Astronomer and mathematician, born in Wallace, Nova Scotia, Canada. He moved to America in his teens and studied at the Lawrence Scientific School, Harvard. Appointed a professor of mathematics by the US navy in 1861, he was assigned to the Naval Observatory in Washington, DC, and later became professor at Johns Hopkins University (1884--94, 1898--1900). His major contribution was the calculation of all the known positions and motions of the bodies of the Solar System and the major celestial reference objects. He also did pioneer work in calculating the Sun's parallax and determining the velocity of light. He also published popular texts, such as Reminiscences of an Astronomer (1903), and three novels, and became the first president of the American Astronomical Society (1899--1905).