Science Questions

How do scientists find the weight of the moon?

Dave - The way they do it now is basically that if you’ve got something that’s orbiting the moon its weight is going to produce gravity. The stronger the gravity the farther you can orbit. As soon as you put a satellite orbiting the moon you can measure its mass quite accurately. By looking at how a satellite’s orbit changes as it goes around the moon you can see tiny variances in mass from mountains and things. You can get very accurate gravitational maps of the Earth and find things like ore bodies. If you’re got very dense rock somewhere then that’s going to pull you down a bit. In the distant past you might have worked it out the size of the tides on the Earth because you know how far away the moon is and how string gravity is on the Earth. By working out how much of an effect the moon has on the water you might get some idea of how much mass must be in the moon.

Chris - You also know that the tides used to be much bigger than they are today because the moon and the Earth used to be much closer together. There are fossilised tidemarks that geologists have uncovered that are metres in height. Because the moon has slowly migrated away from the Earth because it is moving away from us by about 3cm a year. You end up with progressively smaller and smaller tides. We’re now down to the more reassuring several metres rather than the 100s of metres that they were getting at one time.

Dave - That effect is because the tides are slowing down the Earth and also speeding up the moon. You can actually detect the slowing down of the earth because 200million years ago you could tell how many days were in a year. The corals grow a little line every day in a year. You can see the range in a year of maybe 400 days in a year about 200 million years ago.

February 2009