Swift and the Martian moons

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How did Jonathan Swift get knowledge of the two tiny moons some 150 years before they were discovered by the means of a telescope?

Berlin · 2007  Uwe Topper topper

Jonathan Swift

The two minute moons of Mars are quite known to everybody, they have sounding names: Phobos and Deimos. And the enigma of Swift’s knowledge of those moons is well known to all students. In chapter 3 of Gulliver’s travels Swift in 1726 not only mentioned the existence of the two moons but also described their distances to the mother planet and their revolution velocity around it. Those data are mostly termed „exceedingly correct“.

Yet, those moons were for the first time seen in the sky by the American astronomer Hall in 1877 using a new and powerful telescope. And even this point was clear to Swift 150 years earlier: It is only on account of the weak telescopes of his time that the moons are not to be seen, he argued.

When I was a student at High Scool and heard of this enigma, I was rather startled. Nobody knew a satisfying answer, the most frequent one was: „incidentally“ knowledge may pass to persons without any logic. Had the moons been very big, their existence and even movements could have been asserted mathematically, but in this case any kind of calculation was impossible because of the extraordinary small size of the objects.

Some people even made use of this precognition of Swift as a proof of clairvoyance or possibly prophesy or some mystic insight into the real essence of nature, but as scientific researcher I could not admit this type of solution. There remained one possible escape (as I had hinted at in 1977, S. 25): Ancient civilisations might have had technical superiority of the kind we only regained during the last generations (think of the Egyptian technique of cutting very hard stone and polishing it without using steel as is believd commonly), and this knowledge could have passed on by some undercurrent way like a number of other ideas and capacities that are still a riddle to us.

So I was looking some decades (!) for the answer and only step by step found it.

First of all: Swift was not the only one to have foreseen that Mars has two moons. The young Voltaire did so some years before Swift when writing a science-fiction story in which cosmonauts reach Mars and see the two moons with their own eyes. I did not find this text myself but read it in a students guidebook on the planets and their moons by well known astronomer Rolf Müller (1966, p. 82). Yet Müller did not tell where Voltaire might have got his idea from.

Finally I found the initiator of this chain of tradition: It was John Kepler himself!

In his work entitled „Talking to the star messenger“ (1610) where he answers to Galileo’s recent discovery of the four moons of Jupiter – which then was a real surprise to all scientists and a scandalous assertion to the clergy as well – Kepler says: „Wish I had a good telescope so I could anticipate you discovering the moons of Mars which must be there because of ratio.“ And for Saturn he projected six to eight moons as well as supposing that even Venus might have one and possibly Mercury, too.

But which type of ratio did he mean? The commentaor of the new Spanish edition of the Kepler-Galileo communication, Carlos Solís Santos (1984/2007, S. 137) explains that Kepler surmised that the cosmos is structured harmoniously and that therefore all heavenly bodies and their courses behave in harmonious ratios to each other. If Jupiter had four moons and Earth only one, then – Solís suggests while thinking the way Kepler would have thought – the planet between Earth and Jupiter ought to have two moons, as the farthest planet Saturn ought to have six (if the sequence was arithmetic, 1,2,4,6) or eight (if it was geometric, 1,2,4,8). (If the sequence had been 1, 1+1=2, 2+2=4, 4+3=7, then he would have ascertained the seven near to Saturn moons known today.)

As for the two inner planets Venus and Mercury, Kepler was unsure (they couldn’t have half a moon), that is why he anounced it hesitatingly „perhaps“. Anyhow, the suggestion that the other planets must have moons, too, was for Kepler rather obvious, and by no means it is so for us today.

We can suppose that Swift, a very learned man interested in all sciences, had read the widely published letter of Kepler since he gives more hints in his description of the movements of the two Martian moons:

„One of them finishes his revolution in ten, the other in 21 ½ hours, which makes the squares of their revolutions correspond to the cubics of their distances from the center of Mars.“ This is the law of Kepler for planets. Yet for the two Martian moons it is not applicable.

(I am desolate to quote Swift retranslating his lines from the German version, as I have no English original at hand).

As the above mentioned Rolf Müller amply explains, the parameters of the movements according to Swift are not correct; instead of a ratio of 3 to 5 their distances are 1.4 to 3.5 Martian diameters, and instead of the mentioned revolution velocity they are found as 7 ½ and 30 hours.

So I regard this riddle as solved: Human fantasy envisions so many possibilities to explain the world around us that there forcedly have to be always some visions that nearly hit the reality. All other visions are later forgotten, of course. 


Galileo Galilei (1610): Sidereus Nuncius (span. version: La gaceta sideral The Star Messenger) and John Kepler, Conversación con el mensajero sideral (his answer) (span. version, at the care of Carlos Solís Santos, Alianza Editorial Madrid 2007)
Müller, Rolf (1966): Die Planeten und ihre Monde (Springer, Berlin-Heidelberg)
Swift, Jonathan (1726): Travels of Lemuel Gulliver / Journey to Laput
Topper, Uwe (1977): Das Erbe der Giganten (Olten/Switzerland)

Originally written in German. Translated by the author, Uwe Topper, July 2009

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