Life on Earth

What has the size of the moon got to do with anything?

Maybe I spent too long on a train earlier this week but this month's New Scientist provides the answers to 6 questions about our universe and here's my version of one.

Eclipsed

I have only ever witnessed a total eclipse of the sun once in my life - unfortunately I was standing in Slough at the time and for more than one reason I was not in the right place. But a bright day went eerily dark and suddenly the birds began to sing as if it was dawn. I had my little pieces of holed-paper and watched the whole thing happen for nothing.

I am lucky - most never get to see one; thousands chase them.

At the height of 'totality' the fit of the sun and moon is so very perfect that beads of light penetrate the rugged valleys of the moon creating the amazing 'diamond ring' effect.

Coincidence

It is a shocking coincidence that the sun is around 400 times as wide as the moon and it is also about 400 times further away from earth. That makes the two look the same size in the sky which is unique in our solar system of all eight planets and their 166 moons.

The coincidence is very unique. Our moon is different to all the other moons around the planets such as Jupiter, Uranus or Neptune which are thought to have been created by one of two processes - 1) from the accretion of a disc of material in the planet's gravity field (like a miniature version of the solar system forming) or 2) through the gravitational capture of passing smaller bodies. That second form is likely what happened on Mars where it's two satellites are the only other moons in the inner Solar System.

Big Moon in Sky

Our moon is different though. Our moon is too big relative to the earth to be explained by either of those two events. Boffins who know their subject on this, think there is only one real explanation - in the first 100 million years of our Solar System, an object the size of Mars smashed into us and the result was that the shattered pieces congealed and formed two bodies which were held together in their combined gravity.

If you think that is coincidence - then read on.

Life As We Know It

Having such a large moon is very important for life on earth. As the earth spins on its own axis, it has the tendency to wobble like an old drunk which is caused by the gravitational pull by other bodies like the Sun. The moon's gravitational effect on the earth actually dampens that wobble and so it prevents instabilities in the earth's rotation that would otherwise cause far more unstable weather patterns on earth and that would have made it far more tricky to have created the conditions for life to have even started, let alone have evolved, here on earth.

Our position in the Solar System is in what is termed the 'habitable zone' where liquid water is very abundant (ok so we get a bit of snow now and then too) and this happens to be the single most important factor for life to have existed on earth. The presence of a large moon can be argued to have been equally important, and one large enough to cause total eclipses no less.

Knowing what we know about the size of our moon and its effect on life on earth may help in our search for life on other planets outside our Solar System.

Bye Bye Moon

Since the creation of our moon, it has drifted away from the earth at the rate of around 3.8cm per year. It doesn't sound much but it does mean that dinosaurs never got to see the remarkable effect of a total eclipse as the moon was too near and blotted out the Sun completely. Similarly in around 200 million years time, humans (should we survive) would not be able to see a total eclipse as the moon would seem smaller than the Sun in the sky.

It's incredible to think that our specific time is the result of two coincident timescales - that of the recession of the impact-formed moon and that of the evolution of beings intelligent enough to notice it (that would be us humans in case you are scratching your head).

It makes you really wonder about this amazing thing we know as life. As we ponder the wizardry of financial systems and how it can ruin us, it is very sobering to know that but for the moon and its size we may not be here today.

Perhaps that may be the best place to stick the proposed new 'Bad Bank' and the senior executives who created it?