Mercury - Second smallest nearest to sun

General Information

Average distance from the Sun

58 million km
(36 million miles)



Orbiting Speed

30 Miles / second


85 pond kid would weigh
32 pond

Length of year

88 Earth days

Length of solar day

156 Earth days

Rotation period relative to stars

58-65 Earth days

Diameter at equator

4,870 km
(3,030 miles)

Approximate surface temperatures

equator, midday:
420 0C (800 0F);
midnight: -180 0C
(-300 0F)

A day on Mercury (sunrise to sunrise) is almost two Mercurian years long. During a day
Mercury moves nearer the Sun, so the Sun seems to increase in size, and then shrink again.

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.