Uranus is the seventh planet from the Sun and the third largest (by
diameter). Uranus is larger in diameter but smaller in mass than Neptune.
orbit: 2,870,990,000 km (19.218 AU) from Sun
diameter: 51,118 km (equatorial)
mass: 8.683e25 kg
Careful pronunciation may be necessary to avoid embarrassment; say "YOOR a
nus" , not "your anus" or "urine us".
Uranus is the ancient Greek deity of the Heavens, the earliest supreme god.
Uranus was the son and mate of Gaia 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
William Herschel while systematically searching the sky with his telescope on
March 13, 1781. It had actually been seen many times before but ignored as
simply another star (the earliest recorded sighting was in 1690 when John
Flamsteed cataloged it as 34 Tauri). 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' axis is almost parallel to the ecliptic. At the time of
Voyager 2's passage, Uranus' south pole was pointed almost directly at the Sun.
This results in the odd fact that Uranus' 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' 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' atmosphere is about 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 (right). Recent observations with HST
(left) show larger and more pronounced streaks. Further HST observations show
even more activity. Uranus is no longer the bland boring planet that Voyager
saw! It now seems clear that the differences are due to seasonal effects since
the Sun is now at a lower Uranian latitude which may cause more pronounced
day/night weather effects. By 2007 the Sun will be directly over Uranus's
Uranus' 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. Like Jupiter's, they are very
dark but like Saturn's they are 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. 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
Uranus' 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 unaided 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. There are several
Web sites that show the current position of Uranus (and the other planets) in
the sky, but much more detailed charts will be required to actually find it.
Such charts can be created with a planetarium program such as Starry Night.
Uranus has 21 named moons and six unnamed ones:
Unlike the other bodies in the solar system which have names from classical
mythology, Uranus' moons take their names from the writings of Shakespeare and
They form three distinct classes: the 11 small very dark inner ones discovered
by Voyager 2, the 5 large ones (right), and the newly discovered much more
Most have nearly circular orbits in the plane of Uranus' equator (and hence at
a large angle to the plane of the ecliptic); the outer 4 are much more
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
2003U2 75 6 ? Showalter 2003
Belinda 75 34 ? Voyager 2 1986
1986U10 76 40 ? Voyager 2 1986
Puck 86 77 ? Voyager 2 1985
2003U1 98 8 ? Showalter 2003
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
2001U3 4281 6 ? Sheppard 2003
Caliban 7169 40 ? Gladman 1997
Stephano 7948 15 ? Gladman 1999
Trinculo 8578 5
Sycorax 12213 80 ? Nicholson 1997
2003U3 14689 6 ? Sheppard 2003
Prospero 16568 20 ? Holman 1999
Setebos 17681 20 ? Kavelaars 1999
2002U2 21000 6 Sheppard 2003
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' center to the ring's inner edge)
Bill Arnett; last updated: 2003 Oct 11