Ice-Ages

• About every 41,000 years, the earth's spin axis -- its tilt -- varies from about 21.5 degrees to about 24.5 degrees. Currently, it is 23.5 degrees. The more the tilt is the greater the fluctuations in temperatures are from winter to summer. Summers would be hotter while winters would be colder. Since the length of this cycle does not match the 100,000 year glacial cycle, it is assumed this can not be the primary cause of the ice ages.

graphic of earth's tilt

• Over a 100,000 year cycle -- which does match the glacial cycle -- the earth's orbit around the sun changes. It varies from being roughly circular to much more elliptical. Over a 100,000 cycle the earth's orbit cycles between these extremes, stretching more into an ellipse and the becoming more circular. On first analysis, scientists have determined the highly elliptical orbit creates more dramatic changes in the seasons when this cycle combines with changes in the earth's spin axis. The seasonal effects can be dramatic -- especially for whichever hemisphere is tilted more towards the sun in the summer -- when earth is closest to the sun in its elliptical orbit. (The same hemisphere will then be furthest from the sun and tilting away in the winter.) This cycle, I believe, is partly misunderstood and will be explained more in depth shortly.
• The third cycle is the earth's precession, or wobble. Just as a top can wobble as it spins, the earth's tilt wobbles. This precession goes through a 23,000 year cycle. Therefore, as the earth's tilt goes through its 41,000 cycle, the wobble goes through its 23,000 year cycle, therefore this combination changes how the earth's tilt is pointing over the 100,000 year cycle. (When all three combine in the worst possible way the seasonal changes in temperatures would be the most dramatic.)