[migo (OP)]

I was reading a nice article in Wiki about Milankovitch cycles ( newbielink:http://en.wikipedia.org/wiki/Milankovitch_cycles [nonactive]), and one question is still puzzling me: is the average distance of Earth from the Sun taken over 1 year period constant, or changing over time?

Thanks!

[evan_au]

There are several different Milankovitch cycles, of different lengths.
They all transfer angular momentum between the planets of the solar system.
One of these cycles changes the eccentricity of the Earth's orbit, which changes the average distance from the Sun.
But since the Sun's intensity varies as the inverse square of the distance, the difference in temperature between the closest (perihelion) and farthest (aphelian) points on the orbit changes more than you would expect from the change in average distance: about 23% difference when Earth's orbit is at its most elliptical.