Fact 1: In Jonathan Swift’s satirical novel Gulliver’s Travels, the eponymous hero visits the floating island of Laputia, where the local astronomers tell him about Mars’s twin moons. “…the innermost is distant from the center of the primary exactly three of his diameters, and the outermost five: the former revolves in the space of ten hours, and the latter in twenty-one and a half.” Gulliver’s Travels was published in 1726.

Fact 2: During the close approach between Earth and Mars in 1877, American astronomer Asaph Hall discovered Mars’ two moons using the largest refracting telescope in the world at the time, the 26-inch pride and joy of the US Naval Observatory in Washington, D.C. The moons were subsequently named Phobos and Deimos, meaning “panic” and “dread” respectively. They’re the names of the twin horses pulling Ares’ (Roman Mars) chariot. Phobos and Deimos are probably captured asteroids.

The Laputian astronomers claimed that Phobos and Deimos orbit Mars in 10 and 21.5 hours respectively; the actual values are 7.7 and 30.3 hours. Similarly, the Laputians’ values for the distance from the center of Mars was 3 and 5 Martian diameters for Phobos and Deimos; the correct values are 1.4 and 3.5 diameters. How could Swift have known about Mars’ two moons 151 years before they were discovered, and how could he have made reasonable estimates for their orbits?

First off, he didn’t know. No one did until Hall discovered them with a really large telescope. One possibility is that Swift knew of an incorrect interpretation by Johannes Kepler of an anagram devised by his contemporary, Galileo Galilei. In 1610, Galileo wanted to announce his discovery of the rings of Saturn, but was afraid of being preempted. In those pre-arXiv days, you wrote coded letters about your discoveries to your fellow scientists. Kepler, however, misinterpreted Galileo’s code, believing he was claiming to have found two moons of Mars, an impossibility given the small size of Galileo’s telescope.

Or — equally plausible — Galileo had discovered Jupiter’s four largest satellites, now called the Galilean moons (Io, Europa, Ganymede, and Callisto). Earth has one moon, so perhaps (according to Kepler) Mars, which orbits the sun between Earth and Jupiter, has the geometrical mean of 1 and 4, i.e. 2 moons.

And as for the orbital elements for Mars’ moon — the periods and distances from Mars’ center — I think we can safely put Swift’s numbers down to lucky guesses. As blogger and neurologist Steven Novella puts it, “These figures are correct to within an order of magnitude, which is another way of saying that they are wrong.”

Finally, a shout-out to Asaph Hall’s wife, Angeline Stickney, who was instrumental in convincing her reluctant husband to spend time looking for moons of Mars during one of Earth’s closest approaches. In her honor, the largest of Phobos’ crater, about six miles across, is named “Stickney.”

Stickney crater, Phobos. False color image from Mars Reconnaissance Orbiter, 23 March 2008 which was then about 4,000 miles from the moon. (NASA/JPL, public domain)

(Thanks to Total Recall, starring our once-time governator, Arnie, for the title of this piece.)