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Deimos (/ ˈ d aɪ m ə s /; systematic designation: Mars II) [11] is the smaller and outer of the two natural satellites of Mars, the other being Phobos. Deimos has a mean radius of 6.2 km (3.9 mi) and takes 30.3 hours to orbit Mars. [5] Deimos is 23,460 km (14,580 mi) from Mars, much farther than Mars's other moon, Phobos. [12]
The motions of Phobos and Deimos would appear very different from that of Earth's Moon. Speedy Phobos rises in the west, sets in the east, and rises again in just eleven hours, while Deimos, being only just outside synchronous orbit, rises as expected in the east but very slowly. Despite its 30-hour orbit, it takes 2.7 days to set in the west ...
(See Other moons of Earth and Quasi-satellite.) Mars has two known moons, Phobos and Deimos ("fear" and "dread", after attendants of Ares, the Greek god of war, equivalent to the Roman Mars). Searches for more satellites have been unsuccessful, putting the maximum radius of any other satellites at 90 m (100 yd).
However, since the angular diameter of Deimos is only about 1/10 of the angular diameter of the Sun as seen from Mars, it is more natural to refer to it as a transit. The angular diameter of Deimos is only 2½ times the angular diameter of Venus as seen from Earth during a transit of Venus from Earth.
Just like Earth's Moon, both Phobos and Deimos are considerably fainter at non-full phases. Unlike Earth's Moon, Phobos's phases and angular diameter visibly change from hour to hour; Deimos is too small for its phases to be visible with the naked eye. Both Phobos and Deimos have low-inclination equatorial orbits and orbit fairly close to Mars.
Swift is an impact crater on Mars's moon Deimos. It is about 3 km (1.9 mi) in diameter. [2] [3] Swift is named after Jonathan Swift, whose 1726 book Gulliver's Travels predicted the existence of two moons of Mars. [4] Swift is one of two named features on Deimos, the other being Voltaire.
Voltaire is an impact crater on Mars's moon Deimos and is approximately 3 km (1.9 mi) across. [2] [3] Voltaire crater is named after François-Marie Arouet, a French Enlightenment writer who was better known by the pen name Voltaire, who in his 1752 short story "Micromégas" predicted that Mars had two moons.
The two moons of Mars, Phobos and Deimos, are much smaller than Earth's Moon, greatly reducing the frequency of solar eclipses on that planet. [ 1 ] [ 2 ] Neither moon's apparent diameter is large enough to cover the disk of the Sun, and therefore they are annular solar eclipses and can also be considered transits .