Jonathan Swift was inspired by Kepler.
"Some cult literature has sprung up to address how Swift could have known about the Martian moons, since they weren't discovered until 1877. Some
have suggested that Swift himself was a Martian!
In reality, the idea that Mars might have two satellites goes back to Johannes Kepler's 1610 memoir, in which he misconstrued Galileo's anagram to
his friends announcing his discovery of Saturn's rings. The anagram was:
s m a i s m r m i l m e p o e t a l e u m i b u n e n u g t t a u i r a s
the correct solution of which was:
Altissimum planetam tergeminum observavi. "I have observed the highest (most distant) planet [Saturn] to have a triple form." (rings)
Kepler, a born riddle solver, made strenuous efforts to decipher Galileo's string of characters, but he misconstrued the scrambled message to
Salue umbistineum geminatum Martia proles. "Hail, twin companionship, children of Mars", or "I greet you, double knob, children of Mars".
When Kepler originally learned of Jupiter's four moons, and compared this with the Earth's one Moon, he had already predicted from mathematical
progression that Mars must have two moons (by the geometrical progression 1,2,4...). Now Kepler surmised that Galileo had discovered two Martian moons
exactly as he had predicted!"
Kepler got lucky. Jonathan Swift was inspired by Kepler. There is no conspiracy. The moons Swift describes are very similar, but they are not similar
enough to pretend that we had some sort of astronomical secret data. Or else the data would be correct, I would think. besides, why would Jonathan
Swift of all people have this data? I am reading a book about the scientific revolution right now and the people who would have actually had access to
this data, the people in the Royal Society of England of the Academie Royale of France (Halley, Flamsteed, Newton, Huygens, Wren, Cassini, Galileo,
Kepler, etc) would have published this data, because they were all very ravenous about discussing their new discoveries.