Twin moons in the same orbit

Two moons of Saturn Janus and Epimetheus are almost the exact same size and are in almost the exact same orbit. Their orbitsĀ are only about 50 kilometers apart and as one catches up to the other one the gravitation effects cause them to switch places in their orbits. Because of this switching they never actually get very close to each other. These orbits are called horseshoe orbits.

(The Great Courses – Black Holes, Tides, and Curved Spacetime: Understanding Gravity)

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