Author Jonathan Swift (right) had quite an imagination. In his novel Gulliver’s Travels he notes two Martian moons, “the innermost is distant from the center of the primary exactly three of his diameters, and the outermost five.”
There are two notable things about this. First, the writer wasn’t too far off; the real values are actually two and four. More amazingly, Swift wrote his book about a century and a half before American astronomer Asaph Hall recorded the actual discoveries in 1877.
How on Earth did Swift “predict” Phobos and Deimos 151 years before they were first sighted by human beings? One conspiracy theorist is sure the author was born not in Ireland but on the planet Mars. But an intriguing idea comes to us through an error of the great astronomer Johannes Kepler.
Kepler discovered no satellites, but he was the recipient of a mysterious communication from Galileo in 1610:
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.
Was Galileo concerned the Inquisition was monitoring his communication? Or was he one smart guy trying to give another smart guy a little puzzle? This is an anagram. Cover up the rest of this post if you want to take a stab at it. Keep in mind that in Latin the letter “u” may also be used for “v” and “i” may be a “j.”
Kepler unscrambled it and read: Salue umbistineum geminatum Martia proles, which in English is translated, “Hail, twin companionship, children of Mars.”
The problem with anagram communication is that the long ones have more than one solution. Galileo’s opponents may have been kept in the dark on astronomical discoveries, but he also misled the great Kepler, for the puzzle alludes to Saturn, not Mars: Altissimum planetam tergeminum observavi. In other words, “I have observed the most distant planet to have a triple form.” That would be Saturn and its rings.
Fast forward a century: was Swift an amateur astronomer familiar with Kepler’s misinterpretation? We don’t know. I prefer to credit the novelist’s great imagination instead.
As for Professor Hall (left) of the US Naval Observatory, he was better with numbers than letters. He didn’t decipher Galileo’s code; he launched his astronomy career crunching numbers for the Harvard College Observatory in the 1850’s. By the mid-1870’s, he had a wonderful telescope (below, right) at his disposal in Washington DC. The USNO’s 26-inch refractor was the largest of its kind in the world. Viewing when he could as the summer fog occasionally wafted in from the Potomac, he observed and verified two moons of Mars in mid-August 1877.
The discoveries made Hall a sensation both in scientific circles and in the public eye. An American using an American-crafted telescope! Even the British were impressed and the Royal Astronomical Society awarded Hall its Gold Medal.
In the days of Galileo, Cassini, and even Herschel, naming satellites was not such a big deal. Hall himself seemed a little reticent as you can read from an early 1878 paper:
Since there is but little need of names for these satellites, I have delayed making a selection, but to avoid confusion I have chosen the following names: Deimus (sic) for the outer satellite, Phobus (sic) for the inner satellite.
These names were suggested by Mr. Madan of Eton, England. Theyoccur in Book XV of the Iliad, line 119, where Ares is preparing to descend to the Earth to avenge the death of his son. –Bryant translates as follows:
“He spoke, and summoned Fear and Flight to yoke His steeds, and put his glorious armor on.”
From Earth, Phobos and Deimos (Greek for fear and flight/terror) register with magnitudes of 11.4 and 12.5 respectively, a shade brighter than Saturn’s innermost moons Enceladus and Mimas. And a lot closer to Earth. Why weren’t these satellites picked up by the great Herschel? Did he have some sort of bias against the Red Planet?
Mars’ moons are hard to detect for the same reason you can’t see 4th magnitude Ganymede with the naked eye: the brightness of the nearby planet. And the Martian satellites orbit pretty close to their primary. From the ground on Mars, the Spirit rover caught both moons in the Martian evening sky on 30 August 2005:
As described by the NASA-JPL site:
On Mars, Phobos would be easily visible to the naked eye at night, but would be only about one-third as large as the full Moon appears from Earth. Astronauts staring at Phobos from the surface of Mars would notice its oblong, potato-like shape and that it moves quickly against the background stars. Phobos takes only 7 hours, 39 minutes to complete one orbit of Mars. That is so fast, relative to the 24-hour-and-39-minute sol on Mars (the length of time it takes for Mars to complete one rotation), that Phobos rises in the west and sets in the east. Earth’s moon, by comparison, rises in the east and sets in the west. The smaller martian moon, Deimos, takes 30 hours, 12 minutes to complete one orbit of Mars. That orbital period is longer than a martian sol, and so Deimos rises, like most solar system moons, in the east and sets in the west.
Let’s zoom in for some close-ups. First Phobos, courtesy of Mars Reconaissance Orbiter:
Then smaller Deimos, from the Viking Orbiter image archives: