The sun is directly overhead in a bright blue sky, flooding the beach with light. Glittering on the water, it dazzles the eyes of a toddler and his father as they paddle. Some rays hit the sunblock on the little boy's face and skitter away from him, but others soak into his dad's skin. "Gorgeous day," says Dave, an electrician from south London who is just glad the sun is shining on his family trip to the seaside. But how does it do that? "You what?" How does the sun shine, exactly? "Haven't got a clue."
Few of us have. We just expect it to be there, day after day, even when hidden by the weather. We seldom think about the heat on our faces having been born of nuclear fusion in the heart of a far-off star where the temperature is 15 million degrees. Standing on a beach in West Sussex with eyes closed and face upturned, it is hard to absorb the notion that the energy falling all around as heat and light is older than humanity.
Once created in the core of the sun, this energy may take a million years to rise to the surface. It must then travel 93 million miles through space and the atmosphere, weakening all the way, before reaching human skin. This it does quickly. "Listen Dave, that warmth we are feeling left the surface of the sun only eight minutes and 20 seconds ago."
He is not listening, preferring to swim, and who can blame him? The sun is a terrifying power that hangs in the sky, daring us to look. But people who do so go blind. Better to ignore it then. It is hard to do that, however, when you have been sitting in a dark planetarium with a mad-keen scientist who sets your brain on fire with talk of solar storms and flares. If staring at the sun can burn your eyes, considering the scale of the thing can do the same to your brain.
Even Nasa feels it has so much more to learn that a joint project with the European Space Agency will see a new fleet of probes examine the sun over the next few years. Meanwhile, earth-bound scientists can't agree on whether sunlight is good for us.
First they said it was healthy to go brown, then they said we would die of skin cancer and should cover up. That is why Dave's little boy, Harry, is wearing a legionnaire-style hat, sunglasses and full-body swimsuit. It was made in Australia, where the sun is merciless and 50 per cent of the population is at risk of skin cancer - but now even the Australians have changed their minds about sunshine.
The latest research, published last week, reasserts how much we need it to produce vitamin D, which helps fight cancer, diabetes, high blood pressure and arthritis. Up to 60 per cent of British people are short of this vitamin because we spend too long in cars, offices and gloomy weather. So the new creed is that short, protected sessions in the sun are good for you.
"Personally, I would say that between 10 in the morning and four in the afternoon you need to be careful," says Dr John Mason, who takes his children to the beach in the mid-afternoon - but then they are redheads and he is an expert in "this perfectly ordinary, middle-aged star" we call the sun. The resident lecturer at the South Downs Planetarium in Chichester fizzes with facts: the core of the sun is denser than lead, for example, but the rest of it has the consistency of (very hot) thick yoghurt.
"The sun is the ultimate energy source," says Dr Mason, sketching frantically to illustrate his points. "Inside the core, 700 million tons of hydrogen nucleae are fused every second to make 696 million tons of helium." That leaves four million tons a second to become energy. The output of the sun is estimated at 386 billion, billion megawatts.
This fiery ball of gas controls our weather and climate by warming air and water at the equator, which in turn move through the atmosphere and oceans. By feeding plants through photosynthesis it is the ultimate source of all our food. And the sun also provides fuel, besides solar panels. "When we consume fossil-based fuels such as oil, gas or coal we are releasing energy from sunlight stored by micro-organisms millions of years ago," says Dr Mason.
Sunlight is really electro-magnetic radiation, but "the really harmful stuff is absorbed in the atmosphere". That is why the sun is at its most powerful at midday (or one o'clock in British Summer Time) when it is directly overhead and has less atmosphere to act as a filter. Wednesday will be the summer solstice, the time of year when the sun is strongest in this country. The best thing to do then - or any other sunny day - according to the SunSmart campaign run by Cancer Research UK, is to spend lunchtime in the shade, wear a hat, use sunscreen of factor 15 or higher and take extra care with children.
Eric Williams would agree. The 52-year-old enjoyed childhood summers on the South Wales coast "on the beach, all day and every day" without protection from the sun. "You had a little splash of Nivea if you were lucky." He can't prove his skin was damaged by ultra-violet rays in those days, but as children we receive most of our lifetime's exposure to the sun.
