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A planet whose rotation axis were perpendicular to the orbital plane would have an axial tilt of 0°. In our solar sytem, the orbital plane that contains the earth is known as the plane of the ecliptic.
Axial tilt is mainly responsible for the seasons. During part of the year one hemisphere (or the other) is tilted toward the Sun, resulting in longer days and shorter nights; during the rest of the year the hemisphere is tilted away from the sun. Not only does axial tilt cause the hours of sunlight to vary but it can also result in sunlight striking the ground at a more ( summer) or less ( winter) perpendicular angle.
The Earth's axial tilt varies between 21.5° and 24.5° with a 41,000 year periodicity, while the direction of the tilt gradually undergoes precessionPrecession (also called gyroscopic precession is the phenomenon by which the axis of a spinning object (e. a part of a gyroscope) "wobbles" when a torque is applied to it. The phenomenon is commonly seen in a spinning toy top, but all rotating objects can, moving in a slow circle over a period of about 25,800 years. However, other factors may change the axial tilt of Earth (and of other planets).
Through time, axial precession changes the position of the Earth in its orbit at which the seasons occur ( Precession of the equinoxes). This has little effect on the amount of solar influx ( insolationInsolation is the incoming solar radiation that reaches a planet and its atmosphere. Also see diffuse insolation and direct insolation. External link .) during times when the orbit is circular, but can have large effects on the strength of the seasons when the Earth's orbit is highly elliptical (see Milankovitch_cyclesMilankovitch cycles is the name given to the collective effect of changes in the Earth's movements upon its climate. The eccentricity, axial tilt, and precession of the Earth's orbit vary in several patterns, resulting in 100,000 year ice age cycles of th). The effects of axial precession on seasons can be seen in the following chart.
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