Why should the Earth-Sun distance change over such a long time scale?
The Earth’s orbit changes on several time scales, each of which affect the intensity of radiation we observe at Earth. The first change is the winter and summer solstice, where the Sun-Earth distance varies between 91,400,000 miles and 94,400,000 miles. Now on longer time scales, the actual shape of the Earth’s orbit changes every 100,000 years, vacillating between more circular and more elliptical. In this case, when the Earth is closest to the Sun it actually receives 20-30% more sunlight. In addition, the Earth wobbles on its axis every 26,000 years, changing the time at which winter and summer occur. Finally, the tilt of the Earth varies every 40,000 years by about 2 arc degrees, which affects the temperature difference between winter and summer. These changes are termed the “Milankovitch theory” after the geophysicist who first proposed it and are believed to operate together to produce dramatic temperature variations on Earth. You might want to read more about the Milankovitch th
