Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits. The perihelion of Mercury's orbit precesses around the Sun at an excess of 43 arcseconds per century, a phenomenon that was explained in the 20th century by Albert Einstein's General Theory of Relativity. Mercury is bright when viewed from Earth, ranging from −2.3 to 5.7 in apparent magnitude, but is not easily seen as its greatest angular separation from the Sun is only 28.3°. Since Mercury is normally lost in the glare of the Sun, unless there is a solar eclipse it can be viewed only for short intervals before sunrise when it is near its maximum western elongation, or after sunset when near its maximum eastern elongation. At relatively high latitudes such as those of many European and North American population centres, it is even then near the horizon and obscured in a relatively bright twilit sky. However, at tropical and subtropical latitudes, Mercury is more easily seen because of two effects. (i) the Sun ascends above the horizon more steeply at sunrise and descends more steeply at sunset, so the twilight period is shorter, and (ii) at the right times of year, the Ecliptic intersects the horizon at a very steep angle, meaning that Mercury can be relatively high (altitude up to 28°) in a fully dark sky. Such conditions can pertain, for instance, after sunset near the Spring Equinox, in March/April for the southern USA and in September/October for South Africa and Australasia. Conversely, pre-sunrise viewing is easiest near the Autumn Equinox.
Comparatively little is known about Mercury; ground-based telescopes reveal only an illuminated crescent with limited detail. The first of two spacecraft to visit the planet was Mariner 10, which mapped about 45% of its surface from 1974 to 1975. The second is the MESSENGER spacecraft, which attained orbit around Mercury on March 17, 2011, to map the rest of the planet.
Mercury is similar in appearance to the Moon: it is heavily cratered with regions of smooth plains, has no natural satellites and no substantial atmosphere. Unlike the Moon, it has a large iron core, which generates a magnetic field about 1% as strong as that of the Earth. It is an exceptionally dense planet due to the large relative size of its core. Surface temperatures range from about 90 to 700 K (−183 °C to 427 °C), with the subsolar point being the hottest and the bottoms of craters near the poles being the coldest.
Recorded observations of Mercury date back to at least the first millennium BC. Before the 4th century BC, Greek astronomers believed the planet to be two separate objects: one visible only at sunrise, which they called Apollo; the other visible only at sunset, which they called Hermes. The English name for the planet comes from the Romans, who named it after the Roman god Mercury, which they equated with the Greek Hermes (Ἑρμῆς). The astronomical symbol for Mercury is a stylized version of Hermes' caduceus.