Eclipses and Transits

Sam is very interested in everything about astronomy, and has his own telescope, but his real passion is naked eye astronomy, what he and his friends can see from where they live every time they look at the Night Sky. He does not often talk about eclipses of the Sun because the last one visible from where he lives was before he was born and by the time of the next one he and all his friends will probably be grandparents.

But there is usually a total eclipse of the Sun visible from somewhere on the Earth at least once every eighteen months, and I know children from all over the world read these Pages, because they tell me so, so this Page is not for Sam's friends it is for yours. If you like it please tell me - but just one name, your age and the country you live in.

Total and partial eclipses of the Sun

If we could look down on the Solar System from a point a long way above the North Pole we would see the Sun in the middle, with the Earth and all the other planets going round it, the Moon going round the Earth and all the moons of all the other planets going round them, and the Sun, Earth, Moon and all the other planets and all their moons rotating on their axes. This is the way we usually draw the Solar System. In drawings made like this almost everything in the Solar System is moving or rotating anti-clockwise.

Here is a drawing of the orbits of just the Earth and Moon (not to scale of course; we cannot draw a picture of the Solar System to scale, and this is explained in the Page on The Size of the Solar System).

This drawing shows a line passing through the Sun and The Earth. We get a New Moon when the Moon crosses this line on the Sun side of the Earth and a Full Moon when it crosses it on the other side. The Moon goes round the Earth about once every twenty eight days but because the Earth is also moving round the Sun the position of the line between the Earth and Sun is also moving, so we actually get a new Moon about every 29½ days, and of course a Full Moon about 14½ days later. This is more fully explained in the Page on the Moon.

If we look at the orbit of the Earth and Moon edge on rather than from on top this is what we shall see.

The plane of the Earth’s orbit round the Sun is called the ecliptic. The Moon’s orbit is at a slight angle (about five degrees, but here shown greatly exaggerated) to the ecliptic, that is, the imaginary piece of paper we have drawn the Earth and Sun on. This means that at the time of a New Moon, when the Moon crosses the line between the Earth and the Sun, seen from the Earth it does not usually pass directly in front of the Sun but slightly above or below it. As the Moon goes round the Earth it is above the ecliptic for half of each orbit and below it for the other half, so it is only actually on the ecliptic twice each orbit. We get an eclipse of the Sun (a solar eclipse) when, seen from the Earth, part or all of the Sun is covered by the Moon, and this happens when the Moon passes exactly between the Earth and the Sun. This in turn can only happen when the Moon is on or very close to the ecliptic at the time of a New Moon - which is of course why it is called the ecliptic, and why we do not get an eclipse every New Moon.

If we have a point source of light the light will travel outwards from it in every direction. If we put an object in the way of the light it will cast a shadow. The dark area behind the object is called the umbra (from the Latin for shadow). The umbra, and therefore the shadow, has a sharp edge because you can see either all of the light or none of it - “Now you see me now you don’t.”

Usually however a light source has some size so there will be an area behind an object where you can see a part of the light source. This area is called the penumbra.

The umbra and penumbra merge into each other so there is no sharp edge; the width of the penumbra depends on the size of the source, and so will be smaller for a candle than an electric light bulb.

Here the object casting the shadow is bigger than the light source, but of course the Sun is a lot bigger than the Earth and the other planets and their moons.

The umbra comes to a point. To someone at this point the object will exactly cover the light source, that is, the object and the light source appear exactly the same size.

The diameter of the Sun is almost exactly 400 times greater than the diameter of the Moon but it is also almost exactly 400 times further from the Earth. This means that to someone on the surface of the Earth the Sun and Moon appear to be almost exactly the same size. Many people find this hard to believe because when the Sun or Moon is rising or setting it looks much bigger than the Moon or Sun when it is high in the sky, but you can prove it for yourself quite easily.

The Sun and Moon are each about half the width of your thumb when you stretch out your arm. You can try this with the Moon whenever you see it, and you can also do it with the Sun if you are at the seaside or in the countryside and you can see the Sun setting on the horizon, but only if the Sun is actually touching the horizon and appears red and you can see it as a disc. If you cannot see the edge of the Sun it is too bright and you will hurt your eyes.

