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ATMOSPHERE
RINGS
PICTURES
Uranus, the first planet discovered in modern times, was
discovered by William Herschel while systematicly 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).
ATMOSPHERE
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 equator.
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.
RINGS
Like the other gas planets, Uranus has rings. 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. 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' 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.
