American astronomer Asaph Hall discovers Phobos, one of Mars's moons.

An astronomer is a scientist in the field of astronomy who focuses on a specific question or field outside the scope of Earth. Astronomers observe astronomical objects, such as stars, planets, moons, comets and galaxies – in either observational or theoretical astronomy. Examples of topics or fields astronomers study include planetary science, solar astronomy, the origin or evolution of stars, or the formation of galaxies. A related but distinct subject is physical cosmology, which studies the universe as a whole.

Asaph Hall III was an American astronomer who is best known for having discovered the two moons of Mars, Deimos and Phobos, in 1877. He determined the orbits of satellites of other planets and of double stars, the rotation of Saturn, and the mass of Mars.

Phobos is the innermost and larger of the two natural satellites of Mars, the other being Deimos. The two moons were discovered in 1877 by American astronomer Asaph Hall. Phobos is named after the Greek god of fear and panic, who is the son of Ares (Mars) and twin brother of Deimos.

Mars is the fourth planet from the Sun. It is also known as the "Red Planet", because of its orange-red appearance. Mars is a desert-like rocky planet with a tenuous carbon dioxide atmosphere. At the average surface level the atmospheric pressure is a few thousandths of Earth's, atmospheric temperature ranges from −153 to 20 °C and cosmic radiation is high. Mars retains some water, in the ground as well as thinly in the atmosphere, forming cirrus clouds, frost, larger polar regions of permafrost and ice caps, but no liquid surface water. Its surface gravity is roughly a third of Earth's or double that of the Moon. It is half as wide as Earth or twice the Moon, with a diameter of 6,779 km (4,212 mi), and has a surface area the size of all the dry land of Earth.