Precessionthe change in orientation of the Earth's rotational axisalters 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.
Milutin Milankovitch first theorized that precession and two of the Earth's other orbital parameterseccentricity and obliquityaffected climate.
Image and Animation by Robert Simmon, NASA GSFC