Totally Eclipsed

Fact: Solar eclipses are a complete coincidence. 

If you have ever seen a total solar eclipse, you know just how beautiful it is. But it turns out that the perfect match-up between sun and moon that makes it possible is simply the biggest coincidence in the world.

A total solar eclipse occurs when the moon passes between the sun and the Earth, and so blocks the light from the sun. Only a thin ring of brilliant white light (called the ‘diamond ring’) is visible around the edge of the moon, because the moon almost completely obscures the sun. This is despite the fact that the sun, with a diameter of almost 1.4 million km, is millions of times larger than the moon, which has a diameter of just under 3,500 km.

What matters is how big the sun and moon look from Earth – a ratio called the ‘angular size’, which is equal to the size of an object divided by its distance from you . The key here is that the moon is also millions of times closer than the sun, so it looks bigger. Obvious right? But the amazing part comes when you actually do the maths:

The Sun: 1,390,000 km across / 149,600,000 km away = 0.009 (angular size)

The Moon: 3476 km across / 384,400 km away = 0.009 (angular size)

So, by sheer chance, the maths works out so that the sun and the moon appear to be the same size from Earth (although of course those numbers change second by second, so total solar eclipses are quite rare). It is this sheer coincidence, and not by any special laws of the universe, that means we get total solar eclipses. Another example of the human race just getting lucky.

‘Another one of Mr Shaunak’s Little Bites of Science’
Image under Creative Commons license.

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