The outermost of the four giant planets. Neptune is a near twin of Uranus in size, mass, and composition. Its discovery in 1846 within a degree from the theoretically predicted position was one of the great achievements of celestial mechanics. Difficulties in accounting for the observed motion of Uranus by means of perturbations by the other known planets led early in the nineteenth century to the suspicion that a new planet, beyond the orbit of Uranus, might be causing the deviation from the predicted path. The difficult problem of deriving the mass and orbital elements of the unknown planet was solved independently in 1845–1846, first by J. C. Adams in Cambridge, England, and then by U. J. Leverrier in Paris. Adams's result did not receive immediate attention, and so it was Leverrier's solution that led to the discovery of Neptune by J. G. Galle, in Berlin, who found the planet on September 23, 1846, only 55′ from its calculated position.