It is an amazing coincidence that, although the Sun is 400 times larger than the Moon, it is also 400 times farther away. At various times in its orbit, the Moon can pass directly in front of the Sun, blotting out the light from the brilliant photosphere and allowing the fainter, but much more extensive corona to be seen during a total eclipse.
Total eclipses are not that rare - on average 238 per century* - but from any one location you would have to wait nearly 360 years to see one by chance ! [ *Sun In Eclipse - P Moore & M Maunder ]
As it is, the last TE visible from mainland Britain was on 29th June
1927, when the Moon's shadow crossed N.Wales and part of Yorkshire with
totality lasting a mere 25 seconds and seen from a strip of land 50 km
wide.
[ Not counting the eclipse of 30 June 1954 visible from the northernmost
part of the Shetland Isles.]
The next, after 1999, will not be until 23
September 2090
[ although the Channel Isles will get one in 2081 and the Faroe Isles
in 2015.]
In fact there is nowhere else in the Solar System ( so far as I can determine ) where it is possible to see a total eclipse of our Sun which is as perfect ( and long-lived ) as the view we get from the Earth. The nearest is a satellite of Jupiter called Amalthea which transits the Sun in a time of just over 8 seconds ; as it is potato-shaped, Amalthea cannot completely obscure the Sun's disc as seen from Jupiter's cloud-tops, but it can produce an annular eclipse of sorts.
The other satellites are far too large and would cover the Sun's disc - which is only 1/5 of its apparent size as seen from Earth - several times over.
[ Mars has 2 small satellites which transit Sun's disc frequently, Saturn
is too far from Sun]
The apparent size of the Moon varies by about 8 % and the Sun by 2 % :
Table 1 : Apparent
sizes of Sun & Moon
| Angles in min,sec | Maximum | Minimum | Average |
| Solar diameter | 32 ' 35 " | 31 ' 31 " | 32 ' 01 " |
| Lunar diameter | 33 ' 31 " | 29 ' 22 " | 31 ' 05 " |
Mean length of Moon's shadow = 372 000 km
Mean distance of Earth-Moon = 384 000 km
During a total eclipse, the Moon's shadow is just long enough to reach the surface of the Earth. The track of totality is narrow, and cannot be more than 272 km wide. It moves at nearly 1 km/sec over the Earth's surface and the duration of the total phase is never longer than 7 minutes 31 seconds. In 1999 we will get nearly 2 minutes 22 seconds on the centre line near Bucharest in Romania, but only 2 min. 06 sec. in Cornwall {SEE MAP}
During a lunar eclipse by comparison, totality can last up to 1 hour
44 minutes since the Earth's shadow is 9170 km in diameter at the distance
of the Moon ( the Moon is only 3476 km in diameter). Lunar eclipses are
not as scientifically interesting, nor as dramatic as total solar eclipses,
but they are rather more frequent.
Thales of Miletus is said to have predicted the eclipse of 28 May 585BC, which interrupted a battle between the armies of King Alyattes and King Cyaxares, who immediately called a hasty truce. Whether or not Thales did predict the eclipse, he could not have known its true nature - he believed that the Earth was flat and that it floated on a vast ocean. However, he could have known that eclipses follow an 18 year, 11 day cycle known as the Saros. It was certainly known to the Babylonians 2500 years ago through their observations of lunar eclipses.
The Moon takes 29.53 days to go from one New Moon to the next - a period called a lunation. ( Actually the period of the Moon's orbit is 27.55 days [ anomalistic month] but due to the motion of the Earth around the Sun it takes slightly longer for the Moon to return to the same phase.) After 223 lunations, the Earth, Moon & Sun will return to almost the same positions - this takes 18.03 years. [ 239 orbits] This means that any particular eclipse is likely to be followed by a similar one 18y 11d later.
Even this is not an exact match and to complete a full cycle takes 1300 years. At any one time there are 42 Saros series running simultaneously. Each cycle begins with a partial eclipse near one of the earth's poles with successive eclipses reaching lower latitudes and becoming total in most cases, then ending with partial eclipses near the opposite pole. Each cycle consists of between 70 and 85 eclipses. For example, the Saros which included the 11 Jul 1991 over Mauna Kea in Hawaii, began on 22 Jun 1360 and will end on 30 Jul 2622 ; it will have contained 71 eclipses, 45 of which will have been total. The 1999 eclipse is the 21st in Saros 145 which began on 4 Jan 1639 and ends 17 Apr 3009. Next one 21 Aug 2017.
