It happens every 405,000 years. The Earth’s orbit gradually changes shape from almost circular to slightly elliptical over a period of 202,500 years, and then starts returning to form over the next 202,500 years — like a metronome swinging side to side.

Right now, we are in an almost perfectly circular orbit around the sun, and soon — within some thousands of years, that is — we will start moving toward the elliptical.

This happens because of the Earth’s gravitational interactions with other planets, especially Jupiter and Venus — Jupiter because it is very large, and Venus because it is very near.

In new research published in the Proceedings of the National Academy of Sciences, scientists tracked the orbital cycle by analyzing a 1,700-foot-long rock core drilled in the Petrified Forest National Park in Arizona.