Asaph Hall discovers Mars moon Phobos on August 17, 1877

On August 17, 1877, American astronomer Asaph Hall announced the discovery of Mars’s first natural satellite, Phobos. Hall, working at the United States Naval Observatory, had been observing the Red Planet for several years and noted a faint point of light moving in tandem with Mars. The new moon was named after the Greek god of fear, reflecting its small size and rapid orbit.

Hall’s observation was made with a 24‑inch refracting telescope, and the discovery was confirmed by additional observations the following nights. Phobos measures only about 22 kilometers across and orbits Mars in less than a day, making it the innermost of the planet’s two moons. The find expanded knowledge of the Martian system and provided a new target for future planetary studies.

The announcement was quickly published in scientific journals and received attention from the astronomical community worldwide. Hall’s work on Phobos and, later, on Mars’s other moon, Deimos, helped establish the United States as a leading center for planetary research in the late nineteenth century.