US astronomer who discovered the two Martian satellites, Deimos and Phobos, 1877. He determined the orbits of satellites of other planets and of double stars, the rotation of Saturn, and the mass of Mars.
Hall was born in Goshen, Connecticut. Apprenticed to a carpenter at 16, he later enrolled at the Central College in McGrawville, New York. In 1856, he took a job at the Harvard College Observatory in Cambridge, Massachusetts, and turned out to be an expert computer of orbits. Hall became assistant astronomer at the US Naval Observatory in Washington DC 1862, and within a year of his arrival he was made professor.
In 1875 Hall was given responsibility for a 66-cm/26-in telescope, the largest refractor in the world at the time. He noticed a white spot on Saturn which he used as a marker to ascertain the planet's rotational period. In 1884, he showed that the position of the elliptical orbit of Saturn's moon, Hyperion, was retrograding by about 20° per year.
Hall also investigated stellar parallaxes and the position of the stars in the Pleiades cluster.