small telescope show the colors better.^[54] Find a spot with a clear view of the moon away from bright lights. The red stage is the highlight; the partial stages before and after are worth watching too, as the shadow creeps across the moon's face. For photos, use a tripod: the eclipsed moon is far fainter than a normal full moon, so you'll need much longer exposures.

Eclipses and religious practice

Many traditions mark a lunar eclipse: in Hinduism with the Sutak period and in Islam with the eclipse prayer, for example. For those, follow your local panchang, temple, mosque or community; the times on this page are astronomical.

How we calculate

The start and end of each stage come from the positions of the sun, moon and Earth's shadow; our dates and types match NASA's eclipse catalog.^[1] Whether you can see a stage depends only on whether the moon is above your horizon at the time, which we work out for your exact location every two minutes. City data come from GeoNames.^[37]