The Saros is not perfect because the lunar cycles are not totally synchronized.
Consecutive eclipses do not occur at the same location but are shifted
by about 120 degrees in longitude. The match is better over three saros
cycles, although the eclipse tracks are shifted north or south.This 54
year interval is called the Triple Saros. Modern techniques for calculating
eclipses use high precision numerical calculations and are best left to
the professionals such as Fred
Espenak of NASA.
The last total solar eclipse of the Millenium begins in the North Atlantic some 300 km south of Nova Scotia. The Moon's umbral shadow makes contact at 09:30:57 UT and is 49 kilometres wide. Duration of totality is a mere 47 seconds.
First landfall is in the Scilly Isles - the eclipse begins at 08:55:05 UT, that's 09:55 BST. Totality begins at 11:10 BST and lasts for 1 minute 42 seconds; the Sun's altitude is 450. The shadow is 103 km in diameter and moving at 0.91 km/s over the surface of the earth.
In Cornwall, at the BAA site in the grounds of Truro School, first contact is at 09:57 BST. Truro is slightly north of the centre line and so gets 4 seconds less totality than say Falmouth which is almost on it. Totality begins at 11:11:23 when the Sun's altitude is 460 and lasts for just over 2 minutes ending at 11:13:30 BST. The eclipse ends at 12:32.
Plymouth is further North than Truro and so only sees 1 min 42 sec of totality. Torquay is the most easterly point from which the eclipse can be seen from the mainland and it gets 1 min 12 seconds - we'll talk about weather conditions shortly.
London is too far North to see the total eclipse but does get a partial
eclipse of magnitude 0.968. From Manchester the eclipse is only partial
at 90% area eclipsed
The Royal Astronomical Society are meeting on the Isle of Alderney from where there is perhaps the best chance of seeing the eclipse from the Channel Islands. At 1 minute 47 seconds the eclipse is total from 11:15:15 BST with the Sun at 480 altitude.
Although it is not strictly on the path of totality, a good display of Bailey's Beads could be obtained from the northern tip of Guernsey - or if you have a small boat .. ?
Unfortunately, the weather prospects for Cornwall are not that good in August. Meteorological records, as summarised in this table from an article by Howard Miles in the BAA Journal 1995 (105,6), for Falmouth and Penzance show that there is a greater than 50% chance that the sky will be completely cloudy on the 11th. The weather prospects do seem to improve northwards or westwards and are perhaps best from an island where there is less high ground to encourage cloud formation. However, the fact that the weather is rather changeable means that there may be a fortuitous break in the cloud cover between 11 am and 11.30 - just in time to see the total phase! We can live in hope can't we?
2. France & Germany
The southern edge of the umbra reaches the coast of Normandy just as the northern edge leaves England. The Cherbourg peninsula is 30 km south of the centre line and Cherbourg sees first contact at 09:00 UT or 11:00 European Summer Time - remember to add two hours to Greenwich Mean Time for the continent of Europe.
Totality begins at 12:16 European Time when the Sun is 490 above the horizon and lasts for 1 minute 40 seconds. { Fig.7}
Le Havre is the next point of contact in Northern France. Totality begins at 12:20 and lasts for approximately 1 ½ minutes. The Moon's shadow crosses the French countryside 30 km north of Paris which gets a 99.2 % partial eclipse at 12:23 pm before entering Belgium and Luxembourg ( 1.14). Reims gets 1 min 59 sec of totality at 12:24 when the Sun's altitude will be 520.
Metz gets a total eclipse lasting 2 min 13 sec (just 10 seconds short of the maximum for this eclipse) at 12:29. The shadow enters Germany four minutes later where Saarbrucken gets over two minutes of totality. Moving into the Rhine valley, Karlsruhe is also close to the centre line. Strasbourg is a little further south and only gets 1 min 22 sec at 12:30:30 with the Sun 540 above the horizon. Stuttgart, on the centre line sees 2 min 17 sec of totality at 12:34. The Sun's altitude has increased to 55 degrees, the shadow is now 109 km wide and racing over the land at 0.74 km/sec.
Munchen lies 20 km south of centre but its two million citizens will still witness more than two minutes of totality - provided the winds of good fortune bring clear skies on the day. The umbra crosses the border into Austria at 12:41pm.
