Jonathan Swift's Martian Moons Right On 

By Ed Hatton 

The satirical adventure/fantasy novel Gulliver's Travels, by the Irish writer and clergyman Jonathan Swift (1667-1745) is considered a classic of world literature. But did you know that it holds an honored place in the history of astronomy, as well?

Its celestial fame results from a short passage that accurately describes the two moons of Mars. The passage is remarkable because Swift wrote it more than 150 years before the Martian satellites were actually discovered. That singular event took place in 1877, when an American astronomer named Asaph Hall first sighted the two small moons, which he named Phobos and Deimos. 

Amazingly, Swift was right about more than just the number of moons orbiting Mars. He also correctly placed them close to the planet and provided a fairly good estimate of the amount of time it took each small rocky satellite to revolve around Mars. 

Here's the relevant passage from Gulliver's Travels (1726): 

[The astronomers] have likewise discovered two lesser stars, or satellites, which revolve about Mars, whereof the innermost is distant from the center of the primary planet exactly three of its diameters, and the outermost five; the former revolves in the space of ten hours, and the latter in twenty-one and a half; so that the squares of their periodical times are very near the same proportion with the cubes of their distance from the center of Mars. 

Such a precise description of something that hadn't yet been discovered presented a quandary to the experts. How did Swift get it right? 

For years, scholars puzzled over the accuracy of Swift's astronomical descriptions. Was he just a lucky guesser, or did the Dublin clergyman in fact have access to some hidden or secret fount of astronomical knowledge? Some UFO enthusiasts even surmised that Swift's descriptions of the Martian moons could only have resulted from information passed on to him by alien visitors to Earth. 

There is, as it turns out, a less alarming answer. Swift's imaginative story about the Martian moons wasn't the first guess about how many satellites Mars might have. 

Some years before Swift wrote about the travels of Lemuel Gulliver, the German astronomer Johannes Kepler (1571-1630) put forth a theory that Mars had two moons. But even this guess was based on a crude and decidedly unscientific numerological system. 

The notion of "celestial harmony" held that the planets of the solar system possessed an increasing number of satellites as they progressed outward from the Sun. Kepler knew that Venus had no moon and Earth had one. Therefore, the reasoning goes, Mars must have two. Kepler wasn't a total crackpot, though. His studies of the motion of Mars enabled him to formulate his three laws of planetary motion, which were important milestones in the advancement of astronomy. 

So the likelihood is that Jonathan Swift didn't make up the existence of Mars' moons at all, but that he got the idea from Kepler and ascribed it to his fictional astronomers. However, Swift's guesses about the size of the moons and how long they take to travel around the Red Planet were uncannily accurate. And they don't seem to have come from any other source. 

You have to wonder. Perhaps Swift did have a conversation with some extraterrestrial visitors after all... 
 
 

Editor: Author Ed Hatton lives in Kilkenny, Ireland. He and his family lived in Milwaukee for two years in the late 1980s when he was getting a masters degree in history at University of Wisconsin-Milwaukee and my wife, Niamh, was earning a masters in economics.  He has very fond memories of those years, especially of all the used book sales in the churches. "We don’t have anything remotely like that over here – used books are outrageously overpriced."  Hatton also has fond memories of the late, lamented Chili Bowl restaurant. If it was still open, he claims he would definitely have his name on a plate on the wall by now. Hatton can be reached at edsearch.info