Differences between observed and calculated positions of Uranus led researchers to propose another planet farther from the Sun. Observations in 1846 located Neptune.
Urbain Le Verrier and John Couch Adams both carried out relevant calculations, while Johann Galle and others made the observations. The discovery illustrates the partnership between theoretical prediction and telescope observations.
Discovery
Some of the earliest known telescopic observations ever, Galileo's drawings on 28 December 1612 and 27 January 1613 (New Style), contain plotted points that match what is now known to have been the positions of Neptune on those dates. Both times, Galileo seems to have mistaken Neptune for a fixed star when it appeared close—in conjunction—to Jupiter in the night sky. Hence, he is not credited with Neptune's discovery. At his first observation in December 1612, Neptune was almost stationary in the sky because it had just turned retrograde that day. This apparent backward motion is created when Earth's orbit takes it past an outer planet. Because Neptune was only beginning its yearly retrograde cycle, the motion of the planet was far too slight to be detected with Galileo's small telescope. In 2009, a study suggested that Galileo was at least aware that the "star" he had observed had moved relative to fixed stars.
In 1821, Alexis Bouvard published astronomical tables of the orbit of Uranus. Subsequent observations revealed substantial deviations from the tables, leading Bouvard to hypothesize that an unknown body was perturbing the orbit through gravitational interaction. In 1843, John Couch Adams began work on the orbit of Uranus using the data he had. He requested extra data from Sir George Airy, the Astronomer Royal, who supplied it in February 1844. Adams continued to work in 1845–1846 and produced several different estimates for the position of an undiscovered planet beyond Uranus.
Independently from Adams, Urbain Le Verrier developed his own calculations in 1845–1846 that pointed to an undiscovered planet, but aroused no enthusiasm among his compatriots. In June 1846, upon seeing Le Verrier's first published estimate of a suspected undiscovered planet's longitude and its similarity to Adams's estimate, Airy persuaded James Challis to search for it. Challis vainly scoured the sky throughout August and September. Challis had, in fact, observed Neptune a year before the planet's subsequent discoverer, Johann Gottfried Galle, and on two occasions, 4 and 12 August 1845. However, his out-of-date star maps and poor observing techniques meant that he failed to recognise the observations as such until he carried out later analysis. Challis was full of remorse but blamed his neglect on his maps and the fact that he was distracted by his concurrent work on comet observations.
Meanwhile, Le Verrier sent a letter and urged Berlin Observatory astronomer Galle to search with the observatory's refractor. Heinrich d'Arrest, a student at the observatory, suggested to Galle that they could compare a recently drawn chart of the sky in the region of Le Verrier's predicted location with the current sky to seek the displacement characteristic of a planet, as opposed to a fixed star. On the evening of 23 September 1846, the day Galle received the letter, he discovered Neptune just northeast of Iota Aquarii, 1° from the "five degrees east of Delta Capricorn" position Le Verrier had predicted it to be, about 12° from Adams's prediction, and on the border of Aquarius and Capricornus according to the modern IAU constellation boundaries.
In the wake of the discovery, there was a nationalistic rivalry between the French and the British over who deserved credit for the discovery. Eventually, an international consensus emerged that Le Verrier and Adams deserved joint credit. Since 1966, Dennis Rawlins has questioned the credibility of Adams's claim to co-discovery, and the issue was re-evaluated by historians with the return in 1998 of the "Neptune papers" (historical documents) to the Royal Observatory, Greenwich.