Phobos is found closer to Mars

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.

Hall found the inner Martian moon days after identifying Deimos.

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

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.

Observers compared records across locations to improve estimates of solar-system distances.

Photographs preserved a measurable visual record of a rare celestial alignment.