I have seen this topic discussed a bit on the web, but I don't see any clear answers. I takes a lot of energy to make the moon go in a curve, if you had to do it with rockets you would need a lot of fuel. Therefore it takes a lot of energy to make the moon go round the earth for 4 billion years. As the moon and earth are more or less in the same place as they were 4 billion years ago, where does this vast amount of energy come from? Most of the answers I have seen seem to follow the line of: it doesn't actually require energy, because the velocity/potential energy is the same, or because it is like a frictionless pendulum... It seems to me any explantion has to explain why it does take a lot of energy to move the moon if you do it with a rocket, but the exact same "flight path" can be done with no energy spent using gravity. This seems a contridiction. I'm guessing nature has some conservation of energy "gotcha' somewhere, but I can't see where it is.