Mercury is one of the planets that make up the Solar System. It is the closest planet to the Sun, orbiting around it at an average distance of 58 million kilometer (36 million miles). It takes only 88 Earth days to complete one orbit, which means that it keeps overtaking the Earth every 116 days. Astronomers call this lapse of time Mercury's synodic period. Mercury has no natural satellite, or moon, orbiting around it.
Because Mercury is so close to the Sun it can never be very far away from it in the sky as seen from Earth. Mercury can only be observed as a morning star rising just ahead of the Sun or as an evening star setting just after it. The planet always stays low down near the horizon, where the Earth's thick atmosphere serves to obscure much of its detail. Being closer to the Sun than the Earth is, Mercury shows changes of shape called phases, just as the Moon does. When close to the Earth, Mercury is either invisible or appears as a thin crescent, about 1/180 the apparent size of the Moon. When at its furthest, Mercury's full disk can be seen but by then it appears less than half as big.
Conditions on Mercury
Having an average diameter of 4,878 km (3,032 miles), Mercury is probably the smallest of the Solar System's nine planets. (Pluto may be smaller, but its size is still not known for certain.) In terms of size and mass, Mercury is hardly a planet at all; it is only just a little larger than the Earth's Moon. Mercury's mass is about 1/18 that of the Earth, but its density is 5.44 times that of water. Its gravitational pull is therefore quite strong-about 2/5 that of Earth, and in fact the same as that of Mars, a planet nearly 1½ times as big.
Mercury and Relativity
Because it lies so close to the Sun, Mercury's motion is disturbed by the Sun's enormous gravitational influence. This means that Mercury does not follow its expected path.
Facts About Mercury
Average Distance from the Sun: 58 million-km (36 million miles)
Length of Year: 87.97 Earth Days
Period of Axial Rotation: 58.65 Earth days
Diameter: 4,878 km (3,032 miles)
Mass: 0.055 (Earth=1)
Density: 5.44 (water=1)
Average Surface Gravity: 0.38 (Earth=1)
Surface Temperature Range: 400°C (750°F) (sunlit side) -173°C (-279°F) (dark side)
© 1988 Encyclopaedia Britannica, Inc.