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Deimos ("DEE mos") is the smaller and outermost of Mars' two moons, named after Deimos from Greek Mythology. Phobos and Deimos were both discovered by American astronomer Asaph Hall. Deimos was discovered on August 10, 1877.
Deimos is composed of carbon-rich rock, much like C-type (carbonaceous chondrite) asteroids, and ice. It is
heavily cratered. It is probably an asteroid that was perturbed by Jupiter into an orbit that
allowed it to be captured by Mars, though this theory is still in some dispute. Like most bodies of its size, Deimos is highly
nonspherical with dimensions of 15×12.2×11 km.
As seen from Deimos, Mars would be 1000 times larger and 400 times brighter than the full Moon as seen from Earth, taking up a full 1/11 of the width of a celestial hemisphere.
As seen from Mars, Deimos has an angular diameter of no more than 2.5' and therefore appears starlike to the naked eye. At its
brightest ("full moon") it would be about as bright as Venus is
from Earth; at the first or third quarter phase it would be about as bright as Vega. When
Deimos passes in front of the Sun its angular
diameter is only about 2.5 times the angular diameter for Venus during a transit of Venus from Earth.
Unlike Phobos, which orbits so fast that it actually rises in the west and sets in the east, Deimos rises in the east and sets
in the west. However, the orbital period of Deimos of about 30.5 hours exceeds the Martian solar day ("sol") of about 24.5 hours by a sufficiently small amount that it would take 2.7 days between rising and setting.
References
- Robert S. Richardson, If You Were on Mars, Leaflet of the Astronomical Society of the Pacific, 178 (December 1943)
[1]
(1943ASPL....4..214R)
See also
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