Axis direction · Wobble
How precession shifts the seasons
Precession changes where on the orbit each season falls. Earth’s axis slowly turns like the axis of a spinning top, and the orbit’s long axis turns too. Together they shift the seasons relative to Earth’s closest and farthest points.
Climate rhythms near 19,000 and 23,000 years
The axis
It traces a circle in space, like a top’s axis.
The ellipse
Its near point creeps around relative to the stars.
Summer timing
A summer near the Sun is stronger than one far from it.
Two turns that combine
The axis alone takes about 25,800 years to go once around. The ellipse turns as well, and the two together bring the seasons back to the same place on the orbit about every 21,000 years. The climate record shows peaks near 19,000 and 23,000 years.
Why timing changes the sunlight
Earth gets more sunlight when it is closer to the Sun. If northern summer falls at the near end of the orbit, midsummer sunlight gets a boost. Half a cycle later it falls at the far end and the boost becomes a penalty.
At today’s orbit, that swap moves midsummer sunlight at 65°N from 478 to about 510 W/m². At the most stretched orbit it moves it by 115 W/m². The two hemispheres swap in opposite directions: when northern summer is near the Sun, southern summer is far from it.
Why you hear 26,000 and 23,000
About 26,000 years is the axis turning on its own. For climate, what counts is when each season falls relative to the near and far points, and that gives the shorter rhythms.
Both numbers are right once you say which turn you mean.
Try it yourself
Change the orbit
Move all three motions and watch midsummer sunlight at 65°N change.
Questions
Quick answers
Is precession the same as tilt?
No. Tilt is the angle of the axis. Precession is the direction the tilted axis points.
Does it affect both hemispheres the same way?
It affects them in opposite ways. Northern and southern summer fall six months apart, on opposite sides of the orbit.