Saturday, 19 March 2011


Mercury’s apparent magnitude varies between −2.6[9] (brighter than the brightest star Sirius) and about +5.7. The extremes occur when Mercury is close to the Sun in the sky.[9] [10] Observation of Mercury is complicated by its proximity to the Sun, as it is lost in the Sun’s glare for much of the time. Mercury can be observed for only a brief period during either morning or evening twilight. The Hubble Space Telescope cannot observe Mercury at all, due to safety procedures which prevent its pointing too close to the Sun.[84]

Like the Moon, Mercury exhibits phases as seen from Earth. It is "new" at inferior conjunction and "full" at superior conjunction. The planet is rendered invisible from Earth on both of these occasions because of its relative nearness to the Sun. The first and last quarter phases occur at greatest elongation east and west, respectively, when Mercury's separation from the Sun ranges anywhere from 17.9° at perihelion to 27.8° at aphelion.[85][86] At greatest elongation west, Mercury rises at its earliest before the Sun, and at greatest elongation east, it sets at its latest after the Sun.[87]

Mercury attains inferior conjunction every 116 days on average,[3] but this interval can range from 105 days to 129 days due to the planet’s eccentric orbit. Mercury can come as close as 77.3 million km to the Earth.[3] In 871 AD, the nearest approach was the first in about 41,000 years to be closer than 82.2 Gm, something that has happened 68 times since then. After much longer gaps, the next approach to within 82.1 Gm is in 2679, and to 82 Gm in 4487. But it will not be closer to Earth than 80 Gm until 28,622.[clarification needed][88] Its period of retrograde motion as seen from Earth can vary from 8 to 15 days on either side of inferior conjunction. This large range arises from the planet’s high orbital eccentricity.[17]

Mercury is more often easily visible from Earth’s Southern Hemisphere than from its Northern Hemisphere; this is because its maximum possible elongations west of the Sun always occur when it is early autumn in the Southern Hemisphere, while its maximum possible eastern elongations happen during late winter in the Southern Hemisphere.[87] In both of these cases, the angle Mercury strikes with the ecliptic is maximized, allowing it to rise several hours before the Sun in the former instance and not set until several hours after sundown in the latter in countries located at southern temperate zone latitudes, such as Argentina and New Zealand.[87] By contrast, at northern temperate latitudes, Mercury is never above the horizon of a more-or-less fully dark night sky. Mercury can, like several other planets and the brightest stars, be seen during a total solar eclipse.[89]

Mercury is brightest as seen from Earth when it is at a full phase. Although the planet is farthest away from Earth when it is full the greater illuminated area that is visible and the opposition brightness surge more than compensate for the distance.[9] The opposite is true for Venus, which appears brightest when it is a crescent, because it is much closer to Earth than when gibbous.

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