The weather prospects in France and Germany are only slightly better than those of Cornwall. The mean cloudiness in August decreases from 60% near Cornwall to 50% near Paris and the mean number of hours of sunshine increases from 6.5 over France to just over 8 in Austria - with a matching increase in scattered cloud conditions.
The best sites along the centre line in France will be found from Reims to Metz. In Germany the most suitable climatology is from Ulm past Munich to the Austrian border. There is a slowly growing probability of seeing the eclipse through Germany and Austria where a location near the Hungarian border south of Vienna is favored although a location near the German border comes a close second.
3. Austria, Hungary & Romania
Salzburg in Austria will get 2 minutes 2 seconds of totality at 12:39 while Graz near the southern edge of the track will see just over a minute. The presence of all those high mountains could result in quite heavy cloud but certainly the scenery is spectacular - it's worth checking but the Sun should actually be above any nearby peaks at an altitude of 570. Vienna sees only a 99% partial eclipse as it is almost 40 km north of the path.
As the track leaves Austria, it leaves behind the westerly winds which have influence the climate so far and enters a region where the Mediterranean climate has a more pronounced effect. There is little doubt that the best European eclipse conditions will be found in Hungary, Romania and Bulgaria.
Lake Balaton lies wholly within the path of totality and this is a popular resort with warm water and sandy beaches. The former border between the Ottoman and Hapsburg empires ran down the middle and the shores are dotted with ruined castles. It is easily reached from Budapest and the lucky sungazers at the Lake should see an almost maximum 2 min 22 sec of totality at 12:50 - Budapest however will only get a 99% partial eclipse.
The southern edge of the shadow touches northern Yugoslavia before entering
Romania. The instant of greatest eclipse occurs at 11:03:04 UT when the
axis of the Moon's shadow passes closest to the centre of the Earth. At
that moment the epicentre is located among the rolling hills of south-central
Romania near Rimnicu-Vilcea.
[In Eastern Europe, add three hours to Greenwich Mean Time to get 14:03:04
local time.]
Here totality lasts for 2 min 23 sec, the Sun's altitude is 590, the
path width is 112 km and the umbra's velocity is 0.68 km/s.
Bucuresti (Bucharest) is also on the centre line and it will get 2 min 22 sec at 14:07 before the track moves out over the Black Sea and into northern Turkey.
4. Turkey, Iraq, Iran
The next landfall occurs along the Black Sea coast of Northern Turkey. The clear skies and good weather prospects will attract a lot of professional astronomers to this area - so be warned. There is also a great deal of usual tourist traffic. However, the sunny climate is enhanced by sea-breeze circulation of air which promises to keep the skies clear for the duration of the eclipse. The probability of seeing the eclipse rises steadily as the path moves eastwards. {SEE CHART}
The eclipse comes ashore at a remote part of the Turkish coastline near the town of Cide. To the east is the ancient city of Sinop, travel time about 2 hours from Cide. There is 10 km of pebbly beach at Cide which could make a good observing site. Ankara lies 150 km south of the path and witnesses a 96.9% partial eclipse. As the track diagonally bisects Turkey, the centre line duration falls steadily. At 11:29 UT or 14:29 local time, Turhal falls deep within the shadow for 2 min. 15 sec. The shadow briefly enters Syria at 11:45 before crossing into Iraq.
Baghdad is 22 km south of the path and experiences a 0.940 partial eclipse. Although the weather prospects in this region are excellent, the turmoil here makes it a risky option. Even better weather, and a more stable political climate, can be found in Iran, where the track leaves the Zagros Mountains and moves on to the desert plateau. This is an arid area with temperatures approaching 50 degrees Celsius. The low humidity makes the heat somewhat more bearable ( as will the cooling that accompanies the eclipse), but by the time the track reaches the Pakistan border, sultry humidity from the Gulf of Oman will lead to less pleasant conditions.
Skies over Iran are nearly cloudless, except for occasional patches of scattered convective clouds and light showers. These are usually weak enough to succumb to the cooling which comes with an eclipse and which can cause the sky to clear rapidly just before the onset of totality. The highest probablity of seeing the eclipse is reached at Esfahan in southern Iran where there is a 96% chance of seeing the event.
