Historical Organization
United States Naval Observatory
United States Naval Observatory
Details
Organization Timeline
Asaph Hall discovers Deimos
📅 11 August 1877Hall spotted the first of Mars's two known moons; the recorded date is 11 August in US local time.
Mars gains a known moon
📅 11 August 1877Finding Deimos overturned the assumption that Mars had no detectable satellites.
Angelina Hall encourages the continued search
📅 11 August 1877Accounts of the discovery describe Angelina Hall urging her husband to continue looking for Martian moons.
A faint Martian point proves to be Deimos
📅 11 August 1877Repeated observing established that the object was associated with Mars.
Asaph Hall discovers Phobos
📅 17 August 1877Hall found the inner Martian moon days after identifying Deimos.
Mars's second satellite is recorded
📅 17 August 1877Phobos's appearance completed the discovery of the planet's two familiar moons.
Phobos is found closer to Mars
📅 17 August 1877The newly discovered moon orbits nearer to Mars than Deimos.
The Martian system becomes a three-body subject
📅 17 August 1877With both moons identified, astronomers could study their motions around Mars.