What if Earth shared its orbit with another planet?


Nov 15, 2020
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So this article is a bit misleading in the third sentence: "Instead of both worlds moving in a circle around a star ...." The horseshoe orbits come about because the reference frame is rotating. In a non-rotating frame, Janus and Epimetheus both orbit with close to circular orbits, but at different speeds and one closer to Saturn than the other. [Think about riding a carousel and watching two people race around it - one going slightly faster than the carousel and one slightly slower.] When the two moons get close to each other their mutual gravitational causes one to speed up and one to slow down and the moons exchange as to which is in the inner orbit and which the outer. (Think speed skaters in the Olympics when they exchange inner and outer tracks in the crossing straight. [Or roller derby for launching a teammate forward.] ) Thus from a rotating frame of reference it appears to be a horseshoe pattern when they are still traveling in roughly circular orbits.

Note that 3753 Cruithne has a resonant orbit with the earth around the sun. Its path in the sky from earth appears to be a horseshoe orbit. [It is not the same as Janus and Epimetheus, but has some similarities and is still quite interesting.]
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