The positions of the planet Neptune on 23 September 1846, 2010 and 2011 in relation to selected stars in the constellations of Capricornus and Aquarius. Drawing Nick Lomb
The planet Neptune is now regarded as the outermost planet of the solar system. It was discovered in somewhat controversial circumstances by the German astronomer Johann Gottfried Galle on 23 September 1846 at Berlin Observatory.
What was the controversy about? Galle did nothing controversial – he just searched the sky for a predicted new planet at a position sent to him by the director of Paris Observatory – a position calculated by the French scientist Urbain Le Verrier. This was all brilliant work, but the problem that arose was that an Englishman John Couch Adams had also made a similar prediction for the location of the new planet and the English scientists wanted Adams to share some of the credit for the discovery.
According to the story of Adams’ supporters he wanted to give the position that he had calculated to the Astronomer Royal at Greenwich Observatory, but due to the indifference of the Astronomer Royal George Airy no search was made for the planet. Recent historical research, helped by the rediscovery of files “borrowed” from the archives by an ex-director of Mt Stromlo Observatory Olin Eggen, suggests that Adams is due little credit as, unlike Le Verrier, he had not calculated the path of the predicted planet around the Sun.
How did Le Verrier and Adams know that there was an unseen planet in the sky? They knew that the planet Uranus discovered by William Herschel in 1781 was not quite following the path predicted by Newton’s Law of Gravity. Sometimes the planet was moving a little too fast and sometimes a little too slowly. A simple and correct explanation was provided by the gravitational pull of an unseen planet.
The planet takes 164.79 years to circle the Sun. How do we know the period so accurately? After the discovery of the planet, researchers found a number of pre-discovery observations made by people who had observed Neptune without realising that they were looking at a planet. The great Italian scientist Galileo observed Neptune at least twice, on 28 December 1612 and on 27 January 1613, without realising that he was seeing a planet, probably because his tiny telescope did not allow him to make out the disc of the planet.
A full period from the discovery date of 23 September 1846 takes us to the middle of 2011. However, as can be seen from the diagram above Neptune is already close to where it as first observed, a position in the vicinity of the star Deneb Algedi.