In 1877, Mars came unusually close to Earth and telescopes throughout the world were trained on it.
In America an astronomer named Asaph Hall began to search in the vicinity of the planet for satellites. He had at his disposal a 26-inch refractor telescope, at that time, the largest in the world.
After sixteen days of patient search he observed a moon revolving around Mars. On the next day, he found another. Hall named the satellites, Phobos(Fear) and Deimos (Terror) after the two sons of the war-god Ares of Greek mythology. Asaph Hall became famous.
His discovery however posed a riddle to those who had read the book Gullivers Travels by Jonathan Swift.
The book, published a hundred and fifty years earlier, in 1725 has a passage which not only contains a reference to the two satellites but also gives their distances from the planet and their periods of rotation(10 hours and 21 1/2 hours according to Swift. Today we know he was slightly off the mark. The timings are 9 hours 39 minutes for one and 30 hours 18 minutes for the other).
Swift's readers wondered how the author could have known of the satellites of Mars so long before they were discovered. He could not have seen them himself because the telescopes available in his day were not powerful enough to have enabled him to see the moons.
The mystery has never been cleared up.