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7.29 The Discoveries of Uranus and Neptune

Human cultures have been aware of Jupiter and Saturn for thousands of years. These two planets even lend their names to two days of the week, chosen to commemorate the brightest moving objects in the sky. But Uranus and Neptune are so far away and move so slowly that they were not recognized as planets for a long time. Uranus can be seen with the naked eye from a clear, dark site. The earliest recorded observation was by Hipparcos in 128 B.C., who included it in his star catalog. John Flamsteed observed it half a dozen times in 1690 and Pierre Lemonier observed it a dozen times in in the mid 18th Century, but neither astronomy detected its motion so they both thought it was a star. Neptune is just too faint to be seen by the naked eye.

Jupiter, Saturn, Uranus, and Neptune shown to an approximate size scale. Click here for original source URL

A 1998 false-color near-infrared image of Uranus showing cloud bands, rings, and moons obtained by the Hubble Space Telescope's NICMOS camera. Click here for original source URL.

Johann Bode. Click here for original source URL.

William Herschel. Click here for original source URL.

Uranus wasn’t discovered until 1781, when the astronomer William Herschel accidentally detected the planet during an ambitious star-mapping project. Herschel thus became the first known human to recognize a new planet, which was quite a coup and made Herschel famous. Of course, his telescopecouldn’t reveal any details of the planet. Johann Bode (of Bode's rule) suggested that the planet be named for the Greek god Uranus. In Greek mythology, Uranus was the father of Saturn, who in turn was the father of Jupiter. Because of Uranus' lengthy 84-year period of revolution around the Sun, it has made only two orbital circuits since its discovery. Compare this to Mercury, aptly named after the fleet-footed messenger god, which zips around the Sun several times a year.

Neptune, next to earth for size comparison. Click here for original source URL.

The discovery of Uranus led to the discovery of Neptune. Astronomers were surprised to find that Uranus did not move in a perfectly elliptical orbit, as the laws of Kepler and Newton predicted. They recognized that the gravitational attractions of Jupiter, Saturn, and other planets would slightly disturb Uranus' orbit, but even those effects didn’t completely explain the problem. What could be wrong? Researchers in the 1800s hypothesized that the gravitational forces of still another planet, as yet undiscovered, could explain the motion of Uranus. They could even predict roughly where the unseen planet would be, based on the orbits. After several years of complex calculations by British and French mathematicians (all done by hand, of course, since there were no calculators or computers in those days), the new planet was discovered by French astronomers in 1846. With its pale blue color, it was named Neptune, after the god of the oceans. 

The existence and position of an unseen planet was successfully predicted by application of the laws of gravity. Recall that Newton's theory was originally affirmed in a similar way, when it was used to successfully predict the return of Halley’s Comet. This confirmed the power of the scientific method, and the extension of physical laws to apply throughout the Solar System and probably beyond