  Rotation of a body, according to Einstein's theory of general relativity,
generates a "force" on other matter; in Newton's gravitational theory only the
mass of a body produces a force. This phenomenon, due to currents of mass, is
known as gravitomagnetism owing to its formal analogies with magnetism due to
currents of electric charge. Therefore, according to general relativity,
Earth's rotation should influence the motion of its orbiting satellites.
Indeed, we analysed the laser ranging observations of the orbits of the
satellites LAGEOS and LAGEOS II, using a program developed at NASA/GSFC, and
obtained the first direct measurement of the gravitomagnetic orbital
perturbation due to the Earth's rotation, known as the Lense-Thirring effect.
The accuracy of our measurement is about 25%.
