Total Solar Eclipse in Svalbard on March 20, 2015

A total solar eclipse in Svalbard on 20 March 2015 © Thanakrit Santikunaporn/ Shutterstock.com

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Solar eclipse guide: what they are and how to watch safely

An eclipse is dramatic evidence of our place in the solar system, caused when the Sun, Moon and Earth align. It can be awe-inspiring to watch. In this article we explain how solar eclipses happen, when the next one is and how you can view one safely. 

What is a solar eclipse?

A solar eclipse occurs when the Moon passes between the Sun and Earth. When the Sun, Moon and Earth line up exactly, it causes a total eclipse. This is because the diameter of the Sun is 400 times that of the Moon, but coincidentally it is also 400 times further away.

During an eclipse, the Moon temporarily covers the Sun, blocking out the daylight for a short period and casting a shadow on part of Earth. There are two parts to this shadow: the umbra, where sunlight is blocked completely and a total eclipse is experienced, and the penumbra, where only some sunlight is blocked and a partial eclipse is experienced. A total solar eclipse spells a moment of near-complete darkness, while partial eclipses are more like twilight. 

Partial solar eclipses occur multiple times per year, but total eclipses are much less frequent. Total eclipses visible from the UK or Europe are even rarer. 

Diagram showing how the Moon casts two kinds of shadow on Earth during a solar eclipse

Diagram of a solar eclipse. The umbra is the dark centre of the Moon's shadow and the penumbra is the secondary shadow. People in the path of the umbra will see a total eclipse. Image: NASA.

Annular eclipses are another type of solar eclipse. They are nicknamed 'ring of fire' eclipses, although there is not actually a ring of fire but a ring of light. This does not detract from their impressive appearance.

Annular eclipses occur when the Moon's apparent diameter (the width of the Moon as we perceive it from Earth) is less than the Sun's and so the Moon doesn't cover the Sun completely like it does in a total eclipse - we still see its rim.

This type of eclipse occurs because the Moon's orbit around Earth is elliptical rather than perfectly circular, so sometimes the Moon is further away from Earth than at other times and looks smaller.

How often does a solar eclipse happen?

There are two to five solar eclipses each year, with a total eclipse taking place every 18 months or so. Whether you can view that eclipse depends on where you are in the world.

As the Earth rotates, the Moon's shadow on Earth (and the view of the eclipse) travels from west to east. The path of totality is the area in which a full eclipse will be completely visible. Viewers in areas outside the path will see a partial eclipse (or no eclipse at all).

Generally, total solar eclipses are visible every 400 years from any one place. 

Composite image of different stages of a total solar eclipse

The route of the Moon's umbra across Earth is called the path of totality, as it is where the total eclipse will be observed © Owen Production/ Shutterstock.com

When is the next solar eclipse?

The next solar eclipse will be a partial eclipse on 25 October 2022, which will be visible from the UK, Europe, western Russia, the Middle East, western Asia and northeast Africa.

Depending on their location, people living in the UK can expect about 7-29% of the Sun to be covered. Northeast Scotland will be the best place to watch (check other locations on this eclipse map). It is often cloudy in the UK in October, but there's a good chance the Sun might peek through the clouds at some point during the partial eclipse, as it lasts for more than an hour.

The next total solar eclipse visible from the UK will not take place until September 2090.

Some upcoming solar eclipses and the best places to see them

25 October 2022: partial solar eclipse visible from the UK, Europe, western Russia, the Middle East, western Asia and northeast Africa. 

8 April 2024: total solar eclipse visible across parts of North America.

29 March 2025: partial solar eclipse visible from northwest Africa, UK, Europe and northern Russia.

12 August 2026: total solar eclipse visible from the Arctic, Greenland, Iceland, Atlantic Ocean and northern Spain. A partial eclipse should be visible from the UK, with about 90% of the Sun covered.

A full list of eclipses is available on NASA's website.

When was the last total solar eclipse in the UK?

The last total eclipse in the UK was on 11 August 1999. It could be seen over Cornwall and parts of Devon, which were on the path of totality, although cloud cover ruined the view for many areas. Other parts of the UK experienced a partial eclipse.

What phase is the Moon in during a solar eclipse?

Solar eclipses only take place during a new Moon phase. This is because, from our perspective on Earth, during a solar eclipse the Sun is behind the Moon lighting up the side that we can't see, leaving the side facing us in darkness.

Not every new Moon coincides with an eclipse, because the Sun, Moon and Earth need to line up in such a way that the Moon's shadow lands on our planet and this doesn't always happen. This is because the Moon's orbit around Earth is slightly inclined compared to Earth's orbit around the Sun.

If the Moon, Sun and Earth align when the Moon is full, it causes a lunar eclipse. In this case, it is Earth that passes between the Sun and Moon and casts its shadow on the Moon.

