Rotation
The planets also rotate around invisible axes through their centers. A planet's rotation period is known as its day. All planets in the Solar System rotate in a counter-clockwise direction, except for Venus, which rotates clockwise (Uranus is generally said to be rotating clockwise as well though because of its extreme axial tilt, it can be said to be rotating either clockwise or anti-clockwise, depending on whether one states it to be inclined 82° from the ecliptic in one direction, or 98° in the opposite direction). 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. The rotational periods of extrasolar planets are not known; their close proximity to their stars means that hot Jupiters are tidelocked; 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.
