US astronomer who identified and explained the Kirkwood gaps, asteroid-free zones in the Solar System. He used the same theory to explain the nonuniform distribution of particles in the ring system of Saturn.

Kirkwood was born in Hartford County, Maryland, and became a teacher. In 1851 he became professor of mathematics at the University of Delaware; later at Indiana and at Jefferson College, Pennsylvania. From 1891 he lectured in astronomy at the University of Stanford, California.
In 1857 Kirkwood first noticed that three regions of the minor planet zone, sited at 2.5, 2.95, and 3.3 astronomical units from the Sun, lacked asteroids completely. In 1866, he proposed that the gaps arose as a consequence of perturbations caused by the planet Jupiter. The effect of Jupiter's mass would be to force any asteroid that appeared in one of the asteroid-free zones into another orbit, with the result that it would immediately leave the zone.
Kirkwood also carried out research into comets and meteors, made a fundamental critique of French astronomer Pierre Laplace's work on the evolution of the Solar System, and carried out preliminary studies on families of asteroids.