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How big is the moon?

A while ago I had a slightly surreal discussion with a radio presenter and the boss of the UK Space Agency. Us two science types were trying to educate said presenter in one or two basics of solar system science. At one point I asked the presenter a simple question. If you were looking at the full moon and held a coin out at arms length, which coin would be roughly similar in size to the apparent size of the moon?

What would you answer?

The most popular answers are 2p and 10p. Some opt for 5p. In fact it's a bit of a trick question. There are no coins small enough to be the same apparent size as the moon when held at arm's length. The nearest approximation is a hole punch hole. If you hold a piece of punched paper out at arms length, that little hole is about the same size as the moon appears to be.

This explains why photographs of the moon are so disappointing unless you apply a serious zoom. The photo here is a hurried snap taken on my phone because you don't often see the moon and a rainbow simultaneously. The (pretty well full) moon is that little white dot just above the bus shelter and to the right of the centralish tree. The camera tells us what our brain doesn't - it really does look small.

So why does it look bigger - in fact sometimes quite enormous (though never as big as Hollywood suggests)? This is where the UK Space Agency supremo and I deviate. He said it was big near the horizon because of optical effects due to the light passing through more atmosphere. There certainly are optical effects of this kind, but the influence on the apparent size is relatively small. Almost all the variation in the size of the moon (and it pretty well always looks bigger than it should) is down to the way our brain processes images.

We are so familiar with cameras, we tend to think that the eye/brain combo works like a camera. It doesn't. The image that we 'see' is a composite assembled by the brain from a whole host of processes. It is a fake construct. This should be obvious, because we don't see the blind spot, where the optic nerve renders part of the retina inactive, nor do we witness saccades, the fast, jumpy movements our eyes are always making. We see a fake image. One of the modules in the brain recognize shapes - so we can give extra weight to a known shape like the moon. If it is near trees or other relatively close items on the horizon, we tend to see it bigger - but this is only our brain's processor getting things wrong. Seeing really shouldn't be believing.


  1. Love this idea of seeing not being believing. Must slip that into a novel somewhere....

  2. I heard that the brain thinks the sky is shaped like an upturned bowl i.e. overhead objects are closer than objects on the horizon, so it 'corrects' by making things near the horizon look bigger.

    Whether this is true or not, many optical illusions supposedly depend on what the brain "thinks", even when we *know* the reality is different. I wonder if it's possible to train yourself not to see these illusions?

  3. It's an old idea, Sue - it's why some Ancient Greek philosophers argued you should not rely on observations but rather on logic and argument (hence why their science as mostly so rubbish).

    Anon - it's an interesting thought, whether you could train yourself not to see them. I'm not sure how you'd go about it, though. The brain is also quite good at sorting things out - for example if you wear inverting spectacles, eventually the brain will flip the image back the 'right' way up.


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