Hall , Asaph

(1829–1907) American astronomer

Born at Goshen in Connecticut, Hall had to leave school at the age of 13 and support his family as a carpenter, following the death of his father. He educated himself, and his interest in astronomy was strong enough for George Bond to employ him as his assistant at Harvard in 1857. In 1863 Hall became professor of mathematics at the Naval Observatory in Washington. He returned to Harvard as professor of astronomy in 1895.

In 1877 Mars was in opposition to the Sun at a distance of about 30 million miles from the Earth. Hall decided to search for Martian satellites using the 26-inch (66-cm) refractor that the Clark firm had provided for the Naval Observatory. On 11 August he discovered a tiny satellite (the smaller moon) but was then compelled to wait a further six nights for the persistent cloud to clear before he could confirm his sighting and discover a further satellite. Both were very small, having diameters of 17 miles (27 km) and 9 miles (15 km) only. He named the larger ‘Phobos’ and the smaller ‘Deimos’ (Fear and Terror), after the sons of Mars. One curious feature of the two satellites was that Jonathan Swift had spoken of Martian satellites in Gulliver's Travels (1726). Not only did Swift get their number correct but also spoke accurately of their size and orbital period.

In 1876, by noticing a white spot on the surface of Saturn, Hall was able to work out correctly the rotation period as 10.75 hours, which compares well with today's figure of 10 hours 14 minutes (for its equatorial region).