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The closest planet to the Sun.

Prior to 1965, astronomers believed that Mercury's sidereal rotation matched its orbital period of 88 days (synchronousrotation). This belief was reinforced by the chance coincidence of six 58.65-day rotation periods (352 days) closelymatching the synodic period of Mercury's maximum elongation (350 days). Therefore, observers saw the same features at everyopportunity for several years in a row. However, in 1965, G. Pettengill and R. Byce bounced radar signals off the planetusing the 300-meter Arecibo Radio Observatory in Puerto Rico and discovered a rotation rate of 59 days, proving that Mercurywas actually in a 3:2 resonance such that Mercury's day is exactly 2/3 of its 88-day year.

Mercury has been visited by only one spacecraft. Mariner 10 flew by three times in 1974-1975. Unfortunately, the orbitalgeometry was such that Mariner 10 photographed the same face all three times.

Because of torques from theSun, Mercury advances itsperihelion at a rate of 5,600 arcseconds percentury. According to observations, there was an extra 43 arcseconds per century which could not be accounted for. This discrepancy was resolved in 1915, when the theory ofgeneral relativity predicted an additionalperihelion precession of exactly 43 arcseconds per year.

Because of its proximity to theSun, Mercury has 700 K days, while temperatures on its night side are a frigid100 K. Mercury has a heavily cratered surface. Kuiper, the largest crater, is 60 km across, and crater Copley hasbright rays which extend for 400 km. Scarps also abound on Mercury, the largest being Discovery Rupes (550 km long). Although Mercury has plains such as Tir and Odin, it has peculiar hilly terrain on the side of the planet opposite theCaloris Basin, where a largemeteorite is assumed to have impacted. Mercury has no atmosphere, no moons, and amagnetic field only times as strong as Earth's. Its crust and mantle are thought to be composed ofsilicates, and its core is thought to be nickel-iron. Mercury is twice as iron-rich as any other planet.

Sladeet al.(1992) detected permanently shaded ice at the poles of Mercury using synthesis imaging of transmitted radar.This discovery was completely unexpected, but subsequent thermal modelling shows that it is indeed possible for ice topersist stably for billions of years in the shaded portion of craters near the poles.

Mercury transits (passes in front of theSun as seen from theEarth) 5-7 times every 46 years. Thetransits occur during daylight hours around November 10 or May 9 in the following years (where * indicates a Maytransit):

The transits repeat roughly every 46 years after between 5 and 7 transits, the intervals being either 3.5, 7, 9.5 or 13 years. 145 Mercury synodic orbits equal 46 solar years plus 1.1 days. Transits of Mercury can last up to 6 hours.

Earth,Jupiter,Mars,Neptune,Pluto,Saturn,Uranus