The Sun and Moon “look” bigger when they are setting (or rising) because when we are looking at the Sun or Moon and the Earth at the same time our brain cannot work out that they are much further away and so plays a trick on us - an optical illusion. Here is a video of an old Persil advertisement which illustrates this.

This is the best example of how size and distance can play tricks on the way we see things that I have ever come across - thank you Persil! You do get some other rubbish stuff as well but I think this is a small price to pay and you do not have to and should not watch it.

What all of this means is that at the time of a total eclipse the tip of the Moon’s umbra just touches the surface of the Earth.

Inside the umbra there is a total eclipse, but inside the penumbra only a partial eclipse. The total eclipse can be seen in an area about 250 km across and a partial eclipse in an area about 3000 km across. Because the Earth is rotating on its axis and the Moon is moving round the Earth and the Earth is moving round the Sun these areas are also moving very fast (more than 1680 kilometres an hour, depending upon where you are on the Earth’s surface), and in any one place the total part of the eclipse lasts only a few minutes.

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Annular eclipses of the Sun

Up to now we have considered that the Earth and the other planets go round the Sun and the Moon goes round the Earth in circular orbits, but this is not true, they actually go round the Sun in ellipses, with the Sun or Earth at one focus, not in the middle. This is more fully discussed on the Page on Comets

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What this means is that the distance between the Earth and Sun is not constant. The Earth is at perihelion (from the Greek for nearest to the Sun) when it is closest to the Sun, and at aphelion when it is furthest from it - this drawing greatly exaggerates the difference between them for the Earth’s orbit round the Sun.

Similarly, the orbit of the Moon round the Earth is also an ellipse, with a perigee (from the Greek for nearest to the Earth) and an apogee. So if an eclipse of the Sun occurs when the Earth is at its nearest to the Sun and the Moon is at its furthest from the Earth the Moon’s umbra does not quite reach the surface of the Earth and so the Moon appears very slightly smaller than the Sun and does not quite cover it. This is an annular eclipse, from the Latin for a ring, when we can see a ring of sunlight all round the Moon. Sometimes an eclipse starts as total but ends as annular, or visa versa, and astronomers call this a hybrid eclipse.

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Solar eclipses and astronomers

To an astronomer a total eclipse of the Sun is one where the umbra touches the surface of the Earth. Total eclipses are very important to astronomers because they allow them to study the Sun’s corona.

The Sun is shooting plumes of gas millions of kilometres into space but these cannot usually be seen from the Earth because the Sun’s surface is far too bright. But during a total eclipse the surface of the Sun is covered by the Moon and these plumes of gases can be seen, like a crown (corona) surrounding the Sun.

You can of course only see the total eclipse from a very tiny part of the Earth’s surface and very few people will see it, so the astronomers studying the corona must be prepared to go anywhere. Many more people will be in the penumbra and see a partial eclipse. But astronomers reserve the term partial eclipse for one where the umbra passes just over the North Pole or just under the South Pole so part of the Earth’s surface is in the penumbra but none of it is in the umbra.

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To astronomers a hybrid eclipse is one which is visible as a total eclipse in one place and annular in another.

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How often do we get a solar eclipse?

There must be at least two solar eclipses a year but there can be up to five. Most of these are partial but there is usually a total or annular eclipse somewhere on the Earth about every eighteen months. Hybrid eclipses are very much less common. However a total (or annular) eclipse can be seen from only a very tiny part of the Earth’s surface, and so very few people will see more than one during their lifetime. The last total eclipse visible from Southern England was in 1999, the next will be in 2090. There is more about predicting eclipses in the last Section of this Page.

Watching a solar eclipse

It is very dangerous to look directly at the Sun, at the time of a solar eclipse or at any other time.

You can talk about and read about watching a eclipse of the Sun as much as you like and whenever you like, but the only time information about watching one is of the slightest importance to you is when there is one near you. So this Page tells you not what to do now, but what to do then.

When there is a solar eclipse visible in your area most of the local observatories and astronomical societies will hold Open Days, almost always free, when they use their telescopes to project the Sun's image onto a giant screen for you to look at. This is by far the best way of watching it, as there will be experts explaining it and answering your questions. Look for details in your local newspapers and on your local tv.

