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[['''Mercury]] ''' is the smallest planet in the [[Solar System]] and the closest to the [[Sun]]. It orbits the [[Sun]] in 88 Earth days and rotates on its axis every 58.6 Earth days,<ref>"The theory that Mercury is in [[synchronous rotation]] with its orbital motion was shattered in 1965. Astronomers Gordon Pettengill and Rolf Dyce used a radio telescope to make radar observations of Mercury’s spin rate. They found that Mercury’s rotation period is only 58.6 Earth days, not the 88 days accepted for nearly a century." [http://btc.montana.edu/messenger/elusive_planet/modern_times.htm]</ref> which is 3 rotations for every two orbits (called "3:2 resonance"). It can be seen from [[Earth]] only at sunrise and sunset.<ref name="spacemag">[http://www.space.com/spacewatch/mercury_view_030404.html Seeing Mercury at sunset], April 2003</ref>

Mercury has a very elliptical orbit. At its farthest distance from the Sun, it is nearly 50% farther than at its nearest distance to the Sun. <ref>http://astroprofspage.com/archives/139</ref>

It was the study of this planet's orbit that [[Albert Einstein]] claimed to help confirm the [[General Theory of Relativity]].<ref>Testing general relativity [http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=31277 European Space Agency] Accessed July 16 , 2007</ref><ref>Albert Einstein [http://www.phy.hr/ Fizički odsjek] Accessed July 16 , 2007</ref> Specifically, Mercury's [[apsis|perihelion]] precesses by 5599.7 arc-seconds per century, whereas [[Sir Isaac Newton|Newtonian]] physics alone would predict 5558.8 (comprised of the gravitational pull of the other planets on top of [[axial precession]]). This gives a discrepancy of -40.9 between theoretical models and real observations.

Astronomers of the period hypothesized either an [[Asteroid Belt|asteroid belt]] or another planet (which they named [[Vulcan (planet)|Vulcan]]) inside Mercury's orbit. Einstein's second-order correction to Mercury's orbit supposedly accounts for 43 arc-seconds of precession and obviated the need to look for any unseen asteroids or innermost planet.<ref name=Wudka/><ref name=Britannica/><ref name=Hartnett>Hartnett, John. ''Starlight, Time, and the New Physics''. Creation Book Publishers, 2007, pp. 34-36. ISBN 9780949906687.</ref><ref name=Wudka/><ref name=Britannica/> However, the theoretical error in the relativity calculation (stated to be around 0.04<ref name="">Iorio, L., ''On the possibility of measuring the solar oblateness and some relativistic effects from planetary ranging'', 2004</ref>) is much less than the discrepancy which results (2.1 arc-seconds). So, while scientists tell us that the theory of relativity solved the problem perfectly, elementary analysis of the figures indicates that it only luckily landed in the ballpark, and is still out by around 50 times the supposed tolerance in the calculations.

Mercury has more or less no atmosphere, and many craters. <ref>John Gribbin, ''Companion to the Cosmos'' (Little, Brown & Company, 1996)</ref> It has no known moons.