Society for Popular Astronomy

Partial Eclipse of the Sun
Saturday, October 12

Live Images of the Eclipse

Example of what you may see.
© Copyright Robin Scagell, (No copying).

There will be a partial eclipse of the Sun visible from the UK and other parts of Europe on Saturday 12 October 1996. This will be the largest partial eclipse visible from the UK since 1961 - if the weather is clear it should be a memorable sight.

People often remember partial eclipses of the Sun that they saw when they were young, and this should remain in the memories of many youngsters. But as far as the UK is concerned, it's just the prelude to the Big One - the total eclipse due on 11 August 1999. But what is the difference between a partial and a total eclipse? Eclipses happen when the Sun, Moon and Earth are exactly in line. As the Moon goes round the Earth, it occasionally hides the Sun from view, or else it passes through the Earth's shadow. Because its orbit is slightly inclined to the Sun's path through the sky, this does not happen every New Moon or Full Moon.

When it gets between us and the Sun, we see its dark outline pass in front of the Sun, and it is called an eclipse of the Sun. In the other situation, where it passes through the shadow of the Earth, the Moon itself dims and we see an eclipse of the Moon. There was an eclipse of the Moon on 27 September.

Why your location matters

To see an eclipse of the Sun, you have to be in the right spot. The Moon is just over 2000 miles across, so when it passes in front of the Sun it casts a shadow of that size on the Earth. Actually, that is only true near the Equator - if the shadow happe ns to fall nearer the poles, it will be elongated. Within a few hundred miles of the centre of the shadow, the Moon can completely hide the Sun and people in the favoured area see a total eclipse. Outside that area, the Moon does not completely cover the Sun and a partial eclipse is visible.

On 12 October, however, there won't be a total eclipse anywhere because the central part of the Moon's shadow misses the top of the Earth completely. In the UK we will see about half of the disc of the Sun covered, those in the north seeing more eclipsed than those in the south.

When to look

The eclipse takes place on the Saturday afternoon. The exact timings vary slightly depending on where you are, as shown in the following table.

          Eclipse timings for various locations around the UK

Location          Start of      Middle of       End of         Maximum area
                  eclipse        eclipse        eclipse          of Sun
                   (BST)         (BST)           (BST)           covered

Aberdeen           13.49          15.06          16.20              56%
Belfast            13.47          15.06          16.21              52%
Birmingham         13.55          15.14          16.28              51%
Cardiff            13.55          15.14          16.28              49%
Dover              14.01          15.20          16.33              51%
Edinburgh          13.49          15.07          16.21              54%
London             13.59          15.18          16.31              51%
Manchester         13.53          15.12          16.26              52%
Newcastle          13.52          15.10          16.24              54%
Norwich            13.59          15.17          16.31              53%
Plymouth           13.55          15.15          16.29              48%
Southampton        13.58          15.17          16.31              50%

Very soon after the time shown for the start of the eclipse, you should see the Moon's disc start to move onto the Sun. The maximum area, at mid-eclipse, is the actual area of the Sun covered rather than the percentage of its diameter. The the Moon starts to move off again, and its disc will completely clear the Sun by the last time shown.

How the eclipse will look in its early, middle and late stages

How to observe the eclipse

Observing an eclipse of the Sun can be very dangerous if you're not careful. We often underestimate the power of the Sun - following an eclipse in 1984, many people suffered from eye problems as a direct result. The important thing to remember is that just cutting down the light is not enough. Heat rays - invisible infrared light - are just as dangerous, and many dark materials can be transparent to infrared.

There are ways of using telescopes and binoculars safely, but if you do not have these there are a few methods you can use.

Filters

Everyone seems to have heard of using smoked glass, but this is not now recommended because of the infrared problem. Photographic filters come into the same category. Welder's glass is suitable, as long as it is No 14 or darker. At one time, people used dense black and white negatives, but again this is no longer always ideal. Colour film and some modern black and white film, such as Ilford XP-1, use dyes rather than silver, and let through infrared. If you have overexposed and developed silver-based black and white film which is really dense, then observing through one or two thicknesses of the densest part may be adequate.

Well-equipped amateur astronomers use specially made filters over the front of their telescopes, but these are expensive and are not easily available. You can, however, now buy card-mounted spectacles of the same material. They can be obtained from Eclipse 99 Ltd, Belle Etoile, Rue du Hamel, Castel, Guernsey GY5 7QJ. Tel 01481 64847. The price for one set, including a leaflet and postage, is 1.99 UK pounds. Credit cards are accepted. Eclipse 99's e-mail address which is 100334.1671@compuserve.com.

