Every 202,500 Million Years, the Earth Wanders in a New Direction

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 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. The 405,000-year cycle, they found, has held uniform into the very distant past — 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. In core samples taken previously in two areas in the Northeast 2,000 miles away, the sedimentary layers clearly show climate cycles reflecting the 405,000-year cycle of orbital changes, but without the reliable uranium-lead dating of the Arizona sample.

So the scientists linked the climate cycles of the Northeast sediments with the accurate uranium-lead dates from the Arizona core. The timing matched: The variations in both showed that the 405,000-year cycle has been going on precisely as scientists had calculated.

What does knowing this mean? The lead author of the study, Dennis V. 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 dates of fossils, for example.

“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.”