Before the first space-crafts flew to Mercury to take pictures of its surface, some astronomers believed that the planet would have a smooth surface. When Mariner 10 took pictures of Mercury during its flyby of Venus and Mercury in November 1973, scientists discovered that Mercury is the most cratered planet in the Solar System with a surface similar to Earth's Moon.

Mercury has no atmosphere at all. Sunlight reflects off its surface, similar to how light is reflected directly off the Moon's surface. The lack of atmosphere means that sunlight cannot be spread through the atmosphere. The planet's sky is dark, just like the Moon's and, if you were able to stand on the side of the planet not facing the Sun, you would be able to see billions of stars in the sky.

A year on Mercury is only 88 days long, but a full day (from sunset to sunrise) takes 176 days, even though the planet takes 58 days to rotate on its axis. This causes the Sun to rise above the horizon at the beginning of a Mercurian day in the East (as on Earth), then move higher into the sky, move back down towards the horizon at the East, and then quickly move over to the Western horizon to set, after about 60 days!

Mercury (4,878km wide) is slightly bigger than Earth's Moon (3475.6km wide) and is smaller than Ganymede (5,270km) and Callisto (4,890km), two of Jupiter's moons, and Titan (5,150km wide), a moon of Saturn.

Mercury can only be seen just after the Sun has risen and just before the sun sets.

If you were to stand on Mercury, the Sun would appear two and a half times bigger than it appears on Earth.