It seems to have been to Johannes Kepler that it first occurred that the planet Mars ought to have two moons. In any event, in the early 1600s he wrote a letter of congratulation to his colleague Galileo who had discovered the moons of Jupiter, and remarked in passing: "I am so far from disbelieving in the discovery of the four circumjovial planets, that I long for a telescope to anticipate you, if possible, in discovering two around Mars."
Kepler's inference was based on the fact that Earth was known to have one moon, Jupiter four and Saturn five, and the rhythmic order of the universe therefore demanded that the planet Mars have two. Although neither of these twin satellites had ever been spotted, the belief in their existence persisted through the centuries. Voltaire, for example, writing in 1752, tells us that voyagers to Mars "would see the two moons which belong to it, and which have escaped the searches of our astronomers".
The French philosopher Fontenelle mentioned the possibility of two Martian satellites in Entretiens sur la Pluralite des Mondes in 1686; his reasoning was that "the many moons that Nature hath given to Saturn and to Jupiter are a kind of proof that Mars cannot be in want of them."
Even our own Jonathan Swift was aware of these Martian expectations. In Gulliver's Travels he describes a floating island called Laputa that was peopled entirely by enthusiastic and eccentric men of science with their patient wives. Peering through their telescopes, the Laputans "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".
But even if they were suspected to exist, the moons of Mars proved decidedly elusive. The planet's orbit relative to Earth is such that conditions favourable for a sighting occur only every 15 to 17 years. On one such occasion, in 1862, a systematic search was carried out, but yielded nothing.
Fifteen years later, however, persistence was rewarded. A new 26-inch reflecting telescope installed at the Washington Observatory under the stewardship of a young American called Asaph Hall greatly improved the chances of success, and 123 years ago today, at 2.30 a.m. on the night of August 11th/12th, 1877, Hall spotted the first Martian satellite. He observed the second one less than a week later on August 17th, and named the pair Phobos and Deimos, or Fear and Terror, after the mythological sons of Mars, the Roman god of war.