interruptions that appear in the distribution of asteroids where the orbital period of any small body present would be a simple fraction of that of Jupiter. Several zones of low density in the minor-planet population were noticed about 1860 by Daniel Kirkwood, an American mathematician and astronomer, who explained the gaps as resulting from perturbations by Jupiter. An object that revolved in one of the gaps would be disturbed regularly by the planet's gravitational pull and eventually would be moved to another orbit.