Uranus
The Magician
Uranus Facts
- Uranus is the seventh planet from the Sun
and the third largest (by diameter):
- distance from Sun: 2,870,990,000 km (19.218 AU)
- equatorial diameter: 51,118 km; polar diameter: 49946 km
- mass: 8.686e25 kg
Uranus is larger in diameter but smaller in mass than
Neptune.
- Pronounced "YOOR a nus" .
(The older "u RAIN us" is too often interpreted as "your anus" by children
and distracted teenagers.)
- Uranus
is the ancient Greek deity of the Heavens, the earliest supreme god,
who was the father of Cronus (Saturn) and of
the Cyclopes and Titans (predecessors of the Olympian gods).
- Uranus, the first planet discovered in modern times, was discovered by
accident by William Herschel while searching
the sky with a telescope on March 13, 1781; he first thought that it was a comet.
It had actually been seen many times before but ignored as simply another star
(the earliest known sighting was in 1690).
Herschel named it "the Georgium Sidus"
(the Georgian Planet) in honor of his patron,
the infamous (to Americans) King George III
of England; others called it
"Herschel". The name "Uranus" was
first proposed by Bode in conformity with the
other planetary names from classical mythology but didn't come into
common use until 1850.
- Uranus has been visited by only one spacecraft,
Voyager 2 on Jan 24 1986.
- Most of the planets spin on an axis nearly perpendicular to the plane of
the ecliptic but Uranus's axis is almost
parallel to the ecliptic.
At the time of Voyager 2's passage,
Uranus's south pole was pointed almost directly at the Sun.
This results in the odd fact that Uranus's polar regions receive more energy
input from the Sun than do its equatorial regions.
Uranus is nevertheless hotter at its equator than at its poles. The mechanism
underlying this is unknown.
- Actually, there's an ongoing battle over which of Uranus's
poles is its north pole! Either its axial inclination is a bit over 90
degrees and its rotation is direct,
or it's a bit less than 90 degrees and the rotation is
retrograde.
The problem is that you need to draw a dividing line *somewhere*, because
in a case like Venus there is little dispute
that the rotation is indeed
retrograde (not a direct rotation with an inclination of nearly 180).
- Uranus is composed primarily of rock and various ices,
with only about 15% hydrogen and a little helium (in contrast to
Jupiter and
Saturn which are mostly hydrogen).
Uranus (and Neptune) are in many ways similar to the cores of Jupiter and
Saturn minus the massive
liquid metallic hydrogen envelope.
It appears that Uranus does not have a rocky core like Jupiter and Saturn
but rather that its material is more or less uniformly distributed.
- Uranus's atmosphere is abut 83% hydrogen, 15% helium and 2% methane.
- Like the other gas planets, Uranus has bands of clouds that blow around
rapidly. But they are extremely faint, visible only with radical image
enhancement of the Voyager 2 pictures
(picture 4).
Recent observations with HST show larger and
more pronounced streaks. The speculation is that the difference is due to
seasonal effects (the Sun is now at a somewhat lower Uranian latitude which may
cause more pronounced day/night effects).
- Uranus's blue color is the result of absorption of red light by methane in the
upper atmosphere. There may be colored bands like Jupiter's but they are hidden
from view by the overlaying methane layer.
- Like the other gas planets, Uranus has
rings
(picture 7).
Like Jupiter's, they are very dark but like Saturn's composed of fairly large
particles ranging up to 10 meters in diameter in addition to fine dust.
There are 11 known rings, all very faint; the brightest is known as
the Epsilon ring (picture 6).
The Uranian rings were the first after Saturn's to be discovered. This was of
considerable importance since we now know that rings are a common feature of
planets, not a peculiarity of Saturn alone.
- Voyager 2 discovered 10 small
moons in addition to the 5 large ones already known. It is likely that there
are several more tiny satellites within the rings.
- Uranus's magnetic field is odd in that it is not centered on the center of the
planet and is tilted almost 60 degrees with respect to the axis of rotation.
It is probably generated by motion at relatively shallow depths within Uranus.
- Uranus is sometimes just barely visible with the naked eye on a very clear night;
it is fairly easy to spot with binoculars (if you know exactly where to look).
A small astronomical telescope will show a small disk.
Mike Harvey's planet finder charts show the current position of Uranus (and the other planets) in the sky, butmuch more detailed charts will be required to actually find it.
Pictures
- (above) Uranus
175k gif
- Two views of Uranus's darkspot
107k gif
normal and false color views
102k gif;
40k jpg
- False-color view of Uranus
67k gif
- Uranus
71k gif;
17k jpg
Epsilon ring
63k gif;
141k gif;
148k jpg
Uranus's rings (false color)
78k gif;
35k jpg;
211k gif
- rings and shepherds
152k gif;
255k jpg
HST view of rings and atmosphere features
73k gif;
48k jpg
- ... more Uranus images
Movies
- Radio occultation
1050k quicktime
- Sigma Sagitarius Occultation
1300k quicktime
- Historical Background: Discovery of Uranus
3000k AVI
- Magnetic Field of Uranus
3400k AVI
Uranus's Satellites
Uranus has 15 known moons.
- Unlike the other bodies in the solar system which have names from classical
mythology, Uranus's moons take their names from the writings of
Shakespeare and
Pope.
- They form two distinct classes: the 10 small very dark inner ones discovered
by Voyager 2 and the 5 large outer ones.
- They all have nearly circular orbits in the plane of Uranus's equator (and hence
at a large angle to the plane of the
ecliptic).
Distance Radius Mass
Satellite (000 km) (km) (kg) Discoverer Date
--------- -------- ------ ------- ---------- -----
Cordelia 50 13 ? Voyager 2 1986
Ophelia 54 16 ? Voyager 2 1986
Bianca 59 22 ? Voyager 2 1986
Cressida 62 33 ? Voyager 2 1986
Desdemona 63 29 ? Voyager 2 1986
Juliet 64 42 ? Voyager 2 1986
Portia 66 55 ? Voyager 2 1986
Rosalind 70 27 ? Voyager 2 1986
Belinda 75 34 ? Voyager 2 1986
Puck 86 77 ? Voyager 2 1985
Miranda 130 236 6.30e19 Kuiper 1948
Ariel 191 579 1.27e21 Lassell 1851
Umbriel 266 585 1.27e21 Lassell 1851
Titania 436 789 3.49e21 Herschel 1787
Oberon 583 761 3.03e21 Herschel 1787
Distance Width
Ring (km) (km)
------- -------- -----
1986U2R 38000 2,500
6 41840 1-3
5 42230 2-3
4 42580 2-3
Alpha 44720 7-12
Beta 45670 7-12
Eta 47190 0-2
Gamma 47630 1-4
Delta 48290 3-9
1986U1R 50020 1-2
Epsilon 51140 20-100
(distance is from Uranus's center to the ring's inner edge)
More about Uranus and its satellites
Open Issues
- Why doesn't Uranus radiate more heat than it receives from the Sun as the
other gas planets do? Is its interior cold?
- Why is its axis so unusally tilted? Was it due to a massive collision?
- Why do Uranus and Neptune have so much less hydrogen and helium than Jupiter
and Saturn? Is it simply because they are smaller? or because they're farther
from the Sun?
... Sun
... Saturn
... Phoebe
... Uranus
... Cordelia
... Neptune
... 
Bill Arnett; last updated:
1995 July 26