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When Earth casts its shadow on the Moon it can cause quite a spectacle. Find out how often these events occur, and where you can view them from over the next ten years.
You might be familiar with the idea of a solar eclipse: when the Moon passes in front of the Sun from our point of view on Earth, blocking it out and turning day to night for a few minutes on the surface of our planet. But what happens during a lunar eclipse, when will the next one occur and how can you see one?
A lunar eclipse is what happens when, if you were standing on the Moon, you would see Earth block out the Sun. It involves the Moon passing directly into Earth's shadow as all three bodies align - just as part of Earth passes into the Moon's shadow during a solar eclipse - and, in so doing, it causes some odd changes in appearance.
The good thing about all types of lunar eclipse is that, unlike a solar eclipse, they are safe to view with the naked eye. This is because lunar eclipses only reflect sunlight - they don't get any brighter than a full Moon, which you've probably safely observed many times before.
To get the best view you'll need to be on the night side of Earth when one occurs, and you'll need a clear sky that's free of clouds. No specialist equipment is needed. Try to minimise the light in your vicinity and, ideally, watch from a spot where your line of sight won't be obstructed by tall buildings or trees.
A lunar eclipse can last several hours, but the period of totality - when the Moon is completely in Earth's shadow - usually only lasts an hour or so.
A lunar eclipse only occurs during a full Moon, when the Sun, Earth and Moon are all aligned. But despite the Moon only taking 29.5 days to orbit Earth and complete a cycle from full Moon to full Moon, there are only on average about three lunar eclipses every year.
This is because the Moon's orbit around Earth is not in a flat plane - it's angled at about five degrees, which means that the Moon often goes above or below Earth's shadow as it orbits around. As a result, lunar eclipses tend to come in batches when the Moon is at a similar inclination. There were three total lunar eclipses in 2018, for example.
There are three types of lunar eclipse: a total lunar eclipse, a penumbral lunar eclipse and a partial lunar eclipse.
To understand the difference between them, we first need to understand how Earth's shadow works. As our planet blocks out the Sun's light, it actually casts two different shadows. One is a larger shadow that extends away from Earth at an angle, known as the penumbra. Directly behind Earth, however, is a darker and narrower shadow, called the umbra.
This is when the Moon passes into Earth's umbral shadow, which can result in the Moon turning red. This is sometimes called a 'blood Moon', although astronomers aren't super keen on that term, which has more roots in astrology.
The Moon turns red during an eclipse because of how the Sun's light interacts with Earth's atmosphere. As it hits the atmosphere, shorter wavelengths of light such as the colour blue are scattered outwards. Longer wavelengths like red, however, are bent or refracted into Earth's umbra. When these strike the surface of the Moon, they can make it appear red - a similar process to how the sky appears red during a sunrise or sunset.
When the Moon passes into the outer shadow, we call this a penumbral lunar eclipse. There aren't many noticeable effects during a penumbral eclipse. The Moon only gets very slightly darker, and it is normally difficult to notice, even with a telescope.
As its name might imply, a partial lunar eclipse occurs when the Sun, Earth and Moon aren't exactly aligned, so only part of the Moon passes into Earth's umbral shadow and thus only part of it appears red.
Some people have different terms for a total lunar eclipse depending on when it occurs. The distance from Earth to the Moon changes from about 360,000 kilometres to 400,000 kilometres over the course of the Moon's orbit. When the Moon is at its closest, it is slightly larger and brighter in our sky, earning it the moniker of 'supermoon'.
A total lunar eclipse during this time is therefore sometimes called a 'super blood Moon'. On 21 January 2019, however, a total lunar eclipse occurred, unusually, on the first full Moon of the year. As this is known as a 'wolf Moon', that total lunar eclipse earned itself the nickname 'super blood wolf Moon'.
Over the next ten years there will be quite a few lunar eclipses to look forward to. A partial lunar eclipse will occur on 19 November 2021, but in the UK it will barely be visible before the Moon sets in the early morning. It's the total lunar eclipses you'll really want to look out for though on these dates:
16 May 2022: UK, Europe, Africa, Americas
14 March 2025: UK, Europe, Africa, Americas, east Asia, Australia
7 September 2025: UK, Europe, Asia, Australia
31 December 2028: UK, Europe, Asia, Australia, western United States and Canada
Other parts of the world will also have a chance to see several additional total lunar eclipses in the coming years. Here's what to look forward to:
26 May 2021: Americas, east and southeast Asia, Australia
8 November 2022: Americas, Asia, Australia
3 March 2026: Americas, Asia, Australia