Neptune is not visible to the unaided eye and is the only planet in the Solar System found by mathematical prediction rather than by empirical observation. Unexpected changes in the orbit of Uranus led Alexis Bouvard to deduce that its orbit was subject to gravitational perturbation by an unknown planet. The position of Neptune was subsequently calculated from Bouvard’s observations, independently, by John Couch Adams and Urbain Le Verrier after his death. Neptune was subsequently observed with a telescope on September 23rd, 1846^[1] by Johann Galle within a degree of the position predicted by Le Verrier. Its largest moon, Triton, was discovered shortly thereafter, though none of the planet’s remaining 13 known moons were located telescopically until the 20th century.

The planet’s distance from Earth gives it a very small apparent size, making it challenging to study with Earth-based telescopes.

Some of the earliest recorded observations ever made through a telescope, Galileo’s drawings on December 28th, 1612 and January 27th, 1613 contain plotted points that match up with what is now known to be the position of Neptune. On both occasions, Galileo seems to have mistaken Neptune for a fixed star when it appeared close—in conjunction—to Jupiter in the night sky;^[2] 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.^[3] In 2009, a study suggested that Galileo was at least aware that the “star” he had observed had moved relative to the fixed stars.^[4]

In 1845 to 1846, Urbain Le Verrier, independently of Adams, developed his own calculations but aroused no enthusiasm in his compatriots. In June 1846, upon seeing Le Verrier’s first published estimate of the planet’s longitude and its similarity to Adams’s estimate, Airy persuaded James Challis to search for the planet. Challis vainly scoured the sky throughout August and September.^[5]

Meanwhile, Le Verrier by letter urged Berlin Observatory astronomer Johann Gottfried 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 September 23rd, 1846, the day Galle received the letter, he discovered Neptune just northeast of Phi Aquarii, 1° from where Le Verrier had predicted it to be, about 12° from Adams’ prediction, and on the border of Aquarius and Capricornus according to the modern IAU constellation boundaries. Challis later realised that he had observed the planet twice, on 4 and 12 August, but did not recognise it as a planet because he lacked an up-to-date star map and was distracted by his concurrent work on comet observations.[5]

In the wake of the discovery, there was a heated 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.^[6] After reviewing the documents, they suggest that “Adams does not deserve equal credit with Le Verrier for the discovery of Neptune. That credit belongs only to the person who succeeded both in predicting the planet’s place and in convincing astronomers to search for it.”^[7]

Check out the Planetary Bodies Category for similar articles on the planets of solar system!

Sources And Further Reading

[1] =  Hamilton, Calvin J. (4 August 2001). “Neptune”. Views of the Solar System. Archived from the original on 15 July 2007. Retrieved 13 August 2007.
[2] = Hirschfeld, Alan (2001). Parallax: The Race to Measure the Cosmos. New York, New York: Henry Holt. ISBN 978-0-8050-7133-7.
[3] = Littmann, Mark; Standish, E.M. (2004). Planets Beyond: Discovering the Outer Solar System. Courier Dover Publications. ISBN 978-0-486-43602-9.
[4] = Britt, Robert Roy (2009). “Galileo discovered Neptune, new theory claims”. NBC News News.
[5] = Airy, G.B. (13 November 1846). “Account of some circumstances historically connected with the discovery of the planet exterior to Uranus”. Monthly Notices of the Royal Astronomical Society. 7 (10): 121–44. Bibcode:1846MNRAS…7..121A. doi:10.1002/asna.18470251002.
[6] = Kollerstrom, Nick (2001). “Neptune’s Discovery. The British Case for Co-Prediction”. University College London. Archived from the original on 11 November 2005. 
[7] = William Sheehan; Nicholas Kollerstrom; Craig B. Waff (December 2004). “The Case of the Pilfered Planet – Did the British steal Neptune?”. Scientific American. Archived from the original on 19 March 2011.