Jonathan Swift – Author or Astronomer?

Most people would have heard of the wonderful adventure story called Gullivers Travels written by Irish-Anglo author Jonathan Swift.  Few would realise that it was written almost 300 years ago,in 1726. It contains a remarkable astronomical prediction.

The prediction was not written as a prediction but is included in the book as a fact discovered by the Astronomers of the floating island known as Laputa.  In Chapter III of his book, and referring to the Laputian astronomers, Swift writes  “They have likewise discovered two lesser stars, or satellites, which revolve about Mars; whereof the innermost is distant from the centre of the primary planet 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.”

If we convert his narrative description of the orbits of these satellites into numbers, he said that the innermost object was 13,600km distant from Mars and had an orbital period of 10 hours. The actual figures are 6,000km distance and a period of 7.7 hours. He said that the outermost object was at a distance of 27,200km and revolved around the planet in 21.5 hours, whereas the actual figures are 20,100 and 30.3 hours.

Remember that Swift wrote this in 1726 and at that time as far as anyone knew, Mars had no moons. It wasn’t until 1877, a whole 151 years later, that American astronomer Asaph Hall discovered them using the 26-inch refractor of the U. S. Naval Observatory in Washington, D.C.

Title page of first edition of Gulliver's Travels by Jonathan Swift.

Although Swift’s orbital descriptions are not exactly correct they are remarkably close considering that the moons orbit so relatively close to the planet. This is the main reason they had not been discovered earlier. Until Hall, urged on by his wife to look closer to the planet, actually found them, no one else had thought to look so close to Mars. They are so close in fact that they can become lost in the glare of the planet.

How did Jonathan Swift know this when he wrote his adventure story 151 years earlier, or was it simply a guess? We will never know the answer to that. One thing we should remember however was that Swift was also a satirist and other parts of his book are satires on events that were occurring or being discussed in his world at that time. One such example would be a man Gulliver encountered in the Grand Academy of Lagedo.

“He has been eight years upon a project for extracting sunbeams out of cucumbers, which were to be put in phials hermetically sealed, and let out to warm the air in raw inclement summers.” This could have come from a paper by Halley in 1693 on the “Circulation of watery Vapours” and also other pioneering work at the time by people such as Boyle, Hooke, and Hales on the nature of light, heat and the respiration of plants. We know now that cucumbers do work like that – store solar energy through photosynthesis as plant material and release it as heat and food energy when it is consumed.

Getting back to the Martian moons, a possible explanation is that Swift was again borrowing from his knowledge of scientific endeavours of the time and has drawn on Johannes Kepler’s supposition in 1610 after hearing that Galileo had discovered four moons around Jupiter, that Mars might have two moons. His only basis for that was that Earth had one, Jupiter had four (known at that time) and since Mars lay between Earth and Jupiter, it should have two. He also went on to say that Saturn should have six or eight. I suppose if Kepler can have a guess, so can Swift!  

Credit: Ken Glasson for Astro Space News

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