Rotation

The planets also rotate around invisible axes through their centres. A planet's rotation period is known as its day. Most of the planets in the Solar System rotate in the same direction as they orbit the Sun, which is counter-clockwise as seen from above the sun's north pole, the exceptions being Venus^[104] and Uranus^[105] which rotate clockwise, though Uranus's extreme axial tilt means there are differing conventions on which of its poles is "north", and therefore whether it is rotating clockwise or anti-clockwise.^[106] However regardless of which convention is used, Uranus has a retrograde rotation relative to its orbit. There is great variation in the length of day between the planets, with Venus taking 243 Earth days to rotate, and the gas giants only a few hours.^[107] The rotational periods of extrasolar planets are not known; however their proximity to their stars means that hot Jupiters are tidally locked (their orbits are in sync with their rotations). This means they only ever show one face to their stars, with one side in perpetual day, the other in perpetual night