Born in Harford County, Maryland, Kirkwood became professor of mathematics at the University of Delaware in 1851, moving to the University of Indiana in 1856. In 1857 he noted that the asteroids (planetoids) are not evenly distributed in between the orbits of Mars and Jupiter but that there are areas in which no – or very few – asteroids orbit. He showed that these gaps in the asteroid belt – since known as Kirkwood gaps – occur where the period of revolution of an asteroid would have been an exact simple fraction of the Jovian period. Kirkwood explained that any asteroids in these areas would eventually be forced into other orbits by perturbations caused by Jupiter. Similarly he was able to explain gaps in the rings of Saturn (the Cassini division) as being caused by the satellite Mimas. Kirkwood published his findings in The Asteroids (1887).