NEPTUNE,fourth largest of the planets in the solar system, and eighth major planet in order of increasing average distance from the sun.
The discovery of Neptune was one of the triumphs of mathematical
astronomy. To account for perturbations in the orbit of the planet Uranus
the French astronomer Urbain Jean Joseph Leverrier in 1846 calculated
the existence and position of a new planet. That same year the German
astronomer Johann Gottfried Galle (1812-1910) discovered the planet
within 1° of that position.
| BRIEF SURVEY OF NEPTUNE |
| Distance from Sun |
|
| Perihelion (closest) |
4,459,630,000 km (2,771,087,000 mi) |
| Aphelion |
4,536,870,000 km (2,819,080,000 mi) |
| Distance from Earth |
|
| Minimum |
4,305,900,000 km (2,675,600,000 mi) |
| Maximum |
4,687,300,000 km (2,912,600,000 mi) |
| Period of revolution |
164.79 earth years |
| Rotation period |
16.11 hr |
| Eccentricity of orbit |
0.009 |
| Inclination of orbit |
1.769° |
| Mass (earth = 1) |
17.147 |
| Radius at equator |
24,764 km (15,388 mi) |
| Mean density (earth = 1) |
0.317 |
| Surface gravity (earth = 1) |
1.1 |
| Atmospheric pressure at surface (earth = 1) |
>1000 bars |
| Effective temperature |
-214° C (-353° F) |
| Known natural satellites (as of Feb. 2004) |
13 |
The average distance of Neptune from the sun is
4,498,252,900 km (2,795,084,800 mi), and its equatorial radius is
24,764 km (15,388 mi), or about 3.9 times that of the earth. Its volume
is 57.7 times, its mass 17.147 times, and its mean density 0.317 that
of the earth (about 1.76 times that of water). Neptune is never visible
to the naked eye, but it can be observed in a small telescope as a
small, greenish-blue disk without definite surface markings. The
effective temperature of Neptune is about -214° C (-353° F), much like
Uranus, which is more than 1.6 billion km (more than 1 billion mi)
closer to the sun. Scientists assume, therefore, that Neptune must have
some internal heat source. The atmosphere consists mostly of hydrogen
and helium, but the presence of up to 2% methane gives the planet its
striking blue color.
| Principal Satellites of Neptune |
| Name |
Mean distance from Neptune |
Diameter |
| Naiad |
48,200 km (29,950 mi) |
58 km (36 mi) |
| Thalassa |
50,100 km (31,100 mi) |
80 km (50 mi) |
| Despina |
52,500 km (32,600 mi) |
148 km (92 mi) |
| Galatea |
62,000 km (38,500 mi) |
158 km (98 mi) |
| Larissa |
73,500 km (45,700 mi) |
192 km (119 mi) |
| Proteus |
117,600 km (73,100 mi) |
416 km (258 mi) |
| Triton |
354,800 km (220,500 mi) |
2706 km (1681 mi) |
| Nereid |
5,513,400 km (3,425,900 mi) |
340 km (211 mi) |
Thirteen known satellites orbit Neptune, two of
which are observable from earth. The largest and brightest is Triton,
discovered in 1846, the same year Neptune was first observed. Triton,
with a diameter of 2706 km (1681 mi), is only slightly smaller than
earth's moon. It has a retrograde orbit-that is, opposite its primary's
direction of rotation-unlike any other large satellite in the solar
system. Despite its extreme coldness, Triton has a thin nitrogen
atmosphere with some methane and some form of haze, and it displays an
active surface of geysers that spout an unknown subsurface material.
Nereid, discovered in 1949, has a diameter of only about 340 km (about
211 mi). Six more satellites were discovered by the Voyager 2 planetary probe in 1989. Another five moons were identified during 2002-03 by astronomers at observatories in Chile and Hawaii.
Neptune is also circled by at least five thin rings.
Its magnetic field is tilted about 47° to the rotation axis and, like
that of Uranus, displays significant anomalies.