Astronomer. He identified and explained the "Kirkwood Gaps," asteroid-free zones in the main asteroid belt between the orbits of Mars and Jupiter. He used the same principles to explain the divisions between Saturn's rings. He also produced the first convincing evidence that meteor showers are debris from disintegrated comets. Kirkwood was born on a farm and became a teacher. Largely self-taught, he became professor of mathematics at Delaware College in 1851, and served as college president from 1851 until he left in 1856 to take a professorship in mathematics at Indiana University. His career at Indiana spanned 30 years, with the exception of two years, 1865-1867, at Jefferson College in Cannonsburg, Pennsylvania. After he retired in 1886, he and his wife, Sarah A. McNair Kirkwood, moved to Riverside, California. In 1891, at age 77, he became a lecturer in astronomy at Stanford University. In 1849, while he was a high school principal in Pennsylvania, Kirkwood gained fame as "the American Kepler," for his formulation of a law of planetary rotation analogous to Johannes Kepler's law of planetary revolutions. Kirkwood's "law" proved to be wrong, but that did not discourage him from further research. In 1857, Kirkwood noticed that three regions of the asteroid belt lacked asteroids completely. In 1866, he proposed that the gaps were caused by Jupiter's gravitation. The effect of Jupiter's mass would be to force any asteroid that entered one of the gaps into another orbit. The same year, he showed that Cassini's Division, the main gap in Saturn's rings, is caused by the gravitational effects of Saturn's moon Enceladus. In 1872, he was able to demonstrate the effects of three more of Saturn's moons in producing both Cassini's Division and Encke's Division in the rings. Kirkwood published 126 papers and three books. The asteroid 1578 Kirkwood is named in his honor, and so is the impact crater Kirkwood on the lunar far side. He is buried beside his wife and their daughter Agnes, both of whom predeceased him.