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Daniel Kirkwood (September 27, 1814 - June 11, 1895) was an American astronomer.

Born in Harford County, Maryland, he graduated in mathematics from the York County Academy in York, Pennsylvania in 1838. After teaching there for five years, he became Principal of the Lancaster High School in Lancaster, Pennsylvania, and after another five years he moved on to become Principal of the Pottsville Academy in Pottsville, Pennsylvania. In 1851 he became Professor of Mathematics at Delaware College and in 1856 Professor of Mathematics at Indiana University in Bloomington, Indiana, where he stayed until his retirement in 1886, with the exception of two years, 1865-1867, at Jefferson College in Canonsburg, Pennsylvania.

The most significant contribution of Daniel Kirkwood was his study in the orbits of the asteroids. In 1866 he published¹ the discovery of gaps in the distances of asteroids from the sun, named Kirkwood gaps in his honor. He also was the first to suggest that meteor showers are debris from comets.

Kirkwood also identified a pattern relating the distances of the planets to their rotation periods, which was called Kirkwood's Law. This discovery earned Kirkwood an international reputation among astronomers; he was dubbed "the American Kepler" by Sears Cook Walker, who claimed that Kirkwood's Law proved the widely held Solar Nebula Theory. The "Law" has since become discredited as new measurements of planetary rotation periods have shown that the pattern doesn't hold.

In 1891, at age 77, he became a lecturer in astronomy at Stanford University. He died in Riverside, California in 1895.

Altogether he authored 129 publications, including three books. The asteroid 1951 AT was named 1578 Kirkwood in his honor and so was the lunar impact crater Kirkwood, as well as Indiana University's Kirkwood Observatory. He is buried in the Rose Hill Cemetery in Bloomington, Indiana, where Kirkwood Avenue is named for him.

Kirkwood was probably a cousin of Iowa politician Samuel Jordan Kirkwood who became United States Secretary of the Interior under President James Garfield and President Chester A. Arthur.

¹ "On the Theory of Meteors," Proceedings of American Association for the Advancement of Science for 1866 (1867): 8-14. He actually first noticed it in 1857.

• Frank K. Edmondson, "Daniel Kirkwood: Dean of American Astronomers," Mercury Magazine (publication of the Astronomical Society of the Pacific), vol. 29, no. 3 (2000), pp. 26-33.

• J. Donald Fernie 1999, "The American Kepler," The New Scientist vol. 87, no. 5, pg. 398.

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