Fast-spinning young Earth pulled the moon into a yo-yo orbit

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The Earth and moon’s celestial dance was a lot wilder during the pair’s youth.

By simulating the early moon’s orbit, researchers have reconstructed what the moon’s phases would have looked like during the solar system’s early years. The result, presented online March 10 at arXiv.org, reveals a moon that alternated rapidly between its sunlit and shadowy sides and bounced like a ball toward and away from Earth.

Scientists believe that the moon assembled from debris left over from a collision between Earth and a Mars-sized object roughly 4.5 billion years ago. In a 2012 paper in Science, researchers proposed that the Earth spun rapidly during this period. They calculated that gravitational tug from this fast rotation pulled the young moon into an orbit 12 times as oblong as the current lunar orbit.

In the new work, Emil Noordeh of York University in Toronto and colleagues demonstrate that the short orbit of the 24,000-year-old moon produced a lunar cycle between one full moon and the next lasting around 1.5 days.

While spectacular, nothing was around to witness the early moon’s phases: The first life on Earth didn’t appear until hundreds of millions of years later.

PHASE CHANGE The lunar cycle has changed significantly since the moon formed roughly 4.5 billion years ago, as demonstrated in this simulation. On the left, a 24,000-year-old moon moves in a short, highly elliptical orbit. This oblong orbit results in a fast lunar cycle and a moon that bobs toward and away from Earth as the yellow outlines of constellations whip by. On the right, the present-day moon orbits in a relatively circular pattern about once every 27 days.

Credit: E. Noordeh and P. Hall