Who turned out the lights?

I feel it would be inexcusable to let a total eclipse of the sun pass without comment on a physics blog. This is especially true since I am one of the lucky people who have seen a total eclipse – in my case the 1999 eclipse that scythed across Europe.

There will no doubt be millions of photos strewn over the internet, plus movies (have a look at the BBC one here), but I’m sorry to tell you that looking at photos just doesn’t come close to the real thing.

For a start, there is the issue of dynamic range on a camera – it just isn’t possible to set an exposure that captures all the details – either you will over-expose the bits of the corona nearest the centre – or underexpose the bits furthest away – or, most probably, both. The human eye is brilliant at adjusting to light levels – the corona is an amazing collection of wispy strands and patches that are probably better sketched than photographed.

But really it’s the whole experience that you just don’t get by looking at photos – such as the way that the sky dims – almost impercetably at first, such is the ability of the eye to see across many light levels,  but at the last moment it’s like someone turning off a dimmer switch. And seeing stars come out in the middle of the day. One notable experience was, at the end of totality, as the photosphere (the bright bit) poked out from behind the moon, a flock of birds took me by surprise as it took off from the roof of the nearby cafe – they had come in unnoticed to roost when it had got dark.

Total eclipses are not all that rare – they occur every two years or so, but don’t expect one to come to you. Unless you are very lucky, you will have to travel to view them, but it is well worth the experience. Just pick somewhere likely to be sunny.

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