THE Greeks and the ancient Romans were aware of only five planets in the solar system in addition to their own. A seventh was added to the list in 1781 when William, Herschel discovered Uranus, but when its orbit was analysed, it was noticed that it was not behaving exactly as it should there had to be yet another planet somewhere, tugging at Uranus and causing irregularities in its orbits around the sun.
By clever analysis of these observed peculiarities, two astronomers arrived independently, and almost simultaneously, at predicted positions for the suspected eighth planet. The Englishman, John Couch Adams was the first to complete his calculations, but when he sent his results to the Astronomer Royal at Greenwich, no one took much notice. Across the Channel, however, Urbain Jean Joseph Leverrier was more persistent.
In June, 1846, Leverrier completed a brilliant mathematical analysis of the problem, and faced with an apathy among his fellow countrymen similar to that encountered by Adams, he communicated his findings to a German colleague in Berlin. He told Johann Galle exactly where to look, and on the very first night of the search, Galle and his assistants found the new planet exactly where Leverrier predicted. The discovery took place 150 years ago today, on September 23rd, 1846.
Because it appeared to have a slightly bluish tinge, this latest addition to the known solar system was called Neptune, after the Roman god of the sea. But the choice of name was not unanimous. The French, perhaps to make amends for their initial lack of enthusiasm, wanted to call the new planet "Leverrier" - Their English colleagues, however, did not agree: "Mythology is neutral ground," said one of them. "And besides, just think how awkward it would be if the next planet should be discovered by a German, by a Gugge, a Funk, or by our hirsute friend, young Boguslawski."
Although neither of them had the honour of having the new planet named after him, Adams and Leverrier have traditionally been given joint credit for its discovery. But there is a distinct possibility - that more than 200 years before, Galileo Galilei may have been the first to document the planet. In 1980, historians examining drawings by Galileo of the moons of Jupiter, noticed that in January 1613 he had identified through his telescope "a star" that was very close to the position where Neptune would have been around that time. It would have been uncharacteristic of Galileo not to recognise the nature of this moving object in is field of view - but perhaps he just did not.