Solar eclipse 2015: The facts you need to know

On 20 March, the UK will experience one of nature’s most amazing spectacles, a solar eclipse. This happens when the Moon moves in front of the Sun for a few minutes, blocking its light and underneath the Moon’s shadow darkness falls.

Total solar eclipses are rare and in a way it is an incredible stroke of luck that we have them. The Sun’s distance from Earth just happens to be about 400 times the Moon’s distance from our planet. The Sun also happens to be about 400 times larger than the Moon, so thanks to this magic ratio they appear about the same size in the sky, meaning that during an eclipse the Moon can fit precisely over the Sun. We have to say ‘about’ a lot because Earth’s orbit and the Moon’s orbit are not circular but elliptical, meaning sometimes they can be a bit further away, or a bit nearer. This results in the Sun sometimes appearing larger than the Moon during some eclipses, leaving a ring of light from the Sun around the Moon’s silhouette. We call this an annular eclipse.

An eclipse begins at ‘first contact’ when the Moon’s disc first touches the Sun’s disc. You won’t notice a change in the light at this point – in fact it won’t get dark until the Moon has practically covered all of the disc – this is ‘second contact’ when the far limb of the Moon’s disc touches the Sun’s apparent disc. Totality – which is how we describe the Sun being blocked by the Moon – can last for several minutes. ‘Third contact‘ happens when totality ends and the Moon begins to move away from the Sun and daylight returns once more. ‘Fourth contact’ is when the Moon moves completely off the Sun and the eclipse ends. 

The Moon is very slowly moving away from Earth at a rate of 3.8 centimetres (1.5 inches) per year, so eventually it will appear too small to completely cover the Sun. Luckily, this day won’t arrive for at least another 500 million years!

How a solar eclipse forms

Eclipses are all a result of orbits. The Moon orbits the Earth once every 27.3 days. The Earth orbits the Sun once every 365.2 days. Their orbits are elliptical, meaning their distance from their parent body can change throughout an orbit. The tilt of the Moon’s orbit relative to the ecliptic (the path of the Sun through the sky) is 5.1 degrees. A solar eclipse happens only when the Moon crosses the ecliptic at the exact position that the Sun is at that moment in time.

They require syzygy

Eclipses occur during a particular alignment of the Sun, Moon and Earth called syzygy, which is when all three bodies are arranged in a straight line.

There is more than one type of shadow

A shadow is divided into two parts – the umbra and the penumbra. The umbra is the central, deepest part of the shadow. The penumbra is where only part of the source of light is blocked. Total eclipses are seen in the umbra, while partial eclipses are seen in the penumbra.

The Moon’s shadow moves very fast

The Moon’s shadow moves quickly across the face of the Earth, from west to east, faster than the speed of sound – the eclipse shadow at the equator travels at 1,730 kilometres (1,075 miles) per hour. This is because the Moon is orbiting Earth at 3,400 kilometres (2,113 miles) per hour, counterbalanced by the Earth’s rotation at 1,670 kilometres (1,038 miles) per hour. This is also why the Moon moves across the sky faster than the Sun.

The length of totality can vary 

Some eclipses are very short, with totality lasting just a couple of minutes. Others can last six or seven minutes. The reason for the difference is a result of the elliptical orbits of Earth and the Moon. When the Moon is closer to Earth in its orbit, it moves faster. The same for the Earth around the Sun, and this all affects the speed at which we see the Moon move across the Sun during a solar eclipse.

They can create diamond rings

Just at the moment totality begins or ends, a spectacular effect takes place that is called the ‘diamond ring’ – a bright burst of light appears, looking very much like the jewel in a diamond ring. This is caused by sunlight bursting through gaps between mountains on the edge of the Moon.

Eclipses are relatively rare 

On average, total solar eclipses happen every 18 months, although sometimes it can be several years between eclipses. They don’t occur every month because the Moon’s orbit is tilted with respect to the Earth’s orbit around the Sun, so it is only rarely that the Moon’s path across the sky intersects with the Sun’s.

UK solar eclipses are very rare

Total solar eclipses seen from the UK are very rare. The last one was in 1999 and the next won’t be until 23 September 2090, where Cornwall will be in the umbral shadow for two minutes and ten seconds. However, there will be partial solar eclipses visible in 2018 (only Shetland, Orkney and the northern coast of Scotland), 2021, 2022 and 2026.

They must be observed with care

It is very dangerous to look direct at the Sun without using special eclipse glasses or a telescope with a specialist solar filter. This is because the Sun is so bright it can damage your eyesight, or even permanently blind you. Even if 99 per cent of the Sun’s surface is blocked by the Moon, the remaining per cent is still intense enough to burn your retina. So here are some safe options for observing eclipses, or the Sun in general. If using eclipse glasses, check they do not have any damage. Even a pinhole could damage your eyesight. Try projecting the image of the Sun through a telescope and onto a piece of white card. Keep the finderscope covered, in case small children accidentally look through it. Gaps between leaves in trees can also act as natural pinholes to project the Sun’s image. You can also use specialist solar filters and telescopes. Produced by companies such as Coronado and Lunt, these can be a bit expensive but they allow you to view the Sun at other wavelengths of light, such as hydrogen-alpha, which appears orange, blocking out the dangerous light.

Discover more amazing facts you never knew about eclipses in the latest issue of How It Works magazine, on sale now! Pick up a copy from all good retailers, or order it online from the ImagineShop. If you have a tablet or smartphone, you can also download the digital version onto your iOS or Android device. To make sure you never miss an issue of How It Works magazine, make sure you subscribe today!

Plus, take a look at:

Where to see the Total Solar Eclipse

How to view a Total Solar Eclipse

What is a lunar eclipse?

How It Works issue 070 Fusion Power