It was the first discovery of a new planet since ancient times. The first five, Mercury, Venus, Mars, Jupiter and Saturn, are visible with the naked eye, and have been known since early history. Uranus was the first discovered by telescope. Actually, Uranus had been seen before by other observers with telescopes, but had been marked on charts as a fixed star. Herschel found it moved against the background of stars, and at first thought it was a comet. His measurements were used by the French mathematician Pierre Laplace, who worked out its orbit, and proved it was a planet.