Trouble in the Heavens

Astronomers determined that Uranus's orbit around the Sun took 84 years. But after tracking Uranus for several years, astronomers realized something was amiss with their calculations.

According to Newton's Law of Gravity, Uranus should move about the Sun in a certain orbit, whose ellipticity would be determined by its mass, distance from the Sun, and small gravitational effects of Jupiter and Saturn. In other words, Uranus should behave in a predictable manner, if it had any respect for science. It didn't, and scientists were duly perplexed. Was Newton's Law wrong?

In 1824, the German mathematician Friedrich Bessel suggested that the errors in Uranus's motion would be explained by the discovery of another planet farther out. Of course, some argued that Newton's Law was not universal, as previously advertised, but was only valid out to Saturn--beyond that it required modification.

This was deemed a rather compromising solution to an elegant law of physics, and by the end of the 1830s astronomers generally acknowledged that there was an undiscovered planet whose gravitational attraction caused Uranus's troubled motion.

Next: A Discovery and a Scandal

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