Solar Eclipse of 11th August 1999

Introduction
- Figure 1: Eclipse track across UK
Eclipse Path
Eclipse Types
- Figure 3: How eclipses occur
Safety
Photography
Phenomena
Web Links
Further Reading

Introduction

The first total solar eclipse visible from the United Kingdom mainland for more than 70 years occurs on the morning of Wednesday, 11 August 1999, soon after 11:00,a.m. Previous total eclipses visible from the UK this century were those of 29 June 1927 and 30 June 1954; the latter, however, was visible only from the northernmost part of the Shetland Islands. Although the partial phase of the eclipse will be visible from the whole of the British Isles, the path of totality across the country is rather short, being limited to a strip which crosses the Isles of Scilly, most of Cornwall except the northern part, southern Devon and Dorset as far as Torquay, and the Channel Island Alderney.

Figure 1: Total eclipse track, after Espenak & Anderson (1997). (Click on the image for high quality figure.)

Weather prospects for viewing the eclipse are only moderate in the UK, and those wishing to be certain of seeing the event are advised to travel to a part of Europe where cloud is less likely to be a problem.

Eclipse Path

Anyone wishing to view the total eclipse at its best is strongly advised to travel to a place where the Moon completely covers the Sun, as only there will the full beauty of the event be seen. The track of the Moon's shadow on the Earth, known as the path of totality, will pass over many towns and cities of Europe and Asia, thus giving many millions of people the opportunity to witness this rare and wonderful spectacle. Some of the towns include Cherbourg and Reims in France, Stuttgart and Munich in Germany, Timisoara and Bucharest in Romania, Sivas and Diyarbakir in Turkey, Irbil and As-Sulaymaniyah in Iraq, Esfahan in Iran, Karachi in Pakistan, and Baroda and Akola in India.

The maximum possible duration of the total eclipse is 2m 23s, occurring near Rimnicu-Vilcea in Romania. The duration of the total phase of the eclipse ranges from 2m 2s at Falmouth, England, 2m 13s at Metz, France, 2m 17s at Stuttgart, Germany, 2m 7s at Sivas, Turkey, 1m 33s at Esfahan, Iran, 1m 13s at Karachi, Pakistan to 52s at Chandrapur in India.

The times of the total eclipse for selected United Kingdom, Western and Eastern European towns and cities are given in Table 1. Times are given according to British Summer Time (BST); add 1 hour to obtain local times in France, Germany, Austria, Hungary and Romania. The start of the partial phase of the eclipse is denoted `first contact', while `second contact' denotes the start of totality. The total phase finishes a couple of minutes later at `third contact,' while the remaining partial phase of the eclipse can then be viewed up to eclipse end at `fourth contact'. The times of the eclipse for towns and cities where only a partial eclipse is visible are given in Table 2. The northern limit of the eclipse path lies some 70 miles off the coast of southwest Ireland. Consequently, no place in Ireland will experience a total eclipse. At Cork, for instance, the eclipse magnitude will be 97%.

Although Cornwall has some of the best summer weather in England, there is still only a 45% probability of it being clear there at the time of the eclipse. The probability of seeing the eclipse increases the further east one goes. For instance, Munich has a probability of 50%, Bucharest 60%, Sivas in Turkey 75%, and Esfahan in Iran 95%.

Figure 2: Path of totality across Europe, after Espenak & Anderson (1997). (Click on the image for high quality figure).

Table 1: Times of the eclipse to the nearest second (BST) for selected towns and cities lying on the path of totality.

