Planetary perturbations make the earth's orbit about the sun irregular.
This motion of an object in an orbit about a center as called revolution.
The "weightless" condition of astronauts in orbit about the earth arises from just this cancellation of inertial and gravitational forces.
An important task of the space shuttle is to launch a space telescope and place it in an orbit about 500 kilometers above the earth.
The spacecraft will orbit about once every 12 hours and fill in the visual gaps left by it's two earlier flybys and the images snapped by Mariner 10 between 1974 and 1975.
