Robert Simmon, NASA GSFC
Precession—the change in orientation of the Earth's rotational axis—alters the orientation of the Earth with respect to perihelion and aphelion. If a hemisphere is pointed towards the sun at perihelion, that hemisphere will be pointing away at aphelion, and the difference in seasons will be more extreme. This seasonal effect is reversed for the opposite hemisphere. Currently, northern summer occurs near aphelion.
These animations were taken from the Earth Observatory article On the Shoulders of Giants: Milutin Milankovitch.
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