Mercury is the closest planet to the Sun. With a diameter of 4880 km, it is the second smallest. It orbits the Sun in a markedly elliptical orbit (with eccentricity of 0.21) in 88 days. Its distance from the Sun varies between 0.31 and 0.47 Astronomical Units (1 au is the mean distance of the Earth from the Sun). Mercury has a rotation period of 58.6 days which is two thirds of its orbital period.
Most of our knowledge of Mercury's surface was determined during the close approaches of Mariner 10 in 1974/5. The surface looks very like that of the Moon, with many craters of very different sizes and lava plains called maria. There are also shallow cliff-like structures, which are not seen on the Moon, believed to result from wrinkling of the surface as the planet cooled and shrank. The largest craters on Mercury are well preserved and, as they are probably about 3 or 4 billion years old, indicate that there has been no migration of plates (like we see on the Earth) since then.
Mercury has virtually no atmosphere and as it is very close to the Sun has very high mid-day temperatures, approaching 450C, and very cold night temperatures, -180C.
Mercury has a small magnetic field and probably has a large nickel-iron core.
Mercury has no seasons, as the Earth and Mars have; instead it has a seasonal variation with longitude on the planets surface. This is due to the coupling between the rotational and orbital periods. The longitudes near 0 and 180 receive two and a half times as much radiation overall as do those near 90 and 270.
Because Mercury's orbit is well inside that of the Earth, it is never seen far from the Sun in the sky. It can only be seen with the naked-eye when it is at what are called its Greatest Elongations (when it is furthest from the Sun in the sky). Even then it is difficult to see from the UK and a clear horizon view is needed. At best it can be glimpsed as a star-like object low down in the evening or morning sky just after sunset, or before sunrise.
In a small telescope Mercury can be seen (even in the day-time) to have a small disc between 5 and 15 arcseconds across. The disc shows phases, like the Moon's with full at Superior Conjunction (when Mercury is at its furthest from the Earth, behind the Sun) and new at Inferior Conjunction (when it is between the Earth and the Sun).
