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The eighth planet from the Sun, Neptune
was the first planet located through mathematical predictions
rather than through regular observations of the sky.
When Uranus didn't travel exactly
as astronomers expected it to, two mathematicians, working
independently of each other, proposed the position and
mass of another, as yet unknown planet that could account
for Uranus' orbit. Although "the establishment" ignored
the predictions, a young astronomer decided to look
for the predicted planet. Thus, Neptune was discovered
in 1846. Seventeen days later, its largest moon, Triton,
was also discovered.
Nearly 4.5 billion kilometers from the
Sun, Neptune orbits the Sun once every 165 years, and
therefore it has not quite made a full circle around
the Sun since it was discovered. It is invisible to
the naked eye because of its extreme distance from Earth.
Interestingly, due to Pluto's
unusual elliptical orbit, Neptune is actually the farthest
planet from the Sun for a 20-year period out of every
248 Earth years.
Neptune has the smallest diameter of our
solar system's giant gas planets (including Jupiter,
Saturn, and Uranus),
so called because they have no solid surfaces. Even
so, its volume could hold nearly 60 Earths. Neptune's
atmosphere extends to great depths, gradually merging
into water and other "melted ices" over a heavier, approximately
Earth-sized liquid core. Neptune's rotational axis is
tilted 30 degrees to the plane of its orbit around the
Sun. Its seasons last an incredible 41 years. During
the southern summer, the south pole is in constant sunlight
for about 41 years, and in northern summer, the north
pole is in constant sunlight for about 41 years. Neptune's
atmosphere is made up of hydrogen, helium, and methane,
the last of these giving the planet its blue color (because
methane absorbs red light). Despite its great distance
from the Sun and lower energy input, Neptune's winds
are three times stronger than Jupiter's and nine times
stronger than Earth's.
In 1989, Voyager 2 tracked a large oval
dark storm in Neptune's south-ern hemisphere. This hurricane-like
"Great Dark Spot" was large enough to contain the entire
Earth; spun counterclockwise; and moved westward at
almost 1,200 km per hour. Recent images from the Hubble
Space Telescope show no sign of the "Great Dark Spot,"
although a comparable spot appeared in 1997 in Neptune's
northern hemisphere.
The planet has several rings of varying
widths, confirmed by Voyager 2's observations in 1989.
The outermost ring, Adams, contains five distinct arcs
(incomplete rings) named LibertÚ, EqualitÚ 1, EqualitÚ
2, FraternitÚ, and Courage. Next is an unnamed ring coorbital
with the moon Galatea, then Le Verrier, Lassell, Arago,
and Galle. Neptune's rings are believed to be relatively
young and relatively short-lived. Neptune
has 11 known moons, six of which were discovered by
Voyager 2. The largest, Triton, orbits Neptune in a
direction opposite to the planet's rotation direction,
and is gradually getting closer until it will collide
with the planet in about 10 to 100 million years, forming
vast rings around Neptune that will rival or exceed
Saturn's extensive ring system. Triton is the coldest
body yet visited in our solar system; temperatures on
its surface are about -235 ÁC. Despite the deep freeze,
Voyager 2 discovered great geysers of gaseous nitrogen
on Triton.
Related Links:
Source: NASA
Last Updated : 08.20.2003
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Date of Discovery:
1846
Distance from the Sun:
(Semimajor axis of orbit)
4,498,252,900 km
30.06896348 A.U.
Radius:
24,764 km
(3.883 of Earth's radius)
Volume:
44 (Earth = 1)
Mass:
102.44 x 1027 g
Density:
1.76 gm/cm3
Surface Gravity:
1100 cm/s2
Escape Velocity:
23.71 km/s
Sidereal Rotation Period:
0.67125
Sidereal Orbit Period:
164.79132 sidereal years
Mean Orbit Velocity:
5.4778 km/s
Orbit Eccentricity:
0.00858587
Orbit Inclination:
1.76917 degrees
Equatorial Inclination:
29.58 degrees
Atmospheric Temperature (at level with
pressure = 1 bar):
73 K
Major Atmospheric Constituents:
H2, He, CH4
Natural Satellites:
1. Naiad
2. Thalassa
3. Despina
4. Galatea
5. Larissa
6. Proteus
7. Triton
8. Nereid
9. S/2002 N1
10. S/2002 N2
11. S/2002 N3
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