Asaph Hall III

Asaph Hall III (October 15, 1829 – November 22, 1907) was an 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, the son of Asaph Hall II, a clockmaker, and Hannah Palmer. His father died when he was 13, leaving the family in financial difficulty. So Asaph left school to become an apprentice to a carpenter at 16. He later enrolled at the Central College in McGrawville, New York, where he studied mathematics. There he took classes from an instructor of geometry and German, Angeline Stickney. In 1856 they married.

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 the USNO 66-cm/26-in telescope, the largest refractor in the world at the time. It was with this telescope that he discovered Phobos and Deimos in August 1877. He also 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.

Hall retired from the Navy in 1891. He became a lecturer in celestial mechanics at Harvard University in 1896, and continued to teach there until 1901. Asaph Hall died in November, 1907 while visiting his son, Angelo, in Annapolis, Maryland.