Well, not quite, but he came close.

I was recently asked about the claim that Swift wrote about the moons of Mars in Gulliver’s Travels, published in 1726, and that he knew that there were two moons and exactly described their size and orbital period. The implication is that Swift somehow knew about the moons of Mars, Phobos and Deimos, a century and a half before they were discovered. It turns out that his predictions, if you can call them that, were accurate enough to be an interesting coincidence, but not so close that we have to consider it anything but that.

Gulliver’s Travels

If you have never read this book, I highly recommend it. The animated and live-action movies are fun for kids, but they contain almost none of the biting social satire, and generally leave out the most interesting parts of the book.

After Liliput and before Brobdingnag (the diminutive and giant lands shown in the movies), Gulliver visited Laputia, a floating island of crazy scientists. My favorite is the scientist who was trying to figure out how to get sunshine out of food by running it backwards through the digestive system. Gulliver was speaking of the Laputian astronomers when he noted:

They have likewise discovered two lesser stars, or satellites, which revolve around Mars, whereof 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.

Phobos and Deimos

The two moons of Mars, Phobos and Deimos, were discovered by Asaph Hall, Sr. on August 12, 1877. He was actively looking for Martian moons so it was no accident. This was 151 years after Gulliver’s Travels. Phobos means fear and Deimos panic – the names of the horses that pulled the chariot of the God of War – Ares. Ares is the Greek name for the Roman god Mars.

They are both small rocks of similar composition to asteroids, and so one hypothesis is that they are captured asteroids from the nearby asteroid belt. It is also possible that they formed in orbit – this is an unresolved question.

Phobos and Deimos have orbits which are about 1.4 and 3.5 diameters from Mars’ center respectively. The Laputians gave figures of 3 and 5. The periods of Phobos and Deimos are 7.7 and 30.3 hours, respectively, while the Laputians reported 10 and 21.5.

These figures are correct to within an order of magnitude, which is another way of saying that they are wrong. They are reasonable guesses, obviously, but do not betray any special knowledge. If Swift somehow knew what the figures were to any accuracy why wouldn’t he give them? Why get them close but not very close? It is a funny coincidence that he got as close as he did, but given all the opportunities for literary fiction to somewhat loosely match later scientific discoveries, the odds favor the occasional good guess.

It is also possible that Swift followed the basic logic that Mercury and Venus have no moons, the Earth has one, and Jupiter and Saturn have many. Mars is between Earth and Jupiter, so maybe it has two. There is some sense to this logic as it is probable that the farther away a planet is from the sun the more likely it is to hold moons. Too close, and the sun will grab them. The Earth’s moon is likely the result of a chance collision. While Mars is far enough away, and close enough to the asteroid belt, to have picked up a couple of moons.

It is also possible that Swift got the idea from Kepler, who in turn came to the conclusion that Mars had two moons because he misunderstood a cryptic anagram of Galileo’s. According to this reference:

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.” Kepler, a born riddle solver, made strenuous efforts to decipher Galileo’s string of characters, but he misconstrued the scrambled message to mean: Salue umbistineum geminatum Martia proles. “Hail, twin companionship, children of Mars”, or “I greet you, double knob, children of Mars”.

That would be an interesting coincidence also – getting the answer right for the wrong reasons, but nothing extraordinary.

Of course, once the notion that Swift “knew” of the moons of Mars prior to their discovery got out, cranks of all kinds jumped on this misinformation to suit their needs. One interesting one comes from the oxymoronic creationdiscovery website. Here’s what these geniuses have to say:

It is obvious from the results that Swift’s values were not mere guesswork. Seeing the moons of Mars requires substantial telescopic equipment, or a spacecraft flyby such as the Mariner and Viking spacecraft of the 1970s. Measuring the orbital periods of these moons cannot be done from Earth with our best equipment today. Either this was knowledge that God revealed to the ancients, such as Job, or it is possible that Earth people once saw Mars through very sophisticated astronomical equipment, that is beyond anything that we have at present. During the first millennium or so after God created these super intelligent beings called Adam and Eve, many scientific and technological achievements were probably made like this. Again this evidence supports the creation view instead of the evolution view that shows people becoming more and more technically able. In reality in many ways we are less technically able than human beings of thousands of years ago. This is the view that would be predicted by creationist views of history, based on Biblical teachings.

I always admire the almost preternatural ability of creationists to pack so many logical fallacies into so few words.

To honor Swift, and also Voltaire, who wrote in Micromegas, published in 1752, about Mars having two moons, two craters on Deimos are named after Swift and Voltaire. This is a fitting tribute. It is cool that one day people living on Mars will be familiar with many geological features of their world and its moons that have curious names with interesting histories. Future scholars will enjoy knowing how the Swift crater got its name, like people today like such place-name trivia as how the state of Connecticut got it’s name. (It’s the Algonquian name for the Connecticut river – quinnitukqut.)