Solar eclipse: what it is and how to see one

What is a solar eclipse and how do you view one safely? Find out about the celestial mechanics behind these spectacular astronomical events.

A composite images showing the stages of a solar eclipse. Photo by Vishal Bhatnagar/NurPhoto via Getty Images)

One of the most breathtaking astronomical events you can witness is a total eclipse of the Sun, also known as a total solar eclipse. Not only is it an experience you’ll never forget, but it also shows the Solar System in motion through the fortunate alignment of three astronomical bodies.

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The first object is our planet, Earth, which orbits the second object, the Sun. The third object in the equation is the Moon.

A solar eclipse happens when the Sun, Moon and Earth become aligned in space.

Fancy seeing an eclipse for yourself? Find out when the next one is happening with our guide to the 2021 eclipses.

Read our beginner’s guide to the mechanics behind a lunar eclipse.

Eclipse phases Abolfazl Arab, Sistan and Baluchestan province, Iran, 26 December 2019 Equipment: Nikon D7200 DSLR
Eclipse phases Abolfazl Arab, Sistan and Baluchestan province, Iran, 26 December 2019 Equipment: Nikon D7200 DSLR

On Earth we’re lucky that the Moon is just the right size and orbits at just the right distance to make total solar eclipses possible.

You may wonder how they happen at all since the Sun is so much bigger than the Moon.

Well, due to one of the most amazing coincidences in nature, even though the Moon is 400 times smaller than the Sun, the Sun itself is around 400 times further away, which can make the two objects appear the same size.

An annular solar eclipse occurs when the moon's shadow cone doesn't quite reach Earth's surface. Credit: Bairi from Pixabay.com https://pixabay.com/illustrations/annular-solar-eclipse-eclipse-2003461
An annular solar eclipse occurs when the moon’s shadow cone doesn’t quite reach Earth’s surface. Credit: Bairi from Pixabay.com pixabay.com/illustrations/annular-solar-eclipse-eclipse-2003461

However, we don’t get a total solar eclipse every time the Moon moves between Earth and the Sun. The lunar orbit is tilted, so that it sometimes passes above or below the Sun.

And because the Moon’s orbit isn’t circular but elliptical like an oval, when it is furthest from us and an eclipse occurs the Moon is too small to cover the Sun completely.

We then see an ‘annular eclipse’, in which a thin ring of sunlight can be seen circling the Moon.

What happens during a total solar eclipse?

A composite image of the total solar eclipse of 21 August 2017 captured by Luigi Fiorentino from Casper, Wyoming, US.
A composite image of the total solar eclipse of 21 August 2017 captured by Luigi Fiorentino from Casper, Wyoming, US.

When we witness a total solar eclipse it means that we are in the shadow of the Moon and, as the Sun is the bigger object, it makes the shadow of the Moon cone-shaped.

This shadow cone starts out as big as the diameter of the Moon at 3,476km (2,160 miles), but by the time it reaches Earth it is much smaller. The biggest it can get is about 300km (190 miles) in diameter.

Stages of the 20 March 2015 solar eclipse, by Gary Foord, Faroe Islands.
Stages of the 20 March 2015 solar eclipse, by Gary Foord, Faroe Islands.

If you’re lucky enough to be within the zone of the shadow, you’ll see darkness descend as the shadow sweeps across the planet.

The Moon covers the Sun entirely for 7 minutes 31 seconds at most, but you’ll probably see a ‘totality’ lasting between 2 and 4 minutes.

If you’re outside the 300km circle of the shadow cone you’ll only see a partial eclipse because the Moon covers up less of the Sun as you move further away from the track of totality.

Total solar eclipse at Castle Gardens, Wyoming, USA, 21 August 2017 by Alex Conu
Total solar eclipse at Castle Gardens, Wyoming, USA, 21 August 2017 by Alex Conu

Due to the intense light from the Sun, a total eclipse is dangerous to look at. The only time when it’s safe to look directly at the eclipse is during the five or so minutes of totality when the Moon completely covers the Sun.

For the rest of the event you must protect your eyes with specially designed equipment (more on this below). Take due care and you can fully enjoy this marvel of celestial mechanics.

During a total solar eclipse, if you’re in the ‘umbra’ you’ll see the entire Sun being slowly covered by the Moon and get the full glory of totality.

The cone-shaped shadow of the Moon cast by the Sun creates an umbra and penumbra on Earth. Credit: Science Photo Library
The cone-shaped shadow of the Moon cast by the Sun creates an umbra and penumbra on Earth. Credit: Science Photo Library

There’s also an area  around the umbra called the ‘penumbra’, where the shadow isn’t quite so dark.

On the ground this forms a large circular zone where you see more and more of the Sun the further you get from the umbra until you don’t see an eclipse at all.

So whenever people in one location see a total eclipse, those in a large surrounding area will see a partial eclipse. There are a maximum of five solar eclipses in any given year.

Three ways to safely view a solar eclipse

1

Pinhole

A woman projects a pinhole image of a solar eclipse through a piece of tin foil on to paper, California, 20 May 20 2012. Credit: John Walker/Fresno Bee/Tribune News Service via Getty Images.
A woman projects a pinhole image of a solar eclipse through a piece of tin foil on to paper, California, 20 May 2012. Credit: John Walker/Fresno Bee/Tribune News Service via Getty Images.

A safe way of viewing an eclipse is with two pieces of card. Make a small hole in one and hold the other so that the Sun is projected onto it. You can then watch as events unfold.

2

Projecting

The projections of a partial solar eclipse in New Delhi, India, 29 March 2006. CreditL Manan Vatsyayana/AFP via Getty Images
The projections of a partial solar eclipse in New Delhi, India, 29 March 2006. Credit: Manan Vatsyayana/AFP via Getty Images

You can set up binoculars or a telescope on a tripod to capture an eclipse. Hold a piece of card away from the eyepiece so the sunlight is projected onto it, then watch the Moon cover up the Sun.

Do it yourself with our guide on how to build a solar projection screen.

3

Eclipse glasses

Eclipse glasses are perhaps the easiest way of staying safe while observing a solar eclipse. Credit: LeoPatrizi / Getty Images
Eclipse glasses are perhaps the easiest way of staying safe while observing a solar eclipse. Credit: LeoPatrizi / Getty Images

You can now buy safe eclipse viewers that you wear just like sunglasses. They cut out all harmful ultraviolet and infrared rays and 99.9% of the Sun’s visible light.

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Have you ever seen an eclipse, or managed to capture a photograph of one? Let us know by getting in touch via contactus@skyatnightmagazine.com or on Facebook, Twitter and Instagram.