5. Pakistan & India
At 12.22 UT ( 18:52 local time ) the Moon's shadow enters Pakistan and skirts the shores of the Arabian Sea. Karachi is near the centre line and experiences 1 minute 13 seconds of totality with the Sun 22 degrees above the western horizon. The width of the path has by now shrunk to 85 km while its speed has increased to 2 km/s.
The last nation to receive the shadow does so at 12:28 UT (18:58 LT ) as the umbra enters India and sweeps rapidly westward. The centre line duration drops below 1 minute and the Sun to 7 degrees. Calcutta will witness a 0.879 magnitude partial eclipse with the Sun barely 2 degrees above the horizon. Leaving India just north of Vishakhapatnam at 12:36 UT, the shadow sweeps into the Bay of Bengal where it departs the Earth and races back into space ( 12:36:23 UT) not to return until 21st June 2001 when a total eclipse can be seen from South America, Southern Africa and Madagascar.
The eclipse will have lasted for 3 hours 7 minutes and will have covered
a distance of 14 000 miles across 0.2% of the Earth's surface.
Like most astronomical calculations, eclipse predictions are usually
presented in terms of Universal Time. In order to convert eclipse predictions
from UT to local time, you need to know what time zone you are in. For
the total solar eclipse of 1999, the path of totality passes through a
number of time zones and the conversion from UT to local time is as follows:
The visible surface is called the photosphere. At a temperature of nearly 6000 kelvin it is a thin layer, about 500 km thick, from which most of the radiation that reaches the surface of the Earth is emitted. It is the only part of the Sun's atmosphere which can be seen except during a total eclipse ( or without special equipment such as a coronagraph ). Below the photosphere, the ionised gas or plasma is too thick or opaque to allow radiation to pass unhindered. In the deep interior, energy is transported outwards by gamma radiation. It is believed that the temperature at the core reaches 15 million degrees. Photons produced in the core as a result of nuclear fusion reactions, take millions of years to travel in a random "drunkard's walk" to the photosphere. They are continuously being scattered or absorbed and re-emitted by the atoms deep inside the Sun. Above 0.7 of a solar radius, energy is transported mainly by convection currents as the hot gas rises to the surface.
The photosphere lies at the top of the convective zone and its surface is mottled by the tops of convection cells 1000 km in diameter, giving it a granular appearance when viewed in the light of hydrogen atoms using an instrument known as a spectroheliograph. In this instrument, based on the spectrograph, a narrow band of colour is selected from the visible spectrum by passing the Sun's light through a prism. By moving the slit of the instrument across the disk of the Sun, a picture can be obtained in the light emitted by excited atoms in the solar atmosphere.
The Sun radiates a total of 3.86 x 10^26 watts of energy into space at all wavelengths; we receive about 1.4 kilowatts of this energy on average for each square metre of Earth's surface. The brightness of the photosphere, and the effect of scattering of the light by our atmosphere, make it impossible to study the Sun's outer layers other than at a total eclipse of the Sun.
ON NO ACCOUNT SHOULD YOU LOOK AT THE
SUN DIRECTLY, EXCEPT DURING THE PERIOD OF TOTALITY . . . YOU WILL DAMAGE
YOUR EYESIGHT IRREPARABLY !
The Chromosphere
Immediately above the photosphere there is a 500 km reversing layer where the temperature falls from 6000 to 4000 kelvin. Above this is the pinkish chromosphere. Its colour is due to the light emitted when electrons in hydrogen atoms in their second excited state return to their first ( lower ) excited state by emitting photons of wavelength 656.3 nanometres ( 10 - 9 m). This is the well-known hydrogen-alpha spectral line which also colours gaseous nebula with a reddish glow.
The height of the chromosphere varies and it can reach up to 300 000 km above the photosphere. The temperature at the base is around 4500 K, rising to 20 000 K at a height of 1500 km. By the time it merges into the corona the temperature has risen to over a million degrees. The mechanism which heats the corona is not yet well understood.
The chromosphere is usually studied by means of narrow band filters or a spectroheliograph. Activity in the chromosphere takes the form of plages and filaments called spicules or fibrils, which are long tongues of gas like blades of grass rising to 10 000 km above the base of the chromosphere and lasting from 2 to 10 minutes. Flares and prominences can also be seen, suspended high in the corona by loops of magnetic field, usually associated with disturbances of the photosphere such as sunspots. These flares cause disturbances in the Earth's magnetic field which can affect radio communications and satellites. They are best seen during a total eclipse as "red flames" hanging in the inner corona close to the Sun's disc.
