Asaph Hall discovers Phobos, the inner moon of Mars

Background. Asaph Hall III, an American astronomer, was responsible for the United States Naval Observatory's 26-inch telescope in 1875, the largest refracting telescope in the world at that time. During Mars's closest approach in 1877, Hall was encouraged by his wife, Angeline Stickney, to search for the Martian moons. This context led to his identification of Phobos, the innermost and larger natural satellite of Mars.

Discovery. Hall discovered Phobos on 18 August 1877 at about 09:14 GMT at the United States Naval Observatory in Washington, D.C. Contemporary sources using the pre-1925 astronomical convention recorded the time as 17 August at 16:06 Washington Mean Time. Hall had discovered Deimos, Mars's other moon, a few days earlier on 12 August 1877 at about 07:48 UTC, a date that under the pre-1925 convention would be recorded as 11 August at 14:40 Washington mean time.

Instrumentation. The discovery was made using the USNO 26-inch telescope, which Hall had been given responsibility for in 1875. This instrument was the largest refracting telescope in the world at the time. The observatory, established in 1830, remains a leading facility for astronomical data and is located in Northwest Washington, D.C.