Look up this Monday, Americans

Americans north and south will have the best seats in the house for a total lunar eclipse on Monday 21 January.

The event will be visible in almost all of both continents, as well as in Britain and Ireland, north-western France, north-western Spain, Portugal, a small part of west Africa, the eastern Pacific and the north-eastern tip of Russia.

And if you’re going to be in an appropriate place, a lunar eclipse is easy to experience. As Britain’s Royal Astronomical Society notes, “all you have to do is go outside”.

Solar eclipses are dangerous because observing the Sun directly can damage your eyesight, but the light from a lunar eclipse is much fainter and so is completely safe.

A total lunar eclipse occurs when the Earth passes exactly between the Sun and the Moon. The Sun is behind the Earth, and the Moon moves into the Earth’s shadow.

The Moon first darkens slowly as it moves into the penumbra of the Earth, the lighter part of the terrestrial shadow. When the Moon is completely within the umbra, the darker part of the shadow, it takes on a red hue that varies in colour.

Sometimes the eclipsed Moon is a deep red colour, almost disappearing from view, and sometimes it can be quite bright. The colour is due to Rayleigh scattering – where the Sun’s blue light is scattered off molecules in the Earth’s atmosphere – which also happens at sunsets.{%recommended 6642%} 

The Sun’s red light is scattered much less by air, and is bent by the Earth’s atmosphere in a process called refraction, travelling all the way through it to light up the Moon’s surface. 

Today the Moon will enter the penumbra at 0235 GMT and the umbra at 0333. The full eclipse (totality) will begin at 0440, with mid-eclipse (the time when the whole Moon will appear red) from 0512 to 0543. The Moon will exit the umbra at 0651 and the eclipse will end as it leaves the penumbra at 0749.

There are different types of lunar eclipse, but a total eclipse is the most spectacular and is the only type that causes the Moon to appear red.

Lunar eclipses occur several times a year and take place as often as solar eclipses, if all the different types are considered. Whereas you can only see a total solar eclipse if you are in the narrow path of the Moon’s shadow, lunar eclipses are visible wherever the Moon is above the horizon at the time, so each one can be seen from a large area of the Earth. For that reason, they are much more common from any given location.

Lunar eclipses always happen at a full Moon as this is when it moves behind the Earth and into line with the Earth and Sun. A full Moon happens every month, but most of the time no eclipse takes place. This is because the plane of the Moon’s orbit around the Earth is slightly tilted compared to the plane of the Earth’s orbit around the Sun, so the Moon normally passes a little above or below the terrestrial shadow. 

Eclipses only happen when the Moon, Earth and Sun are lined up in all three dimensions.

If you want to know more about eclipses past and future, visit the UK’s Eclipses Online portal.