You can also make a pin-hole camera from an old shoe box to project the image of the Sun onto a piece of white card. You can find instructions on several web sites.

You can also buy special eclipse goggles: these will be on sale in local shops for several weeks before the day, and can be bought on the internet at any time. But do not buy a pair until just before you need them, and throw them away afterwards - to keep the cost down they are made of inexpensive materials and are only intended to be used once, not last a long time.

These three are the only safe ways of observing a total eclipse, and do not let anyone tell you otherwise.

Before the invention of the telescope astronomers had watched eclipses in a camera obscura, Latin for a darkened room, in which an image of the Sun formed by a pinhole in a screen was focused onto a white wall. The image was much bigger than that produced by the pinhole camera described above but not very bright, which is why you had to be in a darkened room to see it properly. You could form an image of almost anything this way, and the camera obscura was the forerunner of photography - hence our word camera.

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Eclipses of the Moon

An eclipse of the Moon (a lunar eclipse) occurs when the Moon passes through the Earth’s shadow. This can only happen when the Moon is on the ecliptic at the time of a Full Moon. (This is more fully explained in the Section on solar eclipses, above.)

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At the distance the Moon is from the Earth the Earth’s umbra and penumbra are much bigger than the Moon. There are therefore three types of lunar eclipse.

A total eclipse does of course go through all three stages in turn. Most astronomers are not very interested in penumbral eclipses and most people do not notice them unless they know that they are happening. If you find a web site or book giving the dates of lunar eclipses penumbral eclipses are not usually included.

Total eclipses of the Moon are much less common than total eclipses of the Sun, but they last for up to ninety minutes and are visible from about half the Earth, so most people will see, or at any rate have the opportunity to see, several during their lifetime. The last one visible in Southern England was on January 31st 2018.

The Moon does not go totally dark because the Earth’s atmosphere is scattering and refracting (bending) the light from the Sun and so some light enters the umbra. Because the blue light is scattered more than the red the Moon appears reddish brown. The actual colour and brightness depends upon the amount of water vapour and dust in the atmosphere: in August 1883 Krakatoa, a volcano in Indonesia, erupted very violently and the dust from it produced some really spectacular lunar eclipses, also lots of wonderful sunsets and very cold summers for several years.

In the Bible and many other old writings there are references to “The Moon being turned to blood”, which is not a bad description of a total eclipse of the Moon.

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Eclipses and Wise Men

Today if there is to be a total eclipse of the Sun visible near where you live, there will be lots about it on tv and in the newspapers for weeks before it happens, and all the shops will be full of “eclipse goggles”. If by some chance you have not heard about it, if you are indoors and you notice it getting cold and dark you will probably just put the lights on and turn up the heat a little; if you are out of doors when it happens you might say “What’s happening?” and someone will be bound to reply “It’s an eclipse of the Sun, didn’t you know?” If there is a total eclipse of the Moon most people who live in towns are unlikely to notice it, and even if they do they will probably not give it a second thought. But it has not always been like that.

Until about four hundred years ago most people, everywhere in the World, lived in villages and worked the land for food. These people may not have understood the finer points of photosynthesis but they did know that it was the warmth of the Sun which made the plants grow. So anything which affected the Sun was for them a matter of life and death. If one day in the middle of the day without any warning the Sun suddenly started getting darker, unless they knew what was happening they would be terrified - and who could blame them? Or who could blame them for being terrified if the Full Moon turned to blood?

It is not difficult to predict eclipses of the Sun and Moon, but you do need to make really accurate measurements of the movements of the Sun and Moon against the fixed stars for a period of at least twenty years. More than four thousand years ago the ancient Chinese, Indian, Persian, Arabic and Egyptian astronomers were keeping stunningly accurate observations of the Sun and Moon for hundreds of years at a time and were able to predict eclipses of the Sun and Moon for several years ahead - many of these records still exist today! There were of course no telescopes until after 1600 CE, so these astronomers used sighting tubes, astrolobes and other instruments instead. But these instruments were very big and expensive and very few people had the skills needed to use them or the mathematical knowledge needed to do the calculations. More to the point, at a time when almost all the population were necessarily peasants working on the land to grow food, very few people could afford the time or the money to study the stars. So astronomers were usually people in the service of rich and powerful people, rulers such as Princes, Kings and Emperors. Because of their very special position as advisers to rulers they were often referred to as Wise Men.