How long does a solar eclipse last?

Earth and the Moon are always moving, so eclipses are temporary events - the longest total solar eclipse in recent history was on 11 July 1991 and lasted six minutes and 53 seconds. It can take several hours for the Moon to pass in front of the Sun, but the moment of total eclipse (called totality) lasts only minutes - up to a maximum of seven minutes and 32 seconds.

The period of totality varies because the apparent sizes of the Moon and Sun change depending on their distance from Earth. Neither the Moon's orbit around Earth nor Earth's orbit around the Sun are circular.

A partial solar eclipse

A solar eclipse can be awe-inspiring to watch © Warachai Krengwirat/ Shutterstock.com

What is it like to experience an eclipse?

The Museum's Scientific Associate Ken Phillips has witnessed four total solar eclipses and taken measurements in three of them. These measurements looked at the fast changes to the solar corona (the hot outer atmosphere of the Sun). Eclipses offer an opportunity to study things that are usually harder to see or hidden by the Sun's glare.

Ken says, 'A total solar eclipse is an amazing and totally unique experience. The previous couple of hours will have seen the Sun gradually get more and more covered by the Moon and the landscape around becomes mysteriously dark, with an eerie yellowish colour.

'The few minutes - it is rarely more than seven and usually much less - when the Sun is totally eclipsed are always utterly memorable. The Sun turns into a black disk but surrounded by the white, wispy strands that make up the outer solar atmosphere, or corona. Pink bumps called prominences are often visible around the edge of the now-eclipsed Sun.

'All too soon the Sun is uncovered and the bright disk shines through.'

What should you be looking for during a total eclipse? Ken outlines what else you could experience during the event:

'Although your attention is taken up by the eclipsed Sun, you might also notice, just as the Sun is about to be totally eclipsed or is emerging from it, fast-moving bands of shadow (called shadow bands) that race across the ground beneath your feet. And bright stars or planets such as Venus and Jupiter might be visible near the Sun in the sky.

'Wildlife becomes confused - birds may stop singing or start flying to their roosts, trying to make out what has happened. It is as if night has suddenly occurred even though minutes before the Sun was shining high in the sky.' 

How to watch a solar eclipse

You should never look directly at the Sun. Even with sunglasses on or through dark material such as a bin bag or photo negative. These filters do not protect your eyes against infrared radiation and can cause permanent eye damage. Never use binoculars or a telescope unless you have a special solar filter fitted or are using them to project the eclipse onto white card.

To watch a solar eclipse you can wear special eclipse glasses or construct a pinhole camera. 

How to make a pinhole camera

  1. Cut a hole in a piece of card.
  2. Tape a piece of foil over the hole.
  3. Poke a hole in the foil with a pin.
  4. Place a second piece of card on the ground.
  5. Hold the card with the foil above the piece of card on the floor to project an image of the Sun onto it. 
A tree's shadow on a pavement which includes multiple crescent shapes

Crescent-shaped shadows are formed by sunlight passing through gaps between tree leaves during a partial solar eclipse. The tree canopy acts like multiple pinhole cameras. © Mariusz S Jurgielewicz/ Shutterstock.com

You can also see the eclipse filtered through the leaves of tree canopies onto the pavement below. This works the same way as a pinhole camera but repeated over and over in the gaps between the leaves.

If you choose to wear solar eclipse viewing glasses, ensure these are authentic and have been purchased from a reputable astronomy supplier. Likewise if purchasing a solar filter for a telescope.

Watch the 2001 solar eclipse during totality: 

2001 solar eclipse during totality

Recorded in Lusaka, Zambia, this video shows the wispy solar corona that becomes easier to see when the Moon covers the Sun during a solar eclipse. A solar prominence is also visible on the left side of the Sun. The glowing 'bump' (actually a loop) consists of plasma, a hot gas containing electrically charged hydrogen and helium atoms and electrons. Video: Ken Phillips.
 

Remember that it needs to be a clear day for the best chance of seeing a solar eclipse, since clouds could block the Sun. However, a total eclipse can still be a memorable event even if it's cloudy, as you will still experience darkness falling suddenly and animals behaving strangely. 

Superstitions surrounding solar eclipses

Throughout history, the rarity of solar eclipses and their ability to seemingly turn day to night has meant that they were seen as a bad omen. Ancient cultures believed that eclipses occurred when a demon ate the Sun or Moon. In Ancient Greece, an eclipse meant a god was angry, and the Pomo indigenous people of northern California believed that a bear had bitten the Sun in a fight causing temporary darkness.

Even today, some people believe that solar eclipses can harm pregnant women. Fortunately, the only risk from an eclipse is to your eyes.