Axial tilt
Planets also have varying degrees of axial tilt, they lie at an angle to the plane of the their stars' equators. This causes the amount of light received by each hemisphere to vary over the course of its year, when the northern hemisphere points away from its star, the southern hemisphere points towards it, and vice versa.
Each planet has seasons, changes to the climate over the course of its year. The point at which each hemisphere is farthest or nearest from its star is known as its solstice. Each planet has two in the course of its orbit, when one hemisphere has its summer solstice, when its day is longest, the other has its winter solstice, when its day is shortest. Jupiter's axial tilt is very small, so its seasonal variation is small, Uranus, has an axial tilt that is so extreme it is virtually on its side, which means that its hemispheres are either in sunlight or in darkness around the time of its solstices. Among extrasolar planets, axial tilts are not known for certain, though most hot Jupiters are believed to possess no axial tilt, as a result of their proximity to their stars.
