Irish clergyman and social and political commentator, best known for his satirical fantasy Gulliver’s Travels, originally entitled Travels into Several Remote Nations of the World in Four Parts ... by Lemuel Gulliver (1726), in which reference is made to two (then undiscovered) moons of Mars. The astronomers on the flying island of Laputia, says Gulliver, have
... discovered two lesser stars, or satellites, which revolve around Mars, whereof the innermost is distant from the center of the primary 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.
When the two Martian moons, Phobos and Deimos, were eventually found, by Asaph Hall at the US Naval Observatory, their orbits proved to be quite similar to those described in Swift’s novel. Phobos is actually 6,000 km from the surface of Mars and revolves around Mars in 7.7 hours, whereas Swift gave the values 13,600 km and 10 hours, respectively. Deimos averages 20,100 km from Mars and orbits in 30.3 hours; Swift gives 27,200 km and 21.5 hours, respectively.
Yes, and “Gulliver” immediately adds, “... 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.”
What’s NOT said, is that the constant of proportionality in Kepler’s Law, which he cites, depends on the mass of the central body, namely Mars. Swift’s numbers show that he had a good estimate in hand, and I’ve always thought he must have had help with this. I believe that he did belong to some sort of intellectual circle of friends, but I’ve never seen any comment from literary sources about Swift’s calculations.