|Mercury is the closest planet to the sun and is the second smallest
of the nine planets. Because of its size, it is fainter than Venus, Mars,
Jupiter and Saturn, which can be seen with the naked eyes. It is best seen
in the Northern Hemisphere in the west at twilight in May, or just before
sunrise in the east in November. These are the times at which Mercury reaches
maximum elongation. But, even at maximum elongation, the planet is never
more 28° from the sun.
Calculated from Mercury's overall density, its core is thought to be 70 per cent composed of iron. Despite its composition of iron, the planet has a slow rotation and relatively weak magnetic field. The planet has a sidereal period of three months. This is the shortest of all the planets in the solar system. As such it has the appearance from Earth of moving faster than the others, a characteristics which led the Greeks to name it Hermes, after the messengers of the gods. The Romans in turn called it Mercury after their own deities' wing-footed messenger.