Expert: Courtney Seligman - 8/8/2009

Question
If I were living during the days of Johannes Kepler (1571-1630) What tools and theorys would I have been able to use to find out the mass of Earth?

Answer
The mass of the Earth is determined by measuring the constant of gravity, G, and comparing its value to the force of gravity on objects of known mass. Even the concept of that constant didn't exist until Newton's time, and its value can still only be estimated to about a hundredth of a percent (that is, about 4 digit accuracy), so in Kepler's day there wouldn't have been any way to accurately measure the mass, no matter what kind of tools were available.

However, good guesses could have been made by measuring the density of rocks of various types, guessing what fraction of the Earth consisted of various density rocks, and multiplying the average density obtained in that way by the volume of the Earth. The result would have been off by at least a factor of two, as the rocks at the surface of the Earth aren't nearly as dense as those inside the Earth, but the fact that the mass is immense could have been calculated fairly easily.