… Kepler had purchased a copy of Lucian’s [1st Century] satirical work on lunar exploration facetiously titled, A True Story. From a scientific point of view the work made no sense: Lucian’s voyage to the moon begins in a whirlwind and concludes by poking fun at the society of his day through a chronicle of “hilarious discussions on the moon.” The flight of Duracotus and Fiolxhilde is also the result of supernatural forces that are no less mystical than the whirlwind conjured up by Lucian.
A second, and more important source of inspiration for Kepler’s moon voyage was Plutarch’s The Face on the Moon, [2nd Century] which Kepler read in 1595. It is a symposium of Greek scientific thought that includes the views of Hipparchus, Aristotle, and Aristarchus of Samos. Extensive speculation on the lunar environment as a possible home for life is presented; and Plutarch even relates the story of a mythical traveler—a Greek Duracotus—who sails to an island whose residents have knowledge of the passage to the moon. Kepler now had the classical precedent he lacked during his student days: he even hoped to publish translations both of Lucian’s and Plutarch’s work with the Somnium to show his debt to these classical writers, and hopefully blunt potential criticism of his own moon voyage. It was a task he did not complete.
Gale E. Christianson, Kepler’s Somnium: Science Fiction and the Renaissance Scientist, Science Fiction Studies, March 1976