# How do they measure the size of a crater?

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I hope this is the right place to post this question ...

I've always wondered how astronomers can work out the size of craters on the moon.

Thanks

Pete

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Trigonometry...

You know how far away the moon is ("d" in kilometers), you measure the angular size of a crater from an image ("Theta" in arcseconds). Size of crater (in kilometers) = d * tan(Theta)

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I suppose the question then is how we know the distance to the Moon. This can be measured using parallax: the principle can be demonstrated by holding a pencil in front of you and looking at it with one eye, alternating between left and right. You see the pencil apparently move in relation to the background. The amount that it "moves" depends on the distance between your eyes and the distance to the object. Two observers far apart on Earth can measure this sort of effect by timing an event such as an eclipse (when the Moon passes in front of the Sun) or occulation (when the Moon passes in front of a star). This was first realised by the ancient Greeks. In modern times the distance has also been measured directly by bouncing electromagnetic signals (radio, laser etc) from the Moon's surface.

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The modern methods, I think, involve redar and, more recently, lasers directed at a mirror left by the Apollo astrononauts. No doubt a more knowledgeable reply will follow.

On an UCLAN course we estimated the height of lunar mountains using trig and the shadows of the mountains.

Olly

Edited by ollypenrice
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The modern methods, I think, involve redar and, more recently, lasers directed at a mirror left by the Apollo astrononauts.

Yes, but you don't need to know the distance to the Moon to the nearest millimetre to get a good estimate of crater sizes.

The best measures now come from images sent back by lunar mapping satellites rather than from ground based photos but the principle is the same. You know the distance (or can work it out by parallax), you know how big a feature appears to be in angular measures, and basic trigonometry fills in the missing numbers.

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Yes, but you don't need to know the distance to the Moon to the nearest millimetre to get a good estimate of crater sizes.

The best measures now come from images sent back by lunar mapping satellites rather than from ground based photos but the principle is the same. You know the distance (or can work it out by parallax), you know how big a feature appears to be in angular measures, and basic trigonometry fills in the missing numbers.

No, but I was replying to the question immediately before mine! I should have used the quote thngy. Sorry.

Olly

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