Asaph Hall discovers Phobos

The 1874 event drew coordinated work across multiple countries and hemispheres.

Hall spotted the first of Mars's two known moons; the recorded date is 11 August in US local time.

Finding Deimos overturned the assumption that Mars had no detectable satellites.

Accounts of the discovery describe Angelina Hall urging her husband to continue looking for Martian moons.

Repeated observing established that the object was associated with Mars.

Phobos's appearance completed the discovery of the planet's two familiar moons.

The newly discovered moon orbits nearer to Mars than Deimos.

With both moons identified, astronomers could study their motions around Mars.

The 6 December transit was the second in the 1874–1882 pair.

NASA reproduces a photograph of Venus crossing the Sun made by students at Vassar College.