Uranus proved to be an unruly planet. It would erratically speed up and then slow down. It refused to cooperate with Newton’s laws of gravity, which accurately predicted motion everywhere from objects here on Earth to the trajectories of planets in space.42 This anomaly led the French mathematician Urbain Le Verrier to speculate about the existence of another planet located beyond Saturn. This planet, Le Verrier surmised, might be tugging at Uranus and, depending on their respective locations, either pulling Uranus forward and speeding it up or pulling it back and slowing it down. Using only
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