The Discovery of Neptune

FAS Astronomers Blog, Volume 32, Number 8.

Neptune is the only planet to be discovered through mathematical analysis. The orbit of Uranus was determined to be perturbed by an unknown object farther out in the Solar System, which astronomers thought might possibly be a new planet. 

In 1792, Jean Delambre published tables of planetary positions. It was soon recognized that Uranus did not quite maintain the positions predicted by Delambre. Alexis Bouvanrd published updated tables in 1821. But, again, Uranus did not quite cooperate and deviated from the predictions. By the early 1830s astronomers started to recognize that there was a planet beyond the orbit of Uranus. Britain’s Astronomer Royal George Biddell Airy published a report referencing this “undiscovered planet” in 1832. 

Joseph Lalande, John Herschel, and Von Lamon all observed Neptune in the late 18^th and early 19^th century – without recognizing it as a planet. However, the discovery of Neptune came down to a mathematical analysis performed by two astronomers: Englishman John Couch Adams and Frenchman Urbain Le Verrier. 

Adams, after reading Airy’s report, began thinking about the possibility of locating a new planet back in 1841. He worked through the calculations over several years finishing in the September-October 1845 timeframe. He sent his results to James Challis, the director of the Cambridge Observatory. Challis, who was overworked at the time, suggested that Adams contact George Airy. Adams tried to visit Airy on three different occasions. Unable to gain a meeting, he left a note summarizing his calculations on October 21, 1845. Airy responded in a letter to Adams with some technical questions, however, Adams never got around to responding. 

Around the same time in the summer of 1845, Le Verrier was asked to pursue similar calculations by François Arago, the Director of the Paris Observatory. He completed his first set in November 1845 and formally announced his results on June 1, 1846. He further revised his calculations in August 1846.

Le Verrier’s calculations soon made their way to England. Airy, recognizing that Le Verrier’s predictions were similar to that of Adams, convinced Challis to begin a search for the new planet. In July 1846, Challis began a somewhat wide and slow plodding search during which he might have identified Neptune a couple of times without recognizing what it was. 

John Herschel, in August 1846, also passed Adam’s information on to William Dawes, a clergyman with a telescope. Dawes, rather than beginning the search, sent the information via a letter to William Lasser, who had a larger telescope capable of locating the new planet. Unfortunately, Lasser had a sprained ankle at the time and evidently lost the letter from Dawes. Therefore, he was unable to undertake the search. 

Adams continued to refine his calculations and sent an update to Airy on September 2, 1846. On September 10, John Herschel presented a talk on the hypothetical planet to the British Association in Southampton. Adams was to have done the same but arrive too late to give his talk.

In the meantime, Le Verrier was having difficulties finding someone to do the search. He eventually caught the interest of two astronomers in Germany who just happen to have updated but unpublish star maps. 

It was Johann Galle along with his student Heinrich Louis d’Arrest at the Berlin Observatory who made the discovery. On September 23, 1846 (and possibly into the early morning of September 24), Galle began searching the sky for the hypothetical planet. He called out the positions of stars while d’Arrest checked the charts. It didn’t take them long to find an object that was not on the map. On September 25, they communicated their finding to Le Verrier and on the same day, the Berlin Observatory announce that the new planet had been definitively identified. It was right where Le Verrier had predicted (within 1 degree). It turns out that Adam’s prediction wasn’t quite as accurate (12 degrees off). 

Once the discovery of Neptune was announced, the British recognized how close they had come to making the discovery themselves and they tried to take some of the credit. John Herschel published an article in October 1846 praising Le Verrier, but also crediting Adams for developing the same prediction. Challis found that he had observed Neptune three times during his 1846 search. In fact, on September 29, he had gone as far as discussing a disk like object he had observed with the Reverend W. T. Kingsley. However, before they could check the observation, clouds moved in, and the opportunity was missed.

It was George Airy that made the strongest case for Adams. Later in October 1846, Airy sent letters to Le Verrier, Challis and Adams to “set the record straight”. On November 13, 1846, Airy read an account of his receiving Adam’s 1845 summary at a meeting of the Royal Society. Based on Airy’s account, Adams was given credit, along with Le Verrier and Galle for the discovery of Neptune.  

However, there is still some controversy. In October 1998, several documents were discovered and returned to the Royal Greenwich Observatory archives. Included in the documents was the note Adams’ left for Airy. NicholaKollerstrom, on reviewing these as well as other documents, concluded that Adam’s predictions for the location of Neptune were somewhat underwhelming. Airy’s 1846 account might have embellished some of the facts or was selective in what he reported. And Adam’s did not come as nearly as close as Le Verrier to predicting the location of Neptune. 

I guess to simplify things, the USGS/IAU Working Group on Planetary Nomenclature, lists only Galle as the discoverer of Neptune. 

Just to make things more interesting, astronomer David Jamieson discovered, in 2009, that Galileo’s notebooks from January 1613 indicate that Galileo, in fact, discovered Neptune, although, he didn’t recognize it as a planet.