What was a good mystery to you after reading the novel gulliver's travels? ? Why ?

I don't have 400 words but even within the satirical fantasy context of the story I recall this question being asked in class,

"In Gulliver’s Travels, Jonathan Swift described the two moons of Mars, Phobos and Deimos, giving their exact size and speeds of rotation. He did this more than 100 years before either moon was discovered."

I still don't know the answer. My proof. said something that didn't sound quite right or I was asleep with my eyes open (I used to be able to do that).