Broadhurst, Clarkson and Fuller can provide safe eclipse viewing filters for £1.95 post free. 63 Farringdon Road, London EC1M 3JB. Tel 0171-405 2156.

Beware! The sun filters supplied with some small telescopes should never in fact be used on the telescope for direct observation. They are intended to be screwed into the eyepiece, but they can overheat and shatter.

Don't look through the viewfinder of your camera if it is pointed at the Sun, even if you have a dense filter over the front of the lens.

Projecting the Sun's image

This is the only safe way to observe the eclipse.

If you have a telescope or binoculars, point it at the Sun. In the case of a telescope, make sure that any small finder telescope is capped, and keep the cover on one half of the binoculars. The easiest way to find the Sun is tilt your instrument to get the smallest shadow.

Hold a piece of white card about 15 cm (6 inches) behind the eyepiece to act as a screen on which you can catch the image. You should see a bright circle of light, probably blurred, on the screen. Focus the instrument until the circle is sharp. This is the disc of the Sun itself. If the eclipse is in progress you should see the Moon as a dark bite out of one edge.

Moving the card farther away gives a larger but fainter image, and vice versa. Fixing the instrument firmly on a tripod helps enormously. The advantage of projection is that many people can see the image at once.

If you have no telescope, you can actually project a faint image of the Sun through a pinhole in the same sort of way. Get a bit of card - a good old cereal packet is fine - and use a drawing pin to make a hole about 1 mm across. Hold your screen about 15 cm behind this and you should see a small image of the Sun on it. Again, the farther away it is the larger and fainter it becomes.

Also see Other Methods of Observing the Eclipse.

Photographing the eclipse

The simplest method is to photograph the projected image. If you have a suitable filter you can put this over the lens of your camera to cut down the light when photographing directly. But bear in mind the warnings about photographic filters letting through infrared.

The camera's standard lens will give only a small image of the Sun. You really need at least a 135 mm telephoto to give a decent sized image.

Getting the exposure right is a big problem. Automatic exposure meters see a small bright disc with a dark surrounding and try to balance the two, overexposing the Sun. Experiment with a stop or two less exposure than the meter suggests.

Things to look for

There may be one or two sunspots - dark areas on the Sun's disc - but at the moment the Sun is at a quiet stage in its solar cycle and there may not be any. If there are, and the Moon's disc happens to cut across one, you should see that the Moon is still dark against the sunspot. This is because the spots are only dark in comparison with the rest of the Sun - they are still quite bright.

You may be able to see that the Moon's edge (technically called the 'limb') is not completely smooth. Its mountains and craters can be seen in profile if you look carefully.

The limbs of the Sun and Moon may seem to be in constant motion, rippling up and down. This is caused by the Earth's unsteady atmosphere.

You should not see any features beyond the Sun's limb. The dramatic pink prominences which you have seen in pictures are far too faint to be visible, and only appear when the Sun is totally eclipsed.

Look beneath trees to see crescent-shaped images of the Sun and mid-eclipse, caused by the gaps between leaves acting as pinholes.

Dos and don'ts

Do make sure that you are properly equipped before the event rather than rely on anything that comes to hand at the last minute
Do tell children and others not to look at the Sun directly
Do enjoy what is a rare and beautiful event
Don't look at the Sun directly, not even through a pinhole
Don't take chances observing through materials that look dark, even if they seem OK at the time. Your eye is not sensitive to infrared so there is no reflex action to prevent damage.

A study of patients who went to just one hospital in Manchester after the 1984 partial eclipse showed that five had looked at the Sun directly, three looked directly though a pinhole, two used black plastic bags, one wore two pairs of sunglasses and another used a camera with two filters attached.

Warning

Neither the Society for Popular Astronomy, nor anyone responsible for the production or distribution of this material, shall be liable for damages arising from the use of this information.

The Society for Popular Astronomy

The SPA is Britain's leading society for beginners wanting to take up astronomy as a hobby. We help enthusiasts of all ages to enjoy the fun of observing, and there are special sections to cover particular areas of interest - including the Sun!

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Astronomy site of the day 10/11/96

Maintained by Michael Oates Email: mike@ph.u-net.com
Last modified 12th Oct 1996