Sun Secrets: Surprising Facts About Solar Energy and Our Nearest Star.The Sun plays a pretty important part in all our lives – whether you’re considering its vital role as the primary source of energy on our planet, looking at its importance for growing food or simply hoping that it’ll shine for your weekend off!

It’s always there and it’s always flinging vast amounts of energy towards us through space (even if it’s often hidden behind grey, British clouds). It’s something we pretty much take for granted – but for all that familiarity, our Sun hides a few surprising secrets, which you just might not know.

The sun is an almost perfect sphere

OK, we all know that the Sun is a pretty big thing. You could (if it was possible) fit over 1,000,000 Earths inside and you’d need nearly 12,000 flattened Earths to cover its surface. What you may not be aware of, however, is that despite its huge size, the difference between its polar diameter and its diameter at the equator is a miniscule six-and-a-quarter miles – over a total distance of 865,374 miles. This makes it one of the nearest things to a perfect sphere observed in nature.

The Earth – 7,926 miles around its equator, but only 7,900 pole to pole – is distinctly egg-shaped by comparison!

The sun is brighter than we thought

Not so long ago, astronomers used to think that the Sun was quite a dim star – and clearly compared with super-bright (though very, very distant) stars such as Betelgeuse or Eta Carina, it is – but now we look on it a bit differently.

If you compared the 50 nearest stars to us – that’s within 17 light years of Earth or roughly 100,000,000,000,000 miles (yes, that really is 14 zeros!) – then our Sun would rank fourth. In fact it is brighter than around 85% of all the stars in the Milky Way, largely because most of them are a type of star called a Red Dwarf.

The sun IS the mass of solar system

It’s no secret that there are a lot of things in the solar system – the Earth and all the other planets, moons, asteroids and other assorted chunks of space-bits – all orbiting the Sun and that they’re spread out over a very, very long distance too. Pluto, for example, orbits at an average of 3,660 million miles from the Sun, and comets range far, far beyond that.

What might come as a surprise is that despite all these bits of rock, gas and ice that make up the Sun’s family, almost all of the ‘stuff’ of the solar system – about 98.8% of the solar system’s total mass – is contained in the Sun itself, and the planet Jupiter alone accounts for almost all of the rest. As for us on planet Earth, well in astronomical terms, we’re almost not here at all!

The Sun is middle aged – and getting hotter!

We tend to think of the sun as really, really old, but it’s only about half-way through what scientists call its ‘main sequence phase’ – which in everyday terms means it’s a mere 4.6 billion years old, with about another 5 billion to go before it will have burned off its stores of hydrogen. Once it has done that, a whole series of things will happen to it, which will see it expand to a Red Giant and then contract back to a white dwarf, before finally fading to black – but don’t worry, that’s not for a few billion years yet.

All the nuclear fusion of hydrogen going on at the heart of the Sun generates a huge amount of heat. The temperature at its surface is around 6,000 degrees C, while at the core it exceeds a staggering 15 million degrees C – and its getting hotter all the time. The Sun’s luminosity increases by about a tenth every billion years – but again, there’s no real cause for concern. It’s not exactly going to affect any of us anytime soon!

A Free kW per metre

Everyone knows the sun throws out lots of energy – this is an energy site, for goodness sake – but just how much may surprise you.

All that activity means that, measured at ‘one Earth’s distance from the Sun’ (officially known as one astronomical unit or AU) the Sun deposits just over one-and-a-third kilowatts of energy on every square metre of surface. This is called the ‘solar constant’ and its accurate value is 1,368 W per square metre.

The solar energy arriving at the Earth’s atmosphere is made up of about 50% infrared (heat), 40% visible light and 10% UV. The atmosphere itself naturally reduces the intensity of this – filtering out seven-tenths of the UV amongst other things – and so the energy actually reaching each square metre on the surface is nearer to 1kw – or at least it is on clear days with the Sun overhead!

Solar Exceeds All the World’s Needs

That 1kW is an average value, of course; the nearer to the equator you live the more solar energy you get, and vice versa – but then that’s not exactly news to anyone living in Britain, now is it?

Even so, that equates to around 100 W per square foot – which taken over a year works out to nearly 440kWh of energy per square foot per year – or a more than a mind-boggling 12,000,000,000kWh per square mile annually!

Bottom line – estimates put the total amount of solar energy hitting the Earth every year at more than 20,000 times greater than the entire energy demand of the world’s whole population. We’ll never be able to harness all of it, of course, but it certainly makes PV and solar thermal systems look a pretty good idea!

Good Old Einstein!

There’s an old saying that there’s no such thing as a free lunch – and all this energy raining down on the Earth is no exception.

At the heart of it all lies the Sun’s ongoing nuclear fusion reaction which sees it convert hydrogen into helium – and produce all that energy – in line with the now iconic E=mc2 equation from none other than science super-brain, Albert Einstein, who showed how matter can be converted into energy.

Keeping the Sun’s powerhouse running, and that 440kWh of energy per square foot per year reaching us here on the Earth calls for more than four million tonnes of matter to be converted every second – or to put it another way, since the Sun first ignited, it has burnt its way through the equivalent of 100 Earth’s worth of mass!

So the next time you wake to blue skies and glorious sunshine, you might want to spare a quick thought for the Sun blazing away up there – and ponder on one last quick thought.

If it suddenly ceased to shine, it would be just over eight minutes before you’d know anything about it – that’s how long it takes the light to travel the 1AU – or about 93 million miles – to Earth (although, amazingly, the actual energy itself may have already taken millions of years to make it from the Sun’s core to its surface).

But please don’t worry – it’s not going to; the Sun will be around for another 7 or 8 billion years yet!