The discovery of Neptune was a result of carefully applied orbital mathematics. Small deviations in Uranus' orbit led astronomers to believe that Uranus was being disturbed in its orbit by some outside, undiscovered, gravitational influence. They surmised that this influence could be due to a large planet outside the orbit of Uranus. In 1845, an Englishman named John C. Adams predicted the position of this unknown planet. His calculations, however, were not taken seriously. As a result, no observational search for the new planet was begun. One year later a Frenchman, Urbain Leverrier, independently calculated the position and requested observational confirmation. He was also largely ignored. Leverrier then sent his work to a German friend in Berlin, J. Galle, who began earnestly looking for the planet and discovered it within hours.
Neptune orbits the Sun at about 30 Astronomical Units. (An A.U. is the average distance from Sun to Earth.) This is equivalent to 4+ billion kilometers, or a little over 2~ billion miles. At this incredible distance, Neptune receives very little energy from the Sun. Astronomers expected to see little in terms of weather or atmospheric disturbances. After all, if the atmosphere is not heated or disturbed in any way, then there is simply no reason for the atmosphere to have any movement. Voyager 2 showed astronomers that Neptune not only had weather, but had the second fastest winds in the solar system, clocked at a steady 1170 kilometers per hour (700 miles per hour). In addition, Neptune has at least two huge circular storms, much like huge hurricanes on Earth. The largest of these storms, dubbed the "Great Dark Spot", is at least 12,000 kilometers (7,500 miles) long and 8,000 kilometers (5,000 miles) wide. Earth would almost fit inside this storm! Voyager 2 also showed white, wispy clouds of methane ice crystals soaring about 50 kilometers (30 miles) above the blue blanket of methane clouds that shroud Neptune.
Just as Uranus' magnetic field is tilted, so is Neptune's. With respect to the planet's rotational axis, it is tilted by as much as 50x, and is offset from the center of the planet by nearly half of the planet's radius. It may be that Uranus and its twin planet, Neptune, have electrically conductive material in shells around their cores instead of in the cores. This magnetic field was very valuable to astronomers as it gave them an opportunity to measure the actual rotation period of Neptune--16 hours and 3 minutes.
Neptune was thought to have small ring arcs around it. But Voyager 2 showed these to be complete rings around the planet with some areas of highly reflective material and some areas of hard-to-see dark material. Voyager 2 discovered a total of three complete rings and a wide disk of boulder-sized material. This disk, or plateau, extends from between the two narrow, outermost rings almost to the inner, broad, "fuzzy" ring.
Perhaps even more interesting than the planet itself are the moons of Neptune, the largest of which is Triton. Triton, just a little smaller than Earth's moon, is unique in that it revolves about the planet the "wrong" way. Every other big moon in the solar system travels around its planet in the same direction the planet rotates, but not Triton. Some of the most exotic and unexplainable terrain can be found on Triton. Almost half of the surface of Triton (the southern hemisphere) is almost perfectly white and reflects almost all of the light it receives. As a result, Triton is almost certainly colder than the planet Pluto. Much of the middle area of Triton is covered by terrain that looks similar to the skin of a cantaloupe.
Voyager 2 found six new moons around Neptune, raising the total to eight. Nereid, the other previously known moon, was demoted to third in size. A larger moon, now only called 1989 N1, would probably have been discovered earlier but its orbit is much smaller than Nereid's and therefore cannot be seen from Earth.
Neptune takes approximately 165 years to make one complete trip around the Sun. Since it was discovered in 1846, it has yet to complete one Neptunian year. In addition Pluto's orbit is so highly eccentric (or oval) that Pluto is currently closer to the Sun than is Neptune. So, until 1999, Neptune will be the farthest planet from the Sun.
Voyager 2 is now traveling out of the solar system with no planned encounters on its itinerary. It will, however, continue to send back messages to home until its power fails and its eyes close. The findings of Voyager 2 are only a small part of the results of its entire mission. Much of the wealth of material sent by this most remarkable of crafts still has yet to be studied. After all, this is only the first glance.
Neptune's Spots: Now You See 'em, Now You Don't
Astronomy Magazine, issues: November and December 1989, and August 1991.
Contemporary Astronomy, by Jay M. Pasachoff, 2nd ed. 1981.
Nova Videos "Neptune's Icy Fury".
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