Interruptions in the distribution of asteroid s in the asteroid belt between Mars and Jupiter that appear where any of these small bodies' orbital periods would be a simple fraction of Jupiter's.
Several zones of low density in the asteroid belt were noticed about 1860 by Daniel Kirkwood (181495), who explained the gaps as resulting from perturbations by Jupiter. Any object that revolved in one of these locations would be disturbed regularly by the planet's gravitational pull and eventually be moved to another orbit.