It happens every 405,000 years. The Earth’s orbit 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.

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 massive Jupiter and nearby Venus.

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. The 405,000-year cycle, they found, has held uniform back to at least 215 million years ago.

By comparing the amount of decay of uranium to that of lead trapped in zircon, the layers in the Arizona core can be dated quite accurately. The rock is 202 million to 253 million years old.

What does knowing this mean? Lead author Dennis Kent, a professor of earth and planetary sciences at Rutgers University, said that it will give scientists a much more accurate method of dating prehistorical events. “The dream is have a framework independent of the fossils that you can plug the fossils into and see more interesting things — the coexistence of disparate forms, or of similar forms widely separated in location. Now we can place things more accurately in time rather than depending on the fossils to tell us what the time is. And this is an interesting time. Dinosaurs and mammals first appeared 252 to 201 million years ago.”