CompuServe Messages

Joe, there is no gravity, the Earth … never mind. Some geophysics books have gravity graphs that show the acceleration of gravity as a function of distance from the center of the earth. As you know, g = G*me/r^2 where big G is the gravitational constant and me is the earth's mass. As you go up, gravity weakens. Gravity meters can easily detect the difference of gravity between the floor and top of your desk. As you go down, gravity increases because you are decreasing r, the distance to the center of the earth's mass. However, as you go down into the earth, the rock above your head increasingly pulls up on you and g weakens until, at the center, the amount of rock "above" you is the same as the amount "below" and you (or rather, your ashes) float. John Sumner, who used to teach geophysics at the University of Arizona, once worked for Phelps-Dodge in the mine at Bisbee and used his gravity meter to detect the presence of dense, metallic ore bodies above the drift. If he was below the ore, the pull of its mass would detectably weaken g at the meter. As an aside, NASA has developed the most fantastic image of the earth using radar altimeter measurements of sea "level" from an orbiting satellite. Sea level ain't level! You can see all the features of the seabed – seamounts, mid-ocean ridges, fracture zones, and trenches – as the surface is molded by various masses pulling on the water.