The seventh planet from the Sun with the third largest diameter in our Solar System, Uranus is very cold and windy. The ice giant is surrounded by 13 faint rings and 27 small moons. One day on Uranus takes about 17 hours (the time it takes for Uranus to rotate or spin once). Uranus makes a complete orbit around the Sun (a year in Uranian time) in about 84 Earth years. Uranus is the only planet whose equator is nearly at a right angle to its orbit, with a tilt of 97.77 degrees. This unique tilt is possibly the result of a collision with an Earth-sized object long ago, and causes the most extreme seasons in the Solar System. For nearly a quarter of each Uranian year, the Sun shines directly over each pole, plunging the other half of the planet into a 21-year-long dark winter. Uranus is one of two ice giants and the second least dense planet in the Solar System.