Did you know that Neptune was discovered by astronomers who were also mathematicians and that they used their math skills to find it? Many astronomers had seen Neptune, but didn't realize they were looking at a planet. Galileo was one of the astronomers who observed Neptune, but thought it was a star. Planets look like disks through telescopes; stars look like points of light. Galileo's telescope was not powerful enough to reveal that the object he saw was a disk.
It was not until the discovery of the planet Uranus by William Hershel in 1781, that astronomers realized there might be another planet beyond its orbit. Uranus' orbit was a bit strange. Astronomers thought, but couldn't prove, that it was caused by the gravitational pull of another planet. A young British astronomer and mathematician , John C. Adams, decided to calculate the location of the mystery planet. In 1843, he set to work on it. According to his calculations, Neptune was about 1 billion miles or 1.6 billion kilometers farther from the Sun than Uranus. In 1845, he had determined the location of Neptune and sent the results to Sir George B. Airy, the astronomer royal of England. Airy took no action with Adams' work.
Although Adams didn't know it, another young astronomer and mathematician in France, Urbain J.J. Leverrier, had also begun to work on locating the planet. In 1846, his calculations lead him to predict Neptune would be found in almost the same area as Adams had predicted. He sent his work to Johann G. Galle at the Urania Observatory in Berlin, Germany. Galle had already charted the stars in the area where Adams and Leverrier predicted Neptune was located. Using Leverrier's prediction, Galle and his assistant, Heinrich L. d'Arrest, found the planet beyond Uranus in September of 1846.
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