Asaph Hall (October 15, 1829 – November 22, 1907) was an American astronomer who is most famous for having discovered the moons of Mars (namely Deimos and Phobos) in 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 married Angeline Stickney.

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 in 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 positions of the stars in the Pleiades cluster.

He won the Gold Medal of the Royal Astronomical Society in 1879. The Hall crater on the Moon is named in his honor.

External links

• http://www.detroitobservatory.umich.edu/JAHH2003/DetroitObservatoryArticle.pdf
• US Naval Observatory Library search for photos