It's not hard to see how an ice age is caused by a temperature drop, creating summers cool enough that the previous winter's snow never melts. Several seasons' snows accumulate and compact to form glaciers. But what turns down the thermostat? The cold facts have been hotly debated, but the theory most widely accepted - the "astronomical" theory - states that three periodic changes in the earth's position relative to the sun seem to have launched ice ages by influencing the amount of solar radiation the earth receives.
Because of the gravitational pull of the sun and moon on the equator, the earth wobbles on its axis like a toy top slowing down. Every 22,000 years or so, it describes a circle in space. The axis also tilts, causing the seasons. When the North Pole tips away from the sun, it's winter in the Northern Hemisphere. Today, the angle of tilt is 23 ½ degrees but every 41,000 years it moves from 22 to 24 degrees and back again. Perhaps the most important cycle is a change in the shape of the earth's orbit - from nearly circular to highly elliptical and back to circular - every 100,000 years due to the gravitational tug of fellow planets. The combined effect of these three cycles is to place the earth farther away from the sun at certain times, cooling the planet into an ice age.
(‘An Incomplete Education’, by Judy Jones and William Wilson)