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There have been many ice ages during the course of the Earth's history, and research suggests that they wax and wane contingent upon the orientation of the Earth to the Sun. One of the factors regarding the strength and amount of solar radiation reaching the Earth is due to it's axial tilt -- currently 23.5 degrees. However, this tilt is not constant, varying from a maximum of 24 degrees to a minimum of 22.5 degrees. At the times of minimal tilt, about every 40,000 years, ice ages appear.
The cycle of ice ages is a bit more complicated by the fact that tilt is not the only criteria -- there's an approximate 23,000 year cycle of the wobble of the Earth, known as "precession," and the eccentricity of the orbit, which means at times the Earth on average is closer or further from the Sun, a cycle of about 100,000 years. These 3 factors seem to determine how severe or mild an ice age can be, if these factors reenforce or mitigate each other.
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