Racing around the Sun once every 87.97 days, Mercury is the fastest planet in the solar system. It is also one of the smallest, and as the planet closest to the Sun, it is often overshadowed by its much larger neighbor. Despite the planet’s small size, its contribution to science is impressive: The fact that Einstein’s Theory of Relativity was able to correctly predict Mercury’s seemingly erratic orbit was one of the first confirmations of the theory’s validity, and a contributing factor in the theory’s early acceptance.

Humans have known about Mercury since at least the third millennium B.C. The Greeks knew the planet by two names: Apollo, when it appeared as a morning star, and Hermes, or the messenger of the gods, when it appeared as an evening star.

One of Mercury’s most noticeable features is its seemingly unpredictable orbit, which takes it as close as 29 million miles to the sun, and as far away from it as 43 million miles. This was a source of frustration among early astronomers, because what they observed did not correspond to what they predicted with their astronomical models. Mercury’s perihelion, or the point in its orbit in which it is closest to the sun, shifts slightly each time, a behavior that cannot be attributed to the gravitational pull of nearby planets. This could not be explained with Newtonian mechanics, the dominant theory of the time. It was not until Einstein’s Theory of Relativity accurately predicted Mercury’s movement that astronomers had a way to understand and forecast the planet’s strange behavior.

Mercury rotates once every 58.65 days, or three times during two of its years, leaving one side exposed to light for long periods of time, and the other side dark. Because of this, the temperature differences between the sunlit side and the dark side are extreme. The side facing the sun reaches temperatures of around 810°F, and the dark side can reach about -290°F, making the planet’s temperature variations the most drastic in the solar system.

Only one spacecraft has visited Mercury. In 1974 and 1975, the Mariner 10 spacecraft flew by the planet three times, sending back several pictures but only mapping about 45 percent of the planet. The pictures revealed a crater-pocked surface similar to that of the moon.

Mercury can be seen either with binoculars or with the naked eye, but because it is so near the sun it may be difficult to distinguish it.







Mercury

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