		1st Contact	2nd Contact	3rd Contact	4th Contact
Country	Town		Totality Begins	Totality Ends		Duration of Totality
England	Bodmin	09:57:51	11:12:22	11:13:41	12:33:00	1m 19s
	Camborne	09:56:58	11:11:02	11:13:03	12:32:03	2m 01s
	Falmouth	09:57:08	11:11:19	11:13:22	12:32:28	2m 02s
	Helston	09:56:54	11:11:02	11:13:03	12:32:09	2m 01s
	Hugh Town, Isles of Scilly	09:55:45	11:09:43	11:11:24	12:30:30	1m 41s
	Newton Abbot	09:59:00	11:14:21	11:14:51	12:34:47	0m 30s
	Penzance	09:56:39	11:10:39	11:12:41	12:31:41	2m 02s
	Plymouth	09:58:17	11:12:54	11:14:33	12:33:54	1m 39s
	Redruth	09:57:04	11:11:09	11:13:10	12:32:11	2m 01s
	Saltash	09:58:16	11:12:54	11:14:29	12:33:51	1m 35s
	Torquay	09:59:02	11:14:08	11:15:15	12:34:58	1m 07s
	Truro	09:57:18	11:11:28	11:13:25	12:32:29	1m 57s
France	Cherbourg	10:00:11	11:16:11	11:17:45	12:38:11	1m 35s
	Le Havre	10:02:03	11:18:49	11:20:20	12:41:14	1m 31s
	Metz	10:09:13	11:27:56	11:30:09	12:51:35	2m 13s
Germany	Augsburg	10:15:26	11:35:53	11:38:10	13:00:04	2m 17s
	Munich	10:16:21	11:37:12	11:39:20	13:01:26	2m 08s
	Stuttgart	10:13:09	11:32:55	11:35:12	12:56:54	2m 17s
Austria	Graz	10:22:09	11:44:57	11:46:09	13:08:56	1m 12s
	Salzburg	10:18:28	11:39:55	11:41:58	13:04:11	2m 02s
Hungary	Szeged	10:30:04	11:53:22	11:55:43	13:17:22	2m 21s
Romania	Bucharest	10:41:25	12:05:48	12:08:10	13:28:44	2m 22s

Table 2: Times of the eclipse for towns lying off the path of totality. The eclipse magnitude is the maximum fraction of the Sun's diameter obscured by the Moon.

Town	Start	Maximum	End	Magnitude (%)
Belfast	10:01:37	11:14:04	12:30:32	89
Birmingham	10:02:52	11:17:59	12:37:00	94
Cardiff	10:00:23	11:15:37	12:35:12	97
Cork	09:56:03	11:09:06	12:27:07	97
Dublin	09:59:43	11:12:49	12:30:17	92
Edinburgh	10:05:47	11:18:04	12:33:45	85
Limerick	09:56:52	11:09:28	12:26:53	94
Liverpool	10:02:51	11:17:02	12:35:05	92
London	10:03:34	11:19:51	12:39:57	97
Londonderry	10:00:59	11:12:42	12:28:32	88
Oxford	10:02:41	11:18:27	12:38:10	96
York	10:05:18	11:19:40	12:37:30	90

Eclipse Types

The Earth orbits the Sun making one complete revolution in one year. Similarly, the Moon is in orbit around the Earth. Occasionally all three bodies, Sun, Moon and Earth lie in a straight line. As it happens, the angular diameters of the Sun and Moon are very nearly the same. The Sun is about 400 times larger than the Moon, but is also about 400 times further away. When all three bodies, Sun, Moon and Earth lie in a straight line with the Moon in the centre, the Moon will completely cover the Sun and we have the phenomenon of a solar eclipse.

Figure 3: How eclipses occur. The Sun (at left) casts a shadow of the Moon on to the Earth (at right). The diagram (not to scale) illustrates the three different types of solar eclipse.

In practice, the angular diameter of the Moon varies depending on its distance from the Earth. For a total eclipse to occur, the Moon's angular diameter must be greater than that of the Sun. The length of totality depends on how much larger than the Sun the Moon is, with the maximum possible duration of totality being about 7m 30s. Occasionally, the Moon is too far away to completely cover the Sun, and in this case the eclipse is called annular. The various types of solar eclipse are illustrated in Figure 3.

Safety

Permanent damage will be caused to the retina of the eye if proper precautions are not taken while observing the Sun. Injuries to the retina can occur without pain and the results may not be noticeable for some hours after the damage has been done. Viewing a partial solar eclipse, annular solar eclipse, or the partial phase of a total solar eclipse should not be attempted without some form of eye protection or the use of an appropriate observing method.

Never stare at the Sun. Even if only 1% of the Sun's disc is visible, it is still bright enough to cause damage. Children, especially, should be well supervised.

