Neptune

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The chance discovery of Uranus in 1781 made it obvious that there might be more planets out there just waiting to be discovered. We now know that Neptune was out there, ahead of Uranus in the sky, and pulling it faster in its orbit than would be expected. Then by the 1840s, Uranus had passed Neptune and started to slow down. This created a nice problem for mathematicians - could they predict if there was really a new planet, based just on the speed of Uranus in its orbit?

Two expert mathematicians began the task independently, without any idea that the other was also at work. The contenders were Urbain Le Verrier, an established French mathematician, and the younger Cornishman, John Couch Adams, in Cambridge. Their task was not easy - without modern computers, the work was tedious and repetitive. Adams began his work in 1843, but didn't come up with any actual positions for a search to be made until 1845. By that time, Le Verrier had announced that he was also making calculations. The race was on. 

A search was begun from Cambridge by Professor James Challis in late 1845, but his star maps were not good enough to identify a new planet straight away so he had to make more observations of the search area. To make matters worse, the first position that Adams provided was wrong. There's a story in Cambridge that Challis eventually observed the right spot, but his wife called him in for a cup of tea. By the time he got out again, it had clouded over and he missed his chance.

Le Verrier announced his predictions on 31 August 1846. But he couldn't get any French astronomers to make a search, so he turned to the German astronomer, Johann Galle, in Berlin. Galle received the letter on 23 September, and started a search that very night using a recently completed star chart. Within just the first hour of searching, he found Neptune!

British astronomers were fuming at the loss of prestige, particularly as Adams had made predictions earlier. They tried to blame Challis, and also the Astronomer Royal, George Airy, whom the diffident Adams had tried to approach in 1845. But Adams' prediction was less accurate than that of Le Verrier, and he himself acknowledged that Le Verrier was the true predictor of the position of Neptune. 

Today we know that both of them were rather lucky in their predictions, but Le Verrier was luckier than Adams in that by chance his was closer to the actual planet. 

Photo: Neptune, photographed by Voyager 2 in 1989.
Credit: JPL