Neptune was found with a pen before anyone saw it through a telescope
Uranus kept drifting off the path astronomers predicted for it, as if something unseen were tugging it. Two mathematicians worked out where that something should be. On 23 September 1846 an observer in Berlin pointed his telescope there and found a new planet within a degree of the forecast position.
Neptune is the only planet discovered by calculation rather than by scanning the sky. After Alexis Bouvard published tables of Uranus's orbit in 1821, later observations kept disagreeing with them, and he suspected an unknown body's gravity. In Britain John Couch Adams worked on the problem from 1843; in France Urbain Le Verrier produced his own prediction in 1845 and 1846, to little enthusiasm at home. Le Verrier wrote to Johann Galle at the Berlin Observatory, and the evening the letter arrived, Galle and the student Heinrich d'Arrest compared a fresh star chart with the sky and spotted the intruder. Its large moon Triton turned up shortly afterwards.
The British had been searching too. James Challis at Cambridge actually recorded Neptune twice in August without realising it, blaming out-of-date maps and his comet work. Galileo had plotted it even earlier, in 1612 and 1613, but took it for a star; it was barely moving at the time.
Credit and naming turned into a Franco-British quarrel. Le Verrier first claimed official approval for the name Neptune, then pushed to call the planet after himself, backed by the observatory director François Arago. Outside France that went nowhere, and after the astronomer Struve endorsed Neptune at the end of 1846, the sea god's name stuck. In Chinese, Japanese, Korean and Vietnamese it is the sea king star. Adams and Le Verrier were eventually given joint credit, though some historians have since questioned Adams's claim.
The planet itself is an ice giant 17 times Earth's mass, with no solid surface, taking 164.8 years to circle the Sun at about 4.5 billion kilometres. Despite the weak sunlight it has vigorous weather, including a Great Dark Spot seen when Voyager 2 flew past on 25 August 1989, still the only spacecraft to visit.
Source: Neptune
