A careful observer discovers Mars's moons.
An American astronomer discovers Mars' moons.
The 19th and early 20th century American astronomer Asaph Hall discovered Mars' small moons, Phobos and Deimos. Observations of the moons' orbits allowed others to calculate the Red Planet's gravity, which is just under half that of Earth's.
Asaph Hall III (October 15, 1829 – November 22, 1907) was an American astronomer who is most famous for having discovered the 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.
Strangely enough, the two moons of Mars had already been described 103 years earlier by Jonathan Swift in his 1726 novel Travels into Several Remote Nations of the World, also known as Gulliver's Travels. In his reference to the astronomers of Laputa in Chapter III, Swift states: "They have likewise discovered two lesser stars, or satellites, which revolve about Mars; whereof the innermost is distant from the centre of the primary planet exactly three of his diameters, and the outermost, five; the former revolves in the space of ten hours, and the latter in twenty-one and a half; so that the squares of their periodical times are very near in the same proportion with the cubes of their distance from the centre of Mars; which evidently shows them to be governed by the same law of gravitation that influences the other heavenly bodies."