The projection method is the safest way to observe the Sun. By means of a pinhole in a piece of card, cast the Sun's image onto a screen about 1 metre away. Another method is to use a small mirror to reflect the Sun's image onto a shaded wall. A third is to use a small telescope or a pair of firmly mounted binoculars to focus a larger solar image on a white screen - but guard against anyone looking directly through the instrument. Tape down the cover on the front lens of any attached finder telescope. Alternatively, proper solar filters made from metalized glass, or mylar, may be used to view the eclipse directly. Solar filters must be placed securely over the front of the telescope so that the Sun's light is filtered before it enters the instrument. Appropriate solar filters may be obtained from photographic suppliers or a variety of other sources, such as Eclipse 99 Ltd., Belle Etoile, Rue du Hamel, Guernsey GY5 7QJ. Do not observe the Sun with materials such as ordinary photographic filters, exposed photographic film, or coloured plastic, as these may not filter out the highly dangerous infrared and ultraviolet radiation from the Sun. Only the total phase of the solar eclipse may be viewed directly without filters.

Photography

A 35mm camera with a long-focus lens, say 200 to 400mm, mounted on a firm support will be sufficient to capture a reasonably large image (1.9 to 3.8mm) of the Sun in eclipse together with its surrounding corona. With a 1000mm lens, the image should nicely fill the 35mm frame. To photograph the partial phase of the eclipse, a mylar or glass solar filter must cover the lens for safety reasons. For information on where to obtain filters, see the suggestions for further reading.

The exposure time depends on the filter/film combination, and for best results one should experiment with various times on the uneclipsed Sun. This requires preparation! Photograph the noon Sun with your chosen filter at a fixed aperture, say f/8 or f/16, and with a range of times from 1/4s to 1/1000s. Select the best image from the developed negative and use the corresponding exposure time and aperture to photograph the partial phase of the eclipse. During the thin crescent phase of the eclipse, that is, shortly before and after totality, increase the aperture by two stops, for example from f/11 to f/5.6. It is advisable to make a series of bracketed exposures, that is, to vary the exposure times on either side of the original settings in case there is haze or thin cloud on the day of the eclipse. For photography during the period of totality, which lasts for about 2 minutes, remove all solar filters. Using ordinary daylight film (e.g. 200 ASA), exposures of around 1/4s at f/8 or f/11 should be sufficient to capture the corona, while the brighter chromosphere requires much shorter exposures of about 1/1000s at f/16 or f/22.

Phenomena

As the Moon passes in front of the Sun, a small `bite' of increasing size slowly develops. Shortly before totality begins, the thin crescent of the Sun will be broken by the irregular edge of the Moon, caused by lunar mountains and craters, producing the effect known as Baily's Beads (a string of points of light). Then, immediately before totality, the Diamond Ring effect occurs, where the last point of light from the Sun produces a stunning effect on photographs.

With the reduced glare of the Sun comes the rare opportunity to view the Sun's outer atmosphere. The inner part of this region, known as the chromosphere, comes into view first, together with the reddish prominences, huge arches of hydrogen gas caused by giant eruptions on the Sun. Further out lies the solar corona, the very high temperature component of the Sun's atmosphere composed of gas streaming away from the Sun.

The lower part of the Sun's atmosphere is characterized by strong magnetic fields and strong turbulence. This is where solar storms are generated. If the storms build up sufficiently, gas can explode away from the Sun and produce a stream of electrically charged particles. These particles may reach the Earth and interact with our atmosphere, causing radio interference and auroral displays. It has been known for several decades that the corona has a temperature of more than one million degrees, although the precise mechanism by which this is maintained remains a mystery. Prior to 1975, the best ultraviolet and X-ray observations of the corona and the boundary layer lying between the corona and the chromosphere, known as the transition region, were obtained by the NASA-launched SKYLAB spacecraft mission. Since then, there have been several important space missions, the most recent being YOHKOH (Japanese for Sunbeam) and SOHO (Solar and Heliospheric Observatory). Astronomers at the Armagh Observatory have been actively involved in both these missions. The new picture which is emerging is one of a finely structured, highly dynamic environment. Further details and links to other sites may be found at G. Doyle's Home Page .

Other occurrences noticeable just prior to, and during totality are possible shadows coming in waves across the Earth's surface, and changes in the bird and animal kingdoms, such as birds beginning to roost for the `night'.

Web Links

There are many sites on the internet specialising in solar eclipses, and some with particular information on the 11 August 1999 eclipse. These include: