Mercury - Second smallest nearest to sun
day on Mercury (sunrise to sunrise) is almost two Mercurian years long. During a
Mercury is the second smallest planet after Pluto, which is half its size. It is the planet nearest the Sun, orbiting faster than any other. A complete journey round the Sun takes Mercury just 88 days at an average speed of about 172,000 km/h (107,000 mph). Earth travels at about two thirds this speed and Pluto at only about one tenth.
Until 1974 almost nothing was known about the surface of Mercury. Whenever it is visible from Earth it is never far from the Sun, low down in the sky. That makes it very difficult to get a good view of Mercury in a telescope.
Astronomers think Mercury has an iron core accounting for about 70 per cent of its mass and 75 per cent of its diameter. This core is a little bigger than our Moon. The mantle surrounding the core is thought to be a layer of rock about 640 km thick.
A solar day on Mercury, from sunrise to sunrise, lasts about six Earth months
During a day on Mercury, Mercury's distance from the Sun varies between 70 million km (435 million miles) and 56 million km (35 million miles). An observer on Mercury would see the Sun increase in size and then decrease again. At midday on Mercury, with the Sun overhead, the temperature reaches around 420 0C. Three Earth months after sunrise, the Sun sets and the cold night begins. With no atmosphere to act like a blanket and hold in the warmth, the temperature plunges to -180 0C.