Who discovered Mars’s two moons, Phobos and Deimos, in 1877?

Asaph Hall discovered Mars’s two moons, Phobos and Deimos, in 1877.

Hall made the discoveries at the United States Naval Observatory in Washington, D.C., using the observatory’s 26-inch refractor telescope. He found Deimos on August 12, 1877, and Phobos six nights later, on August 18. Their names come from Greek mythology: Phobos and Deimos were associated with fear and terror and accompanied the god Ares, the Greek counterpart of Mars.

The discoveries settled a long-standing astronomical question. Earlier observers had searched for Martian moons, and some writers had even guessed that Mars possessed two satellites, but no telescope had confirmed them. The moons are small, irregular objects rather than spherical worlds. Phobos is the larger and closer moon, while Deimos is smaller and farther away.

A common mix-up is crediting William Herschel, who studied Mars extensively but did not discover either moon. Hall’s observations remain the accepted discovery of both natural satellites.