degree by the independent calcula- tions of the English mathematician J. C. Adams and his French counterpart, U. J. Leverrier, both of whom had used Newtonian mechanics to analyze unexplained perturbations in the orbit of Uranus. 
    Much of our knowledge about Neptune’s surface comes from Voyager 2, which arrived there in 1989. This Voyager image shows many previously unknown features. Most prominent is the large dark area near the edge of the planet. Aptly named the Great Dark Spot, it is a huge storm—the size of our entire planet—high in Neptune’s atmosphere. The white wisps associated with the spot are cirrus clouds of methane, which condense on the storm’s