Milankovitch Cycle

Milankovitch cycles. Source: UCAR

A Milankovitch cycle is a cyclical movement related to the Earth’s orbit around the Sun. There are three of them: eccentricity, axial tilt, and precession. According to the Milankovitch Theory, these three cycles combine to affect the amount of solar heat that’s incident on the Earth’s surface and subsequently influence climatic patterns.

Eccentricity

The path of the Earth’s orbit around the sun is not a perfect circle, but an ellipse. This elliptical shape changes from less elliptical (nearly a perfect circle) to more elliptical and back, and is due to the gravitational fields of neighboring planets (particularly the large ones – Jupiter and Saturn). The measure of the shape’s deviation from being a circle is called its eccentricity.

That is, the larger the eccentricity, the greater is its deviation from a circle. Thus, in terms of eccentricity, the Earth’s orbit undergoes a cyclical change from less eccentric to more eccentric and back. One complete cycle for this kind of variation lasts for about 100,000 years.

Axial Tilt

We know the earth is spinning around its own axis, which is the reason why we have night and day. However, this axis is not upright. Rather, it tilts at angles between 22.1-degrees and 24.5 degrees and back. These angles are measured between the angle of the axis to an imaginary line normal (perpendicular) to the Earth’s plane of orbit. A complete cycle for the axial tilt lasts for about 41,000 years.

Greater tilts mean that the hemispheres closer to the Sun, i.e., during summer, will experience a larger amount of heat than when the tilt is less. In other words, regions in the extreme upper and lower hemispheres will experience the hottest summers and the coldest winters during a maximum tilt.

Precession

Aside from the tilt, the axis also wobbles like a top. A complete wobble cycle is more or less 26,000 years. This motion is caused by tidal forces from the Sun and Moon.

Precession as well as tilting are the reasons why regions near and at the poles experience very long nights and very long days at certain times of the year. For example, in Norway, the Sun never completely descends beneath the horizon between late May to late July.

The Milankovitch Cycles are among the arguments fielded by detractors of the Global Warming concept. According to them, the Earth’s current warming is just a part of a series of cyclical events that take thousands of years to complete, and hence cannot be prevented.

You can read more about milankovitch cycle here in Universe Today. Here are the links:

• Solar Variability Most Likely Not the Cause of Global Warming
• Researchers Say Sun Cycle Alters Earth’s Climate

There’s more about it at USGS and NASA. Here are a couple of sources there:

• The Sun and the Climate (a pdf file)
• Milutin Milankovitch

Here are two episodes at Astronomy Cast that you might want to check out as well:

• The Sun, Spots and All
• Distance in Space, Changing Earth’s Orbit, and Different Sized Stars

References:
NASA Earth Observatory
NOAA Website