Two years ago Mr Williams went to hospital for the removal of a lump on the back of his neck which three separate doctors thought was a harmless cyst. Tests revealed it to be a large and rare cancer. His next operation was to remove tissue and muscle from a wide area around the tumour. The cancer could have attacked his internal organs but so far it has not returned. "I'm not going to die this year, anyway," says Mr Williams. "I am fair-haired and I was always very careful in adulthood. I certainly have not burned since I was a teenager, but this happened to me. My daughters have got the message now."
More men need to do so. A thousand of them die in Britain every year from melanoma - more than women - yet they are far less likely than their female partners to check moles, see a doctor or even wear sunscreen in the first place. "It's not hot," says Dave back on the beach at Selsey, his shoulders already pink. "There's a breeze. I'll put some on in a minute." His son is protected though.
Attitudes change every generation. My grandfather used to sunbathe at Brockwell Park Lido, one of those places built between the wars to encourage working-class people out of their bad housing for a while and into the healthy, life-enhancing sunshine. Coco Chanel had made the tan fashionable, but Grandad probably wasn't influenced by her when he rubbed cocoa butter or olive oil into his skin.
My father seemed to think little of being burned during long holiday swims in the sea. My wife followed the Eighties trend for sun worship on the beach, but she got the message to be careful in the Nineties. Our children never play in the sun without cream, nor do their friends. Parents with offspring who are darker than they were in winter are practically accused of abuse. Still, the idea persists that a tan equals health and beauty - particularly, ironically, if everyone knows you sprayed it on.
Along the coast from Selsey Bill, where Dave was paddling, is a hump on a clifftop called Belle Tout. The name comes from the Anglo-Saxon words for the lookout place of the god Baal, and some people believe the sun was once worshipped here. That makes sense: you can see it rise in the east and set in the west from here, and in the darkest night the sky, sea and earth present a sheer black face that would make anyone pray for the dawn.
Tonight the sun disappears quickly into a haze of cloud, but fingers of reddish pink light still push into the sky before it goes. When the sun is low on the horizon its light has more atmosphere to get through, so the colours that linger are those with the longest wavelengths. The last to go, flashing right at the end, is green. Then we few strangers who gathered for sunset walk away, to the pub and to homes, to switch on headlights and reading lamps. We are not left in the dark so often these days. Nor do we sacrifice human life to the sun as the ancients did, except when we are too stupid to respect its power and slap on cream but let our skin fry instead. Not that the sun cares. That is why it has been such an enduring symbol of power, of the unknown, of the mysteries of the universe and of God. Whatever we do, however clever or stupid we are, none of us can stop the sun from burning.
The heart of the sun
The core is a super-dense ball of gas where hydrogen becomes helium at 15,000,000C. Energy made here moves slowly in the less-dense gas of the radiative zone, bubbles up through the convection zone, then reaches the surface or photosphere.
The magnetic binding
The movement of gas below the surface creates a powerful magnetic field. Rather than having two magnetic poles like the Earth, the sun has many. This image from the TRACE solar project shows opposing magnetic poles in red and green.
The cooler surface
The temperature at the photosphere is only about 5,500C. Even cooler patches called sunspots are created by strong magnetic activity and are visible from Earth as dark blotches. This Nasa ultraviolet image shows arcs of gas around a sunspot.
The volatile atmosphere
Above the surface is the corona, where the temperature can be 1,000,000C. Eruptions of electrically charged gas often break far out from the surface but are pulled back by the magnetic field. This ultraviolet image shows a loop 30 times wider than the Earth.
Solar Facts: The sun is...
4.6 BILLION YEARS OLD
92.96 MILLION MILES AWAY FROM THE EARTH
15 MILLION DEGREES CENTIGRADE AT THE CORE
92.1 PER CENT HYDROGEN (AND 7.8 PER CENT HELIUM)
99.86 PER CENT OF ALL THE MASS IN THE SOLAR SYSTEM