A lunar eclipse

A total lunar eclipse can be a spectacular sight to behold © NASA/Rami Daud

Lunar eclipse guide: What they are, when to see them and where

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?

What is a lunar eclipse?

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.

This total lunar eclipse was seen on 21 January 2019 from Italy © Giuseppe Donatiello via Wikimedia Commons (CC0 1.0)

Is it safe to look at a lunar eclipse?

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.

How to get the best view of a lunar eclipse

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.

Why isn't there a lunar eclipse every month?

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.

A lunar eclipse lasts several hours. The period of totality in a total lunar eclipse, when the Moon turns red, lasts about an hour. © Laura Ngo via Flickr (CC BY-NC-ND 2.0)

What are the different types of lunar eclipse?

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.

What is a total lunar eclipse?

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.

National Geographic share their guide to a lunar eclipse:

What is a blood Moon?

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.

What is a penumbral lunar eclipse?

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 impossible to notice, even with a telescope.

 A lunar eclipse occurs when the Moon passes into Earth's shadow © NASA

What is a partial lunar eclipse?

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.

What is a super blood wolf Moon?

Some people have different terms for a total lunar eclipse depending on when it occurs. The distance from Earth to the Moon changes from 363,000 kilometres (226,000 miles) to 384,000 kilometres (239,000 miles) over the course of the Moon's orbit. When it is at this closest point, 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'. 

It can be difficult to notice a change on the Moon during a penumbral lunar eclipse © O'Dea via Wikimedia Commons (CC BY-SA 4.0)

When to see the next total lunar eclipses in the UK

Over the next ten years there will be quite a few lunar eclipses to look forward to. A partial lunar eclipse will be visible from the UK on 16 July 2019, although it might be hard to spot as it will be very close to the horizon.

In 2020 there will be four penumbral lunar eclipses over the UK and other locations, although you might struggle to notice them. 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

Total lunar eclipses viewable elsewhere in the world over the next 10 years

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

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