Discovery of the Martian moons

The discovery of the two moons of Mars, Phobos and Deimos, occurred in 1877 when American astronomer Asaph Hall, Sr. identified them after a long search, although their existence had been speculated before.

Early speculations

The possibility of Martian moons had been speculated before Hall's discovery. The astronomer Johannes Kepler (1571-1630) even predicted their number correctly, although with faulty logic: he wrote that since Jupiter had four known moons and Earth had one, it was only natural that Mars should have two. [http://www.mathpages.com/home/kmath151.htm MathPages - Galileo's Anagrams and the Moons of Mars] .]

Perhaps inspired by Kepler, Jonathan Swift's novel "Gulliver's Travels" (1726) refers to two moons in part 3, chapter 3 (the "Voyage to Laputa"), in which the astronomers of Laputa are described as having discovered two satellites of Mars orbiting at distances of 3 and 5 Martian diameters, and periods of 10 and 21.5 hours, respectively. This corresponds to the actual orbital distances and periods of Phobos and Deimos of 1.4 and 3.5 Martian diameters, and 7.6 and 30.3 hours, respectively. Swift's prediction is regarded as a coincidence since no telescope in Swift's day was powerful enough to discover the satellites.

Voltaire's 1750 novel "Micromégas", about an alien visitor to Earth, also refers to two moons of Mars. Voltaire was presumably influenced by Swift. [William Sheehan, " [http://www.uapress.arizona.edu/onlinebks/mars/chap14.htm The Planet Mars: A History of Observation and Discovery] "]

Discovery

Hall discovered Deimos on August 12, 1877 at about 07:48 UTC and Phobos on August 18, 1877, at the US Naval Observatory in Washington, D.C., at about 09:14 GMT (contemporary sources, using the pre-1925 astronomical convention that began the day at noon, give the time of discovery as August 11, 14:40 and August 17 16:06 Washington mean time respectively). [cite web |url=http://adsabs.harvard.edu//full/seri/Obs../0001//0000181.000.html |title=Notes: The Satellites of Mars |publisher=The Observatory, Vol. 1, No. 6 |date=September 20, 1877 |pages=181–185 |accessdate=September 12 |accessyear=2006 ] [cite web |url=http://adsabs.harvard.edu//full/seri/AN.../0091//0000013.000.html |title=Observations of the Satellites of Mars |author=Hall, A. |publisher=Astronomische Nachrichten, Vol. 91, No. 2161 |pages=pp. 11/12–13/14 |date=October 17, 1877, signed September 21, 1877 |accessdate=September 12 |accessyear=2006 ] Morley, T. A.; [http://adsabs.harvard.edu//full/seri/A+AS./0077//0000220.000.html "A Catalogue of Ground-Based Astrometric Observations of the Martian Satellites, 1877-1982"] , Astronomy and Astrophysics Supplement Series (ISSN 0365-0138), Vol. 77, No. 2 (February 1989), pp. 209–226 (Table II, p. 220: first observation of Phobos on 1877-08-18.38498)] At the time, he was deliberately searching for Martian moons. Hall had previously seen what appeared to be a Martian moon on August 10, but due to bad weather, he could not definitively identify them until later.

Hall recorded his discovery of Phobos in his notebook as follows: [cite web |url=http://adsabs.harvard.edu//full/seri/MNRAS/0038//0000205.000.html |publisher=Monthly Notices of the Royal Astronomical Society, Vol. 38, No. 4 |pages=pp. 205-209 |title=The Discovery of the Satellites of Mars |date=February 8, 1878 |accessdate=September 12 |accessyear=2006 ] quote
:I repeated the examination in the early part of the night of [August] 11th, and again found nothing, but trying again some hours later I found a faint object on the following side and a little north of the planet. I had barely time to secure an observation of its position when fog from the River stopped the work. This was at half past two o'clock on the night of the 11th. Cloudy weather intervened for several days.:On 15 August the weather looking more promising, I slept at the Observatory. The sky cleared off with a thunderstorm at 11 o'clock and the search was resumed. The atmosphere however was in a very bad condition and Mars was so blazing and unsteady that nothing could be seen of the object, which we now know was at that time so near the planet as to be invisible.:On August 16 the object was found again on the following side of the planet, and the observations of that night showed that it was moving with the planet, and if a satellite, was near one of its elongations. Until this time I had said nothing to anyone at the Observatory of my search for a satellite of Mars, but on leaving the observatory after these observations of the 16th, at about three o'clock in the morning, I told my assistant, George Anderson, to whom I had shown the object, that I thought I had discovered a satellite of Mars. I told him also to keep quiet as I did not wish anything said until the matter was beyond doubt. He said nothing, but the thing was too good to keep and I let it out myself. On 17 August between one and two o'clock, while I was reducing my observations, Professor Newcomb came into my room to eat his lunch and I showed him my measures of the faint object near Mars which proved that it was moving with the planet.:On August 17 while waiting and watching for the outer moon, the inner one was discovered. The observations of the 17th and 18th put beyond doubt the character of these objects and the discovery was publicly announced by Admiral Rodgers. |

The names, originally spelled "Phobus" and "Deimus", respectively, were suggested by Henry Madan (1838–1901), Science Master of Eton, from Book XV of the Iliad, where Ares summons Fear and Fright. [cite web |author=Hall, A. |url=http://adsabs.harvard.edu//full/seri/AN.../0092//0000031.000.html |publisher=Astronomische Nachrichten, Vol. 92, No. 2187 |title=Names of the Satellites of Mars |date=March 14, 1878, signed February 7, 1878 |pages=47-48 |accessdate=September 12 |accessyear=2006 ]

References