Wobble of Earth's Axis
The third orbital change that Milankovich studied is called precession,
the cyclical wobble of Earth's axis in a circle. The motion is like
a spinning top when it is about to fall over. One complete cycle
for Earth takes about 26,000 years.
By itself, the wobble
of Earth's axis does not directly cause temperature changes like
orbital shape and tilt. But it changes the portion of the orbit
at which a given season occurs - that is, it changes when a particular
season will occur.
Tilt At Present Time (Now)
Earth's Tilt 13,000 years
In the upper part of
the diagram above labeled "Now," Earth's axis is tilted
so that summer occurs in the northern hemisphere on the left side
of the Sun and winter on the right. In the lower part labeled "In
13,000 years," the axis has moved to where summer in the northern
hemisphere occurs on the right side of the Sun and winter on the
So what! You say. If
Earth's orbit is nearly circular, then it makes no real difference.
But if Earth's orbit is slightly elongate, and perihelion occurs
on the right side of the Sun (as shown), then Earth is slightly
farther from the Sun on the left side. In this case, northern summer
are slightly cooler and northern winters slightly warmer on the
"Now" part of the diagram. But in 13,000 years, the axis
will have shifted so that summer in the northern hemisphere will
occur on the right side of the Sun where Earth is closer, so summers
will be hotter, and winters will be colder (and longer).
This is important because
glaciers need land on which to form. Most of the land surface on
Earth is now in the northern hemisphere, so when the wobble has
moved Earth's axis so that northern winters occur on the cooler
part of the orbit, glaciers will tend to grow.