In every age there has never been any shortage either of simple and gullible people or of dishonest people willing to take advantage of them. Once astronomers began to be able to predict eclipses, inevitably there would be some who would take advantage of this knowledge for their own purposes, for example “Unless you give me lots of money, at half past nine tonight I will turn the Moon to blood.” By about the 12th century CE the Islamic astronomers were publishing annual almanacs giving details of eclipses and much other information about the movements of the Sun, Moon and Planets for the next twelve months, and obtaining money from people in this way became more difficult.

Almanacs were very important to sailors and people in the desert trying to navigate by the Sun and Moon and stars, which is of course the main reason why they were produced. Christopher Columbus took one to the Americas with him, and this saved his life: do a Web Search on “Christopher Columbus + Lunar Eclipse” for information about this.

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Transits of Mercury and Venus

Venus and Mercury are planets orbiting the Sun, like the Earth, but they are nearer to the Sun than the Earth and so must at some stage cross a line between the Sun and the Earth. This Paragraph is mainly about Venus, but Mercury behaves in a similar way.

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The Earth goes round the Sun in about 365 days whereas Venus, being closer to the Sun, goes round it in about 222 days. (Mercury, being closer still, goes round the Sun in about 88 days.) So Venus crosses the line between the Earth and the Sun about every 583 days. However the orbit of Venus is at an angle of about 3 degrees to the eclipic so it does not pass directly between the Earth and the Sun every time it crosses the line between them, but only when it is on the ecliptic.

A transit of Venus occurs when Venus passes directly between the Earth and the Sun. Venus is about the same size as the Earth but is much further away than the Moon so appears very much smaller and does not come any way near to covering the Sun. You cannot of course see the transit directly because the Sun is far too bright, but you can see it quite clearly, as a black dot moving across the Sun’s disc, if you project the Sun onto a screen, as for an eclipse of the Sun.

Unlike eclipses of the Sun and Moon, the early astronomers had no way of predicting transits of Mercury or Venus with a sufficient degree of accuracy - that had to wait until after Kepler had published his First Law of Planetary Motion, which said that the planets went round the Sun not in circles but in ellipses. The Islamic astronomer al-Fabari may have observed the transit of Venus in 910 CE, but no more were observed until after the publication of Kepler’s Laws of Planetary Motion in 1609.

Transits of Venus are very uncommon: the pattern is two eight years apart and then not another one for more than a hundred and five years. There was a transit of Venus in 2004 and then again in 2012, but the next one will be in 2117.

Kepler’s First and Second Laws were published in 1609. Kepler had predicted the transit of Venus in 1631, but this occurred during the night in Europe and was not observed. The first transit to be observed through a telescope (by projecting it onto a white card of course) was in 1639, by the English astromomer Nicholas Horrocks (1618 - 1641). He was only 21 at the time, and died less than two years later, but he is now regarded as the father of English astronomy. The next transits were in 1761 and 1769, and astronomers from all over Europe were very excited about these. At that time ships on long sea voyages navigated using the Sun and Moon and stars, and for this they needed accurate astronomical tables. It was believed that if astronomers observed the transit of Venus from several different places and compared the results they would be able to produce more accurate tables. Sadly it was cloudy almost everywhere in Europe in 1761 and so no observations were made. But for the 1769 transit Captain James Cook was sent to the South Pacific to set up an observatory on the island of Tahiti. You can find out more about this by doing a web search on “James Cook + Transit of Venus.”

Transits of Mercury are much more common, with one every seven to ten years, but Mercury is very much closer to the Sun than Venus, and so moves more quickly, and it is very much smaller. Mercury is so tiny and moves so quickly that its transits are not very interesting to watch and most astronomers do not bother with them.

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© Barry Gray Revised April 2021