Neptune is the eighth planet from the Sun. It was named after the Roman god of the sea. Because of Neptune's faintness, it was not known as a wandering star to ancient peoples. It was discovered on September 23, 1846 by German astronomer Johann Galle. Neptune's existence was predicted from the orbital variations of Uranus by French astronomer Alexis Bouvard. In 1843, British mathematician John Couch Adams calculated a position for the planet. It was eventually found within one degree of the position predicted by calculations by French mathematician Urbain Le Verrier in 1845-1846. Galileo actually made the first drawing of Neptune when it was very close to Jupiter on December 28, 1612. He did not recognize it as a planet however, even though he may have noted that it moved from an earlier observation. Like Jupiter, Saturn and Uranus, Neptune is a gas giant composed mostly of hydrogen and helium. It also contains hydrocarbons, and ices made of ammonia and methane. Like Uranus, it is sometimes classified as an "ice giant." It takes about 164.79 Earth years for Neptune to revolve around the Sun. It rotates once on its own axis every 16 hours, 6 minutes and 36 seconds. Neptune has a diameter of 49,528 kilometers (30,775 miles). Neptune orbits the Sun at an average distance about 4.5 billion kilometers (2.8 billion miles), about 30 times farther than the Earth. It comes to opposition with the Earth every 367.49 days.
Neptune's apparent diameter varies from 2.2 - 2.4 arcseconds, and it's apparent magnitude varies from 7.8 to 8.0. Neptune is theoretically bright enough to be seen with the unaided eye from an extremely dark-sky observing site. although there are no reliable reports of anyone having seen it. Photographically in amateur equipment, Neptune is a small blue disk with no surface details as seen in the image at the top of the page shot with an 11-inch telescope. The color comes from the methane in Neptune's atmosphere. In 1989, Voyager 2 recorded some banding and a storm, named the "Great Dark Spot," that was about half the size of the Great Red Spot on Jupiter. Later observations in 1994 by the Hubble Space Telescope revealed that the Great Dark Spot was gone, but other smaller dark spots were detected. Like the other outer planets of Jupiter, Saturn and Uranus, Neptune has a ring system. It is too faint to be seen or photographed by amateur astrophotographic equipment. Neptune's Moons Thirteen known moons have been discovered orbiting Neptune so far. The largest is Triton, which as a retrograde orbit, indicating that it is probably an object captured from the Kuiper belt. Sky & Telescope's Triton Tracker gives the location of Neptune's moon Triton. The other moons are Naiad, Thalassa, Despina, Galatea, Larissa, Proteus, Nereid, Halimeda, Sao, Laomedeia and Psamathe. These moons are 19th magnitude and fainter. It is possible to photograph the Triton at magnitude 13.5 with amateur equipment. The problem, again, is the great brightness difference between the planet and the moon. You will need a longer exposure to record the moons, which will cause Neptune to overexpose. Compositing different exposures can compensate for this problem. Tips for Photographing Neptune
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