Galileo may have been an extreme case, but for some eclipse watchers, 11 August will turn to tragedy, just because they don't understand that the sun's powerful UV (ultra violet) rays can cause permanent eye damage. This week, the Government has issued a warning, about the dangers of looking directly at the sun in the eclipse. The Department of Health, with the Royal College of Opthalmologists, the College of Optometrists and the Royal National Institute for the Blind, has produced a leaflet, Observing Solar Eclipses - Taking Care of Your Sight, giving advice on viewing the eclipse safely.
Each time there's an eclipse someone, somewhere, looks at the sun with no eye protection: records show that people have suffered varying degrees of solar damage throughout the ages. During Britain's last total eclipse in 1927, more than 20 people were blinded because they didn't follow safety guidelines. During the partial eclipse in Manchester in 1984, 11 people were left with permanent sight loss.
Staring at the sun causes two distinct types of injury. "If you look at the sun as Galileo did - through some kind of gathering object, such as binoculars - you literally cook the retina of the eye," says Jonathan Dowler, consultant opthalmic surgeon at London's Moorfields Eye Hospital. "This is known as solar retinopathy or thermal injury caused by the infra red rays of the sun.
"If you simply look at the sun for any length of time you are more likely to suffer photochemical injury which is a lot less well understood. However, we do know it may only take seconds for the UV rays to cause severe and lasting damage to the eye."
The problem with photochemical eye damage is that there is no immediate pain or loss of vision. Children are particularly at risk because they tend to have larger pupils and clearer lenses which allow plenty of UV light into the back of the eye. "They also focus extremely well, which means that if they are a bit long sighted they can bring the image of the sun into focus on the centre of the retina - which is exactly where you don't want it," says Dowler.
With long-term solar damage, black spots may appear in the central vision or there may be problems distinguishing colours. "We know about this because Bangladeshi children used to play a game to see who could look at the sun the longest. People who moved to this country from Bangladesh years ago still come to us now with small spots on the back of their eyes that are caused by photochemical damage," says Dowler.
Solar injury has also been reported in some religious sects. A group of children in Yugoslavia in 1988 believed they saw a divine vision in the sun, stimulating pilgrimages from across the world. "People stared at the sun to try and reproduce the effect which resulted in a range of solar eye injuries," says Dowler.
"People with eye conditions; older people who have had cataract surgery; people taking certain medicines, painkillers or drugs; and anyone drinking alcohol need to be careful during the eclipse. There is no treatment which has any effect once solar damage has occurred."
So how can we look at the eclipse without frying our retinas? The Internet is bursting with all manner of protective eyewear and solar shields claiming to filter the harmful rays and making it safe to look directly at the sun. Fight for Sight, a charity which campaigns against blindness, is anxious that enthusiastic eclipse watchers may end up confused by all the solar paraphernalia on offer. "The safest way to experience the eclipse is to turn your back on it and use a pinhole projector to project it onto another piece of card, wall or screen," says spokesperson Rebecca Muller.
To make your own projector, get a pin and poke a 1mm-wide hole in a piece of cardboard. Stand with your back to the sun and project its light through the hole onto a white surface about two metres away. This is a two-centimetre image of the sun.
Alan Frame, of ASF Marketing Ltd, which distributes one of the many solar eclipse viewers on offer, believes Fight for Sight is unrealistic if it thinks people will turn their back on the most breathtaking astronomical event of their lifetime. "Few people will want to miss it, and if people are going to look at the eclipse anyway, they may as well do it safely by using a solar shield," he says.
Dr Ralph Chou is associate professor of optometry at Canada's Waterloo University in Ontario, and is an expert on solar eclipses. "Of course looking at the sun is a risky business and we need to take precautions. However, as long as these shields are used properly - which means following the instructions to the letter - they are 100 per cent safe," he says.
Dowler is sceptical. "Astronomers will always say it's perfectly reasonable to look through special viewers because they do it all the time. The problem is that most of the solar shields on offer are not top astronomical quality."
Dr Chou does have reservations about children using the solar shields. "I don't believe children under 10 should use them, and children who are over that age should be supervised by an adult," he says. "Teenagers are a whole different ball game. They are the ones who are most likely to watch the eclipse, and the most likely to ignore all the warnings and do something daft."
There are two types of shield on offer. One is made from an aluminised polyester material similar to that used in sophisticated astronomical equipment, and reflects the light away from the eyes. The other is made from dark plastic and literally absorbs the sun's light. "Either will do the job, as long as you follow the instructions," says Dr Chou.
Before you attempt to use a solar shield, Dr Chou recommends checking it first in a dark room, by holding it up to a very bright light bulb: "As long as there are no pinpricks, the lens material has not been damaged in any way, and the frame is sturdy enough, you're ready to go".
For a copy of `Observing Solar Eclipses - Taking Care of Your Sight', call 0171-935 0702. National Eclipse Line: 0345 600444. UK eclipse website: www.eclipse.org.ukReuse content