An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian
فرهنگ ریشه شناختی اخترشناسی-اخترفیزیک
M. Heydari-Malayeri - Paris Observatory
HomepageMercury 1) Tir; 2) jivé, simâb Fr.: Mercure 1) The closest → planet to the
→ Sun and one of five planets visible with the
naked eye. The → greatest elongation
of Mercury is about 28°,
making it visible at most about 112 minutes after sunset or before sunrise.
It lies at a mean distance of about 0.39
→ astronomical units from the Sun.
Mercury is just 4,879 km in diameter, about 2.6 times smaller than
the Earth. Its → orbital period is 87.97 Earth days.
Mercury has a high → density, 5.4 g cm-3,
with only the Earth having a higher density among the planets. This is largely due to
Mercury being composed mainly of heavy metals and rock.
One → solar day on Mercury lasts the equivalent of 176
Earth days while the sidereal day (the time for 1 rotation in relation to
a fixed point) lasts 59 Earth days. Mercury is nearly
→ tidally locked to the Sun and over time
this has slowed the rotation of the planet to almost match
its orbit around the Sun. Mercury also has the highest orbital
→ eccentricity of all the planets with its distance
from the Sun ranging from 46 to 70 million km. Mercury has just 38% the
→ gravity of Earth, this is too little to maintain
an atmosphere against → solar winds, which blow it away.
The surface of Mercury which faces the Sun has
temperatures of up to 427°C, whilst on the alternate side this can be
as low as -173°C. Mercury's core has more iron than any other planet in the
→ solar system. This has to do with its formation and early
life. If the planet formed quickly, increasing temperatures of the
evolving Sun could have vaporized much of the existing surface,
leaving only a thin shell. From L. Mercurius "Mercury," the Roman god, originally a god of tradesmen and thieves, from merx "merchandise." 1) Mid.Pers. Tîr the name
of the planet Mercury, O.Pers. proper noun *Tira-dāta-
"given by Tir" (Hellenized Tiridates),
Mid.Pers. Tîr.dât the name of three Parthian Kings; Av.
Tīro.nakaθwa-. |
Mercury's tail dom-e Tir Fr.: queue de Mercure A narrow and elongated structure of glowing → sodium gas associated with Mercury. Mercury's thin atmosphere contains small amounts of sodium that glow when excited by radiation from the Sun. Solar photons also liberate these molecules from Mercury's surface and pushes them away. Because Mercury's gravity is too weak to hold a permanent atmosphere, when atoms are evaporated from the planet's surface, some of the atoms form a tail that points away from the Sun. In particular, the yellow glow from sodium → D line is relatively bright. First predicted in the 1980s, the tail was first discovered in 2001 (A.E. Potter et al., 2001). Many tail details were revealed in multiple observations by NASA's robotic → MESSENGER spacecraft that orbited Mercury between 2011 and 2015. |
transit of Mercury gozar-e Tir Fr.: transit de Mercure The crossing the face of the Sun by the planet Mercury, as seen from Earth. Because the plane of Mercury's orbit is not exactly coincident with the plane of Earth's orbit, Mercury usually appears to pass over or under the Sun. On the average it occurs 13 times each century when the Earth is near the → line of nodes of Mercury's orbit. The three last transits were on 2003 May 07, 2006 November 08, and 2016 May 09. The next one will be on 2019 November 11. The first observation of a transit of Mercury was on November 7, 1631 by Pierre Gassendi. On June 4, 2014 NASA's Mars rover Curiosity photographed a transit of Mercury, marking the first time such a phenomenon has ever been imaged from the surface of a planet other than Earth. See also → black drop. |
