The Earth's Changing Orbit
On June 21, 1987, at a little past 4 in the afternoon by most Alaskan clocks, the earth's North Pole will be pointed as nearly directly at the sun as is possible, given its angle with the plane of the earth's orbit. The sun will be at its highest in the sky, the days at their longest, the incoming solar energy at its greatest. Clouds willing, Alaskans will be enjoying warm summer weather, in spite of the fact that less than two weeks later, late in the afternoon of July 3, the earth will be at its farthest from the sun for the year. In effect, the earth's distance from the sun and the progress of the seasons are working against each other in the Northern Hemisphere -- but this is not always the case.
If the earth and the sun were the only bodies in the solar system, the earth's orbit would not change. However, the moon and the other planets are constantly pulling on the earth. The result is that the angle between the earth's axis and the plane of its orbit, the season at which the earth is closest to the sun, and the degree to which the earth's orbit is elongated all vary slowly over time. Right now, the elongation of the orbit (the eccentricity) is rather small -- about 1.7%. This results in the sun being about 7% brighter at the earth when it is closest, on January 4, than when it is most distant.
Twelve thousand years ago, when the glaciers of the last great ice age were melting, the eccentricity was a bit higher, about 2%. At the same time, the earth was closest to the sun in June, and most distant in January, so the sun was about 7 % brighter in June than it is today. At various times in the last million years, however, the eccentricity has been much higher -- as much as 6%, which would make the sun almost 25% brighter at perihelion (when the earth is closest to the sun) than at aphelion (when the earth is farthest from the sun). During the interglacial before the last ice age, about 125,000 years ago, the eccentricity was about 4%. The times of largest eccentricity tend to be about 100,000 years apart.
Changes in the season when the earth is closest to the sun are faster. It takes about 23,000 years for the season of perihelion to move through the year, though the exact period depends on the eccentricity.
The angle between the earth's equator and the plane of its orbit, the inclination, also changes with time. Right now it is about average, at 23 degrees 27 minutes. Ten thousand years ago it was 24 degrees 15 minutes. The Arctic Circle was at 65 degrees 45 minutes N, and the midnight sun was visible to the earliest Alaskans from Cleary Summit, just north of Fairbanks. When the last great ice sheets were starting to form, 28,000 years ago, the
inclination was only 22 degrees 11 minutes.
Is it just a coincidence that ice sheets melted when the inclination and eccentricity were high and perihelion occurred in summer? A Serbian scientist of the 1920s, Milutin Milankovitch, thought not, and worked out a theory connecting the global climate with changes in the eccentricity, the inclination, and the season of perihelion. For many years his theory was ignored in this country, but since new dating methods and ocean cores have improved our knowledge of climatic history, it has turned out that the main periods of climatic change -- 23,000 years, 41,000 years and about 100,000 years -- match very closely the periods of the changes in the earth's orbital elements.
There are still questions about exactly how changes in the earth's orbit affect the climate, and especially in why the climatic changes match in the Northern and Southern Hemispheres. Nevertheless, there is now widespread belief among scientists that the Milankovitch variations, as they are often called, do indeed play a key role in climatic change.