Mercury

 Hurtling round the Sun at more than 50 km per second, speedy Mercury is the smallest of the solar system's planets - a tiny, searing world ever so slightly larger than Earth's moon. 

The little planet takes a mere 88 (Earth) days to orbit the Sun, but 59 days to rotate on its axis - exactly two-thirds of its year. Astronomers used to think that the planet was "tidally locked", just like the Moon around Earth, presenting only one face to the Sun. However, this configuration of day and orbit reduces the tidal forces on the planet almost as much. Mercury's orbit is strange in other ways - it is more elliptical (stretched) than that of any other true planet's and is also tilted from the plane of the solar system at an angle of eight degrees. The long end of the ellipse "wobbles" slowly around the Sun, partly due to the gravity of the other planets, but also due to the way that the huge mass of the Sun itself distorts space around it. 

Before Einstein discovered general relativity, some astronomers tried to explain Mercury's "wobble" as the work of an even smaller planet closer to the Sun. The combination of long day, short year, high speed and elliptical orbit can cause some very strange effects on the surface.