Asaph Hall discovers Deimos

A powerful geomagnetic storm followed; the sequence later helped develop the study of space weather.

The UTC catalog fixes the transit on 9 December, giving astronomers another rare timing opportunity.

Observing campaigns used cameras as well as telescopes to document Venus crossing the Sun.

Teams sought to estimate the Earth–Sun distance from observations at widely separated locations.

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

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.