The rotational axis of the Earth is tilted from the vertical, but the angle of tilt—called the obliquity—also varies. This affects the way sunlight strikes the Earth. At present, the angle between the rotational axis and the plane of the ecliptic is about 23.45°, but over a period of about 42,000 years the obliquity changes from 22.1° to 24.5° and back again.
The change is significant because it is the Earth's obliquity that produces our seasons in the first place. If the rotational axis were perpendicular to the plane of the ecliptic (obliquity = 0°) there would be no seasons. As the diagram shows, increasing the obliquity has the effect of increasing
Hipparchus and the precession of the equinoxes
the amount of solar radiation falling over high latitudes in summer. Consequently, summers are warmer, but winters are cooler. In other words, the seasons become more extreme. Milankovitch also studied the cycle of changing obliquity and its effects.
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