Friday, 4 March 2011

That 'Oh bugger' moment

It's almost impossible to write a whole book without the odd mistake creeping in. But what really winds me up is when you have a book that isn't printed yet, but has gone past the point of no return, then find out that something you put in it (believing it to be true) is wrong.

So here I am, innocently reading for review a book called What if the Earth had two Moons? when I come across a denouncement of the usual explanation of why the tides are the way they are. As an author, when you can point out a commonly held misunderstanding, it's very satisfying. So, for instance, when I was able to write in The Man Who Stopped Time that most websites and many books got it wrong in ascribing the mechanism for us seeing cinema as moving pictures to 'persistence of vision' (a Victorian concept that was just plain wrong) I felt rather smug. But here was a book denouncing the explanation I'd given for the tides in a book due out this April.

My version of the tides (not to scale)
As it turns out, it wasn't as depressing as I thought. Where the persistance of vision explanation of movies is totally and utterly wrong, the explanation of the tides is really just an over simplification. Which isn't quite so bad. Ignoring the effect of the Sun (which I mentioned) I had put the tides down to the water on the side nearest the Moon where the gravitational pull is stronger being pulled more towards the Moon, making a bulge on that side, plus the water furthest from the Moon bulging away because the pull is weaker. This is true, but also there is also an extra force contributing to the tide due to the Earth's movement around the 'barycentre', the centre of mass of the Earth/Moon system, providing a fairground ride push on the oceans. Oh well. You can't win 'em all.

One small consolation. I've just spotted a mistake in What if the Earth had Two Moons? The author says 'We can only see objects today that are within 13.7 billion light years of Earth.' This would be true if the universe weren't expanding. But because of this the objects we can see whose light has been travelling nearly 13.7 billion years are actually getting on for 40 billion light years away. I don't say this to get my own back, just to point out how easy it is to slip up.


  1. Ah yes, that second tide. As you say it's all down to simple physics and the combination of forces.

    It would be really interesting to model the effects of a second moon though - I feel a horrible geek mode coming on now....:-)