While we're all familiar with the axis of the earth pointing toward the North Star (Polaris) at an angle of 23.45° and that the earth is approximately 91-94 million miles from the sun, these facts are not absolute or constant. The interaction between the earth and sun, known as orbital variation, changes and has changed throughout the 4.6 billion year history of our planet.
Eccentricity is the change in the shape of the earth's orbit around the sun. Currently, our planet's orbit is almost a perfect circle. There is only about a 3% difference in distance between the time when we're closest to the sun (perihelion) and the time when we're farthest from the sun (aphelion). Perihelion occurs on January 3 and at that point, the earth is 91.4 million miles away from the sun. At aphelion, July 4, the earth is 94.5 million miles from the sun.
Over a 95,000 year cycle, the earth's orbit around the sun changes from a thin ellipse (oval) to a circle and back again. When the orbit around the sun is most elliptical, there is larger difference in the distance between the earth and sun at perihelion and aphelion. Though the current three million mile difference in distance doesn't change the amount of solar energy we receive much, a larger difference would modify the amount of solar energy received and would make perihelion a much warmer time of the year than aphelion.
On a 42,000 year cycle, the earth wobbles and the angle of the axis, with respect to the plane of revolution around the sun, varies between 22.1° and 24.5° . Less of an angle than our current 23.45° means less seasonal differences between the Northern and Southern Hemispheres while a greater angle means greater seasonal differences (i.e. a warmer summer and cooler winter).
12,000 years from now the Northern Hemisphere will experience summer in December and winter in June because the axis of the earth will be pointing at the star Vega instead of it's current alignment with the North Star or Polaris. This seasonal reversal won't happen suddenly but the seasons will gradually shift over thousands of years.
Astronomer Milutin Milankovitch developed the mathematical formulas upon which these orbital variations are based. He hypothesized that when some parts of the cyclic variations are combined and occur at the same time, they are responsible for major changes to the earth's climate (even ice ages). Milankovitch estimated climatic fluctuations over the last 450,000 years and described cold and warm periods. Though he did his work in the first half of the 20th century, Milankovich's results weren't proven until the 1970s.
A 1976 study, published in the journal Science examined deep-sea sediment cores and found that Milankovich's theory corresponded to periods of climate change. Indeed, ice ages had occurred when the earth was going through different stages of orbital variation.
For More Information
Read related articles in the Climate section of my site.
Hays, J.D. John Imbrie, and N.J. Shackleton. "Variations in the Earth's Orbit: Pacemaker of the Ice Ages." Science. Volume 194, Number 4270 (1976). 1121-1132.
Lutgens, Frederick K. and Edward J. Tarbuck. The Atmosphere: An Introduction to Meteorology.