The corona
The Sun's outer atmosphere is extremely thin, but it is also extremely hot. Although it shines with the brightness of the full moon during the period of totality, its atoms are millions of degrees hotter than those of the photosphere.
The mechanism which heats the corona has been a mystery for decades but it probably involves the unstable magnetic fields which drive the flares and prominences high into the corona from the photosphere.
Recently discovered solar tornadoes may also shed light on this mystery.
Certainly, solar flares which occur at times of greatest solar activity cause the emission of x-rays and ultraviolet rays to increase dramatically in a timescale of 5 minutes and the resulting auroral displays and communications problems on Earth testify to the enormous amounts of energy released.
The appearance and extent of the corona varies with the 11 year sunspot cycle.
You MUST use eye protection during all stages of the eclipse EXCEPT for the period of totality when no protection is required.
Use a ND 5.0 filter over your telescope or camera or in front of your eyes.
DON'T FOCUS THE CAMERA WITHOUT PROTECTION.
Mylar filters must be fixed securely to prevent them being dislodged.
Don't use all your film during the first half - save it for totality. You can use up any unexposed film after the most important bit is over. It looks almost the same. ( Take a shot of 1st contact though and note the precise time.)
As the shadow of the Moon approaches, note the changes in lighting. A wide-angle lens or video camera can record these subtle changes.
Look for stars appearing in the sky - Venus and Mercury should be brilliant on either side of the Sun. Perseid meteors or even a comet may be seen.
Take some wide-angle shots of the surrounding landscape. In the last few minutes before totality the sky takes on a steely blue tinge shading into orange and red at the horizon.
Automatic exposure cameras will record the changing light level which falls off roughly in proportion to the area of the Sun - remember that even the last chink of sunlight can damage your eyes. Street lights operated by light sensors may turn on up to 1 minute before totality so keep away from them. They will probably remain on for up to 2 minutes afterwards.
{ Research by Dr. John Mason at Feb 26 eclipse in Venezuela suggests that as light levels drop below 100 lux the lights may turn on. During totality light levels may be as low as 10 lux - well within the capabilities of modern CCD chips in camcorders.}
Look for crescent images on the ground formed by natural pinholes such as the branches of trees. A unique photographic opportunity here.
2. Shadow bands
If the sky is crystal clear ( ! ) you may see a diffuse pattern of light and dark bands racing over the landscape up to 1 minute before and after totality. These shadow bands were first recorded by Hermann Goldschmidt in 1820 and are not always seen. Their exact nature remains a mystery.
They are best seen against bright surfaces such as whitewashed walls, although some observers prefer to view them against a grey background. Their orientation and separation vary according the the observer's position relative to the centre line of the eclipse. They are a considerable challenge to the photographer but more observations with timings are needed to determine precisely what causes them.
It is probable that the cause is related to the sudden cooling and reheating of the atmosphere as the Moon's shadow obstructs the light and heat from the Sun. This can amount to several degrees and variations in air temperature and density could give rise to a similar phenomenon.
To record these elusive bands on film or video, a large sheet of grey card should be placed on the ground, with some method of marking its orientation i.e. north-south line. A video recording with audible time signal could be used to estimate their speed and wavelength which is thought to range from about 2-5 centimetres.
Do not let this distract you however from the main event which is about to take place above your head !
3. Baily's Beads & Diamond Ring
As the Moon's irregular leading edge covers the disc of the Sun, brilliant shafts of sunlight seep through gaps between surface features such as mountains and crater rims. These can form spectacular glittering chains of lights around the limb known as Baily's beads.
They were first recorded by Edmond Halley in 1715, but it was Francis Bailey who gave the first really detailed description of them following an annular eclipse of the Sun in 1836. { QUOTATION }
As the last chink of sunlight is extinguished, a spectacular diamond ring effect is seen. This is much more spectacular at the end of totality as the Sun reappears from behind the trailing edge of the Moon. It lasts for only a few seconds and is well worth capturing on film, but you have to be quick as you will need to replace your eye protection immediately the DR is over.
