Dear All, A friend asked me last night why you don't get an eclipse of the sun every month. He and his flatmates had been trying to get to grips with the problem by use of a tomato and an orange, but still weren't too sure what was going on. So last night my starter for 10 was...
My tentative explanation was that the moon's orbit was not in the same plane as the earth's orbit around the sun. Thus there would be times when the moon was sun side of the earth, but its shadow was over or under it. My guess was that the moon orbits around the earth's axis, which is inclined to its orbit about the sun. Is this roughly correct? I know how hard it is to try to describe things like this without the use of a few good diagrams, so please keep any explanations as simple as possible! The person in question was reading a GCSE course book which had obviously simplified the layout of planets and moons to such an extent that, with everything in one plane, there should have been an eclipse every month! I guess that's the trouble with basic texts - a bit of simplifying here and there to make things easy for the kids to understand, and it's not until years later (if at all) that you realise you were duped. Of course one thing led to another and after my initial explanation, the next question was how many eclipses the earth gets in a year. At this point I admitted my ignorance of the full facts, but said I knew a group of people who would know chapter and verse on the matter! I guess at the end of the day I might have to buy him Meuus' (?) book on Clestial Mechanics - that should keep him quiet for a while! Anyway, some basic information on the above questions would be much appreciated, so get your thinking caps on as to how to explain it in simple terms. Best wishes, David Higgon
