Solar Eclipses
(e.g., Friday 20 March, 2015)

Why Do They Occur and What Will Happen?

A solar eclipse occurs when the Moon passes between the Sun and the Earth and obscures the Sun for a viewer on Earth. It is a partial or total solar eclipse, depending on whether the Sun is partially or totally covered up. A total solar eclipse is the most amazing natural event I have ever witnessed
(see here my description and photos of the first one I saw in Guadeloupe).
In a partial eclipse you have the build-up and aftermath, but not the incredible climax of a total eclipse.


The shadow cast on the Earth by the Moon, seen from space.

From space, you can see the shadow of the Moon moving across the surface of the Earth at 1700 mph. In most of the shadow (several thousand km across), you would see part of the Sun covered up, but near the centre (in a narrow region a few hundred km wide) you would see the whole of the Sun covered up and would witness a total eclipse.

For the eclipse on March 20, 2015, the central region traces out a narrow track 460 km (290 miles) wide. The track starts near Greenland, heads across the North Atlantic, crosses the Faroes Islands and Svalbard (an island to the north of Norway in the arctic circle) and ends up at the north pole. A partial eclipse will be experienced across the whole of Europe and the Middle East, as well as north and central Asia.

An animation of the motion of the Moon's shadow can be seen here.


A map of the start times of the eclipse.

In the UK, starting at 8.20 am in Cornwall, 8.30 am in St Andrews, 8.35 am in NE Scotland and 8.45 am in London, the Moon will gradually move across and cover up more and more of the Sun for about an hour. At the peak at 9.35, 84% will be covered in London, 95% in St Andrews and 98% in the Isle of Lewis, so the further north you are the more Sun will be covered. At the peak, you will just see a small crescent of Sun remaining, and so the sky will be darker and it will feel cooler. Then, for another hour, finishing at 10.30 or 10.45 (depending where you are) the Moon will gradually uncover the Sun again.

An animation of what the eclipse should look like seen from Edinburgh can be seen here.
If it is cloudy, you can watch it live here.
For details and animation from your location in the UK, see here.
For suggestions as to where to see the eclipse, see here.

How Rare is a Solar Eclipse?

It is an amazing coincidence (which occurs nowhere else in the solar system) that the Moon and Sun appear almost exactly the same size in the sky. Although the Moon is in reality 400 times smaller in diameter than the Sun, on average it is 490 times closer to the Earth. But the Moon is very slowly moving away from the Earth (due to tidal friction) at a rate of 3.8 cm per year, so after 500 million years it will be 17,000 km further away and there will be no more solar eclipses, because the Moon will apear too small to cover up the Sun.

If the orbit of the Moon were in the same plane as the orbit of the Earth, then there would be 1 solar eclipse per month when the Moon lies between the Sun and the Earth. However, the two orbits are inclined to one another by 5 degrees, and so it turns out that there is roughly one total solar eclipse per year (70 per century) somewhere, lasting about 3 minutes (the one on March 20 lasts 2 min 47 secs), but often they are in a remote part of the Earth.

For a particular location on the Earth, a total solar eclipse takes place roughly every 400 years. The last one was in 1652 in Edinburgh (1715 in London), whereas the next one is in 2135 (2151 in London). The last one in the UK was in 1999 in Cornwall and South Devon (and before that in 1954, 1927 and 1925), and the next one in North America will be in 2017, but the next in Central Europe will be in 2081 and we shall have to wait until 2090 for the next UK total eclipse (or 2026 for a partial eclipse).

How Can You Observe an Eclipse Safely?

It is very important to stress that, at all times during a partial solar eclipse, you must on no account look at the Sun direct, because the Sun can burn your retinas and produce permanent damage, even blindness, even at the peak when only 5% of the Sun is visible.
*DON'T look directly at the Sun.
*DON'T use sunglasses.
*DON'T look at the Sun through binoculars or a telescope.
*DON'T look through the viewfinder of a camera or video camera.
*DON'T use any type of filter other than a special solar filter. (For more details, click here.)


A simple pinhole camera

There are two safe ways to observe an eclipse. The first is to use special eclipse glasses that are designed to cut out most of the sunlight. The second is to project an image of the eclipsed Sun onto a wall or a piece of paper (see here and also here). So, to make a pinhole projector from 2 pieces of paper or card, simply make a fine pinhole in the first sheet and shine the Sun through it on to the second sheet a few feet behind it, so that the image of the Sun is projected onto the second sheet through the pinhole. As you move the second sheet further away, so the image of the Sun grows bigger. Then you can easily watch the eclipse proceed.

What is the Effect of a Cloudy Day?

In order to see an eclipse of the Sun, you need to be in a place with a clear view of the Sun, so if you are in a big city you may like to go to a park, river bank or rooftop. If there is thin cloud, so that the disc of the Sun is still visible, then you can watch the eclipse fine. But if the cloud covering the Sun is thick, then you won't see the Sun but will notice the sky become darker. So let's hope that it won't be cloudy.


The corona during a total solar eclipse (courtesy M Druckmueller)

How Different is a Total Solar Eclipse?

A total solar eclipse is a truly amazing experience, as the whole solar disc is covered up and the solar corona is revealed - the atmosphere of the Sun glowing with the same brightness as a full Moon, so that now you can view it with the naked eye safely. The main disc of the Sun is a million times brighter than the corona, and so, unless you go into space and use a special telescope, you can only see the corona when the disc is completely covered. The corona reveals a beautiful, complex structure determined by the magnetic field. It is called "corona" from the latin for crown, and was observed in the ancient world by the Egyptians, Chinese and Greeks. Unfortunately, we won't be able to see it on Friday, but, if you ever do have the chance, I recommend you take it.

Historical Aspects

One of the earliest records of an eclipse of the Sun from 1375 BC in the city of Ugarit, Mesopotamia, states "On the day of the new moon in the month of Hiyar, the Sun was put to shame and went down in the daytime, with Mars in attendance". There are stories and tales from most cultures, many of them centring on a creature having a meal. The ancient Chinese believed a huge dragon was consuming the Sun, as did the Greeks and Romans. Indeed, the Chinese for eclipse is "shih" meaning "to eat".

Other Snippets of Information

Academics have tried to infer the date of Christ's crucifixion if the darkness that clouded the sky on Good Friday was due to an eclipse. Since it took place during Passover, which is at the time of full Moon, when the Earth is between the Sun and Moon, this would have been a lunar rather than a solar eclipse. From this, Colin Humphreys and W Waddington calculated the crucifixion date as 3 April 33 CE.
(Click here for more details.)
Muslims have special prayers during an eclipse when they thank God for his control over the Sun and Moon.
The word eclipse comes from the ancient Greek word ekleipsis, meaning abandonment or omission.

Web Sites for Further Information:

BBC information for 20 March, 2015.
More details of the eclipse.
A "complete guide" to the March 20, 2015 solar eclipse.
NASA's March 20 eclipse page.
Wikipedia solar eclipse of March 20, 2015
Lists of solar eclipses from UK, AD 1000-2091

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