The Moons of Mars

On the 11th August 1877 the American Asaph Hall, astronomer at the US Navel* Naval Observatory in Washington, first observed the moons of Mars, Phobos and Deimos, with the observatory’s 26-inch refractor telescope, then the largest of its type in the world. It would take many more observations over the following days and weeks before he could confirm his discovery but 11th August is the official date of the first sighting.

The astronomers of Laputa had discovered the moons long before Hall as related to us by Captain Lemuel Gulliver in part three of his “Travels into Several Remote Nations of the World”,

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 its diameters, and the outermost five; the former revolves in the space of ten hours, and the latter in twenty-one and an half; so that the squares of their periodic times are very near to the same proportion with the cubes of their distance from the centre of Mars, which evidently shows them to be governed by the same law of gravitation, that influences the other heavenly bodies.

Since Hall’s discovery of the real moons of Mars there has been much speculation about this passage of Swift’s, with the conspiracy theorists having a field day. Much is made of the supposed accuracy of the orbital periods, in reality 7 and 30 hours but firstly the differences are fairly large and secondly Robert C. Merton has shown that the figures Swift uses are probably based on speculations of Newton’s concerning the general behaviour of satellites. As to the number of satellite it is possible that Swift is here referring to a speculation of Kepler’s, whose third law he describes at the end of the quoted passage.  Kepler was obsessed with the concept of a mathematical God and was permanently searching for mathematical patterns in the structure of the cosmos. When Galileo and Marius announced their discoveries of the four moons of Jupiter Kepler speculated that the four outer planets all had companions or satellites, a word that he coined, the Earth 1, Mars 2, Jupiter 4 and Saturn 8 thereby creating a nice mathematical series of the sort that he loved.

Whatever the truth of the matter with the moons of Mars, we have a case of nature imitating art.

*American do spend a lot of time staring at their belly-buttons but in this case it was the astronomical sailor boys who were intended 😉

1 Comment

Filed under History of Astronomy

One response to “The Moons of Mars

  1. ckc (not kc)

    I think I’d like to visit the US Navel Observatory, but I’m a bit (omphalo)skeptical.

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