Precession—When
is Equinox?
or Why the Earth’s Orbit is a Daisy
Precession—When
is Equinox?
or Why the Earth’s Orbit is a Daisy
The third aspect of the Milankovitch Cycles is the precession of the equinoxes, which refers to perihelion moving later in the year. In 1250, perihelion coincided with the winter solstice (the point in time when the northern hemisphere tilts directly away from the Earth). Since then, perihelion has been moving, causing the seasons to shift forward. Now perihelion occurs twelve days after solstice, which means that the Earth is closest to the sun twelve days after its pole tilts directly away from the sun. |
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Over a period of 19,000 to 23,000 years (it varies because the gravitational force of Jupiter distorts the expected period of 25,780 years), perihelion will move from December 21 all the way around back to December 21. Evidence of this change can actually be noticeable over the life of a long lived person. As a person in the northern hemisphere ages, spring will begin to seem cooler and summers will end later. When perihelion coincides with the summer solstice 12,000 years from now, summers in the northern hemisphere will be hotter than they are now. |
image from Wicander, Historical
Geology, p 473. |
Since perihelion precesses, the Earth’s orbit is not a closed ellipse. Instead, it rotates about one focus drawing a path around the sun that looks like a rosette or a daisy. |
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