The 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.

Earth's Tilt At Present Time (Now)

Earth's Tilt 13,000 years from Now 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 left. 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. Back