How we have found the Mass of the earth?
Ancient scientists had no idea of the mass of the earth; although Eratosthenes came up with a good figure for its size, the mass and corresponding gravitational force were not things that he appears to have considered. The mass of the earth can be determined from the fundamental gravitational constant, and the period of satellites (such as the moon); the trick is to measure the gravitational constant. The classic way of doing this is to prepare a bar with a lead ball at each end, and support it by a quartz thread at the center of the bar; two other lead balls are brought into proximity with the balls on the bar, and the slight gravitational attraction between these will cause a bit of rotation of the bar. A tiny mirror attached to the bar reflects light onto a graduated scale so that the rotation can be measured.
