On Aug. 17, 1877, American astronomer Asaph Hall discovered Mars's moon Phobos.

Hall was working at the U.S. Naval Observatory in Washington, D.C. and looking through a 26-inch refractor telescope — which was then the largest in the world — when he first spotted the moon. Six days earlier, he had discovered another, smaller moon that he first described as "a faint star near Mars." Upon further inspection, not only did he realize that this star was actually the Martian moon Deimos, but he also discovered a second, larger moon now known as Phobos. He announced both discoveries the following day on Aug. 18.