The discovery of Neptune was the greatest advance in nineteenth century's Astronomy and one of the greatest achievements of human thinking.
On March 13th 1781, the British astronomer of German extraction, William Herschel, discovered the first new planet, unknown from ancient times. He named it Georgium sidus (George's star), in honor of king George the third. The planet's name was later changed to Uranus.
Portrait of Sir William Herschel. Royal Astronomical Society (by courtesy of SEDS).
Forty years later, it was noticed that Uranus had not moved exactly as predicted by Newton's Mechanics: its real position differed from its theoretically computed position in one to two arc minutes, about fifteen times less than the visible diameter of the Moon. But even such a small error was not acceptable.
A photograph of Uranus (by courtesy of NASA).
The French astronomer Alexis Bouvard used the Newton equations to calculate the movement of Uranus around the Sun, adding the disturbances induced by the presence of the giant planets, Jupiter and Saturn. The remaining planets then known (Mercury, Venus, the Earth and Mars) are too far away and too small for their effect on Uranus to be noticeable. The anomalies in the orbit of Uranus had two possible explanations:
A photograph of John Couch Adams (by courtesy of SEDS).
At the same time, the French astronomer Urbain Jean Joseph Le Verrier had had the same idea. His calculations were finished two months later than those of Adams, and he published them at the end of 1845. In 1846 he sent his results to the German astronomer Johann Gottfried Galle, at the Observatory of Berlin. After only one hour of search, Galle found the new planet one grade apart from the location pointed out by Le Verrier.
Portrait of Le Verrier (by courtesy of SEDS).
The discovery caused a sensation. For the first time, a planet had been found by simple reasoning. Newton's Mechanics was established. Adams received a belated recognition for his achievement, although Le Verrier took the glory, for he had published. In 1861, John Couch Adams was appointed director of Cambridge Observatory, the same place held by the man who took no notice of him.
A photograph of Neptune (by courtesy of NASA).
The following simulations show what would have been the position of Uranus in the absence of Neptune, and how the presence of Neptune makes it different. You can start each simulation by pressing the Continue button to see there is a slight difference in Uranus' position after 30 years.
Last modified 21/12/99 by Juan de Lara ( Juan.Lara@ii.uam.es, http://www.ii.uam.es/~jlara) need help for using this courses?.