Exposure tables for Baily's Beads and the diamond ring are produced by Fred Espenak { SEE TABLE }
4. Totality
For those precious two minutes of totality the first priority must be to enjoy the spectacle - do not spend all of it making exposures which may or may not capture the full beauty of this event. Look at the eclipsed Sun - do not worry about eye protection now. There may be an eerie silence as the birds stop singing but you may hear gasps of wonder as your companions gaze upwards. The landscape around you has taken on an eerie twilight and you may see the shadow of the Moon "set in the sky" like a dark corridor.
The corona will appear as a pearly white mist around the Sun. Is it symmetrical or irregular? How far from the Sun does it extend ? Are there coronal streamers ? Prominences - Baily's "red flames ". Can you see the bright planets ? Any comets ? What about the Persied meteor shower?
When you have looked - now you should start your program. Having set everything up well beforehand, go through your list of exposures. Don't forget to bracket your exposures of the corona. Take some wide angle shots of the landscape with the eclipsed Sun in the darkened sky. Try to capture the bright planets beside it. If you are lucky you will get a Persied fireball right in the middle. Save at least a couple of frames for the diamond ring as it reappears at the end of the eclipse and re-load if necessary for the final partial phase. DON'T FORGET THE EYE PROTECTION!
For most people this will be academic - you probably already own the camera you will use to photograph the eclipse and, unless you were thinking of buying one anyway, the one you do have will almost certainly do very well.
However, the most useful camera is probably an SLR - single lens reflex body with interchangeable lenses. To get a decent sized image of the sun on a 35mm frame you need at least a 400 mm focal length and probably something nearer 1000 mm would be better. A standard 50 mm lens will give a tiny image 0.5 mm across.
Avoid using zoom lenses - too many elements and more chance of multiple
images especially of the diamond ring.
DO NOT LOOK THROUGH THE VIEWFINDER unless you have a filter over the
lens and never filter the light after it has gone through the camera. Don't
leave it pointed at the Sun either as the heat can damage the mirror -
it won't burn a hole in the shutter.
You need to do some basic testing of the camera systems and make some
trial exposures on the film of your choice -
50 to 100 ASA is quite fast enough as there will be plenty of light.
To test exposures for the corona - use the full Moon as a harmless alternative.
What is right for the full Moon will be nearly right for the inner corona
but the fainter outer corona will need longer exposures so bracket widely
and select the best ones - keep notes, you may get to see more than one
total eclipse!
Remember to use a solar filter up to the moment the diamond ring appears and then remove it. Put it back after the Diamond Ring has reappeared.
A good sturdy tripod or some other means of fixing the camera will pay dividends and check the lens for focus at infinity - use brightly lit landscapes to see if it will be in focus for those few moments of totality.
A cable release is really essential - or some other means of firing the shutter without touching the camera; use the timer !
Don't use FLASH!! It will have no effect on the Sun's image - but it will seriously annoy those around you if you ruin their dark adapted vision as well as your own.
Suggestions for eclipse photography
You may already know what you want to do on eclipse-day. Even so, it is worth double checking that your trust camera will still do what you want when you need it to. Run through everything you intend to do beforehand - make a check list of things you need and ensure that they are at hand on the day itself.
Apart from photographing the beauty of the totally eclipsed sun itself, there are a number of other things you could attempt.
If you have more than one camera and tripods to match:
Set up your "BIG" lens and get it in focus ready for the main event. Cover the lens with a suitable filter and leave it alone.
Use a second camera with wide angle or even a fish-eye lens to capture the scenery as the eclipse progresses. A video camera might fit the bill here.
Another camera with a 50 mm lens could be set to capture several separate images of the Sun as it moves across the sky - this demands regular exposures to be made say every 5 or 10 minutes, not forgetting to change the exposure for the one in the middle ! A practice run at this one is a very good idea - just watch out for those solar rays.
If using a video for totality - use a solar filter up to the moment when the DR appears then remove it. The camera will automatically adjust its exposure but you will have to refit the filter as the total phase ends. Practise doing this efficiently - use an easily removed filter system, not a screw threaded one.
A fun camera can capture the people and the place and may even get a picture of those crescent pinhole images under the trees.
Watch out for :
The eye is a remarkable organ. It is capable of withstanding the brightness of the mid-day Sun in summer ( with a little help from "Foster-Grant" now and then) and also of seeing under conditions of almost pitch darkness. The range of brightnesses spans 10 orders of magnitude. It is important to take care of our eyes and not to subject them carelessly to levels of intensity with which they cannot cope.
