Asaph Hall discovers Martian moons Phobos and Deimos

On August 17 and 18, 1877, American astronomer Asaph Hall discovered the two moons of Mars, Phobos and Deimos. Working at the U.S. Naval Observatory with a 26-inch refractor telescope, Hall first identified Deimos before discovering the larger moon, Phobos.

Phobos is the largest Martian moon, measuring approximately 10 by 14 by 11 miles. It holds the distinction of being the closest moon to its planet in the solar system, orbiting Mars at a distance of about 3,700 miles and completing a full orbit in roughly 7 hours and 39 minutes.