Neptune, the next-farthest planet from the Sun, is in some ways a twin of Uranus. It is slightly smaller -- diameter 49,500 km -- and shows the same featureless disk through amateur telescopes, with a stronger blue-green tinge because there is more methane in the atmosphere above its clouds. Magnitude 7.8 at brightest, it is completely invisible to the naked eye, but can be followed in binoculars when one knows where to look. Unlike the accidental discovery of Uranus, Neptune's existence was predicted in advance. Astronomers found that Uranus was not keeping to its expected course, and some suspected that it was being pulled by the gravity of an as-yet-unseen planet. In France, mathematician Urbain Le Verrier calculated the new planet's position in 1846, beating the English-man John Couch Adams who had been working on the same problem. On September 23 that year, astronomers at the Berlin Observatory found Neptune close to Le Verrier's predicted position. Neptune crawls around the Sun in 165 years at an average distance of 4500 million km, 30 times farther away than the Earth. Being so distant from the Sun, it is a very cold and dark planet. Unlike bland Uranus, Neptune has been found to possess markings in its clouds similar to those of Jupiter. A cloud feature called the Great Dark Spot, similar in nature to Jupiter's Great Red Spot, was revealed by the cameras of Voyager 2 when it reached Neptune in August 1989. This spot gradually drifted towards the planet's equator and had disappeared by the time the Hubble Space Telescope looked at Neptune in 1994, although similar spots have appeared since. But the planet's main distinction is its two curious outer satellites, Triton and Nereid. Triton, the largest of Neptune's eight known moons, is in a retrograde (east to west) orbit at a distance of 355,000 km. Tidal forces from Neptune mean that Triton's orbit is gradually shrinking, so that the moon will spiral closer to the planet until it is broken up in the distant future. A shattered Triton will form a far more substantial set of rings around Neptune than the thin, faint ones that currently encircle the planet, for Triton is 2700 km in diameter, over three-quarters the size of our own Moon. The outermost satellite, Nereid, is much smaller, only 340 km across, but it has an exceptionally elliptical orbit that carries it between 1.4 and 9.7 million km from Neptune. Something seems to have disturbed the satellite system of Neptune.