The solar radiation which reaches the surface of the Earth has an average power of 1.4 kilowatts per square metre ( 0.14 W/cm2). This is equivalent to one single bar electric fire on every square metre. This varies with latitude and season, and of course with cloud cover. Absorption and scattering in the atmosphere, together with attentuation in the tissues of the eye reduce this by about 50% which makes the effective value of sunlight reaching the retina about 0.07 W/cm2.
In bright light our iris closes, reducing the diameter of our pupil to about 2.8 mm. This results in an area of 0.06 cm2 and gives a figure of 4.2 milliwatts of energy reaching the retina. However, the eye lens concentrates this energy into a smaller area. The image of the sun is about 0.15 mm in diameter giving an area of 0.018 square millimetres. The intensity of light which is focussed on to the retina is thus ;
Ir = 0.24 W/mm2 or 23.8 W/cm2
SOLAR ENERGY REACHING THE RETINA
Solar constant ( radiation at all wavelengths ) = 4.0 x 10 26 watts
Solar energy per square metre of Earth's surface. = 1.4 x 10 3 watts
Attentuation by atmosphere & eye = 50 %
Area of the eye's pupil ( bright light ) = 0.06 cm 2
Energy reaching retina per second
= 4.2 milliwatts
( in bright light conditions )
Area of image of Sun on retina = 0.018 mm 2
Intensity of solar radiation focussed on retina
= 23.8 watts / cm 2
DAMAGE TO THE TISSUES OF THE RETINA RESULTS WHEN 4.9 W/ cm 2 FALLS ON IT FOR 30 SECONDS.
THEORETICALLY - A QUICK GLANCE AT THE SUN WILL DO NO LASTING HARM . . . BUT DON'T TRY IT!
TO OBSERVE THE SUN SAFELY, IT IS NECCESSARY TO REDUCE BOTH VISUAL AND INFRA-RED RADIATION BY A FACTOR OF 100 000.
Experimental evidence suggests that damage to the retina will occur
when energy falls for 30 seconds at a rate of 4.9 W/cm2
( but not if it is 2.8 W/cm2).
It should therefore be possible to glance momentarily at the Sun without damaging your eyes, but only for a second or two at the most.
It is certainly NOT SAFE to gaze at the Sun nor to look at its image through any optical instrument which concentrates its light and heat further. This includes camera viewfinders, binoculars and telescopes.
To view the Sun safely, it is necessary to reduce the intensity over the range of wavelengths which cause harm, i.e. from 300 nm to 2000 nm ( the near UV to the near infra-red). For safety, a figure of 5% visible and 5% IR should be achieved ( or a combination not exceeding 10% of total e.g. 2% IR and 8% visible.)
Reducing the invisible IR component is of paramount importance as you do not need to feel the heat to cause damage - there are no pain receptors in the retina!
The total intensity should be reduced to below 2.8 W/cm2 . In practice this means reducing the incident radiation to below 2% for complete safety.
There are a number of alternative methods for doing this - only some of which can be recommended.
Pinhole camera - perhaps the safest method is to use a small pinhole in a sheet of cardboard. A second sheet placed about 0.5 m behind the first will give an inverted image of the Sun about 4 mm across. Natural pinholes made by tree branches are an alternative.
Eyepiece projection - telescope owners can arrange to project an image of the Sun on to a sheet of card or paper - binoculars can also be used in this way. Shield the direct rays with another sheet around the front of the instrument.
Mylar filters - provided you use the right stuff, mylar coated with a layer of aluminium and two layers sandwiched together to protect the coating, this type of filter reduces the light intensity 100 000 times at all wavelengths. Cheap and cheerful.
Welder's glass - comes in 18 different densities; number 14 is the minimum required. Goggles also can be worn but check they are intact and of the correct shade number - check for cracks.
Stanford Solar Viewer - recent innovation using robust cast resin meets required safety standards and is neutral density filter of photographic quality. More expensive but can be used with optical equipment - e.g. Cokin system.
Useful Internet Sites:
SOHO - http://umbra.nascom.nasa.gov/eit/EIT.html
and FRED ESPENAK's ECLIPSE PAGE -
Useful books:
The Sun in Eclipse - Patrick Moore & Michael Maunder (Springer-Verlag 1998)
Guide to the 1999 Eclipse - Steve Bell
(Royal Greenwich Observatory 1996 )
