Uranus
ATLAS OF THE UNIVERSE
U
ranus, third of the giant planets, was discovered by
William Herschel in 1781. Herschel was not looking
for a planet; he was engaged in a systematic ‘review of
the heavens’ with a home-made reflecting telescope when
he came across an object which was certainly not a star. It
showed a small disk, and it moved slowly from night to
night. Herschel believed it to be a comet, but calculations
soon showed it to be a planet, moving far beyond the orbit
of Saturn. After some discussion it was named Uranus,
after the mythological father of Saturn.
Uranus is just visible with the naked eye, and it had
been seen on several occasions before Herschel’s discovery.
John Flamsteed, England’s first Astronomer Royal, even
included it in his star catalogue, and gave it a number:
34 Tauri. However, a small telescope will show its tiny,
greenish disk. The equatorial diameter is 51,118 kilo-
metres (31,770 miles), rather less than half that of Saturn;
the mass is over 14 times that of the Earth, and the visible
surface is made up of gas, mainly hydrogen together with
a considerable amount of helium.
Irregularities in the movements of Uranus led to the
tracking down of the outermost giant, Neptune, in 1846.
In size and mass the two are near-twins, so that in some
ways they may be considered together even though there
are marked differences between them. As a pair, more-
over, Uranus and Neptune are very different from Jupiter
and Saturn, quite apart from being much smaller and less
massive; it has been suggested that they are intermediate
in type between the hydrogen- and helium-rich Jupiter
and Saturn on the one hand, and the oxygen-rich metallic
planets on the other. According to the so-called three-layer model of Uranus, there is a silicate core surrounded
by an ocean of liquid water which is in turn overlaid by
the atmosphere; on the more convincing two-layer model
there is a core surrounded by a deep layer in which
gases are mixed with ‘ices’, mainly water, ammonia and
methane. Above this comes the predominantly hydrogen
atmosphere, together with around 15 per cent of helium
and smaller quantities of other gases. It is not easy to
decide just where the ‘atmosphere’ ends and the real body
of the planet begins; neither is it certain whether there is a
sharp boundary to the core.
What is certain is that Uranus, unlike Jupiter, Saturn
and Neptune, has no appreciable source of internal heat.
This means that the temperature at the cloud-tops is much
the same as that of Neptune, even though Neptune is so
much further from the Sun.
Uranus is a slow mover; it takes 84 years to orbit the
Sun. The rotation period is 17 hours 14 minutes, though,
as with the other giants, the planet does not spin in the
way that a rigid body would do. The most extraordinary
feature is the tilt of the axis, which amounts to 98 degrees;
this is more than a right angle, so that the rotation is tech-
nically retrograde. The Uranian calendar is very curious.
Sometimes one of the poles is turned towards the Sun, and
has a ‘day’ lasting for 21 Earth years, with a corresponding
period of darkness at the opposite pole; sometimes the
equator is presented. In total, the poles receive more heat
from the Sun than does the equator. The reason for this
exceptional tilt is not known. It is often thought that at an
early stage in its evolution Uranus was hit by a massive
body, and literally knocked sideways. This does not sound
very likely, but it is hard to think of anything better.
Significantly, the satellites and the ring system lie virtually
in the plane of Uranus’ equator.
(En passant, which is the ‘north’ pole and which is
the ‘south’? The International Astronomical Union has
decreed that all poles above the ecliptic, i.e. the plane of
the Earth’s orbit, are north poles, while all poles below the
ecliptic are south poles. In this case it was the south pole
which was in sunlight during the Voyager 2 pass of 1986.
However, the Voyager team reversed this, and referred to
the sunlit pole as the north pole. Take your pick!)
No Earth-based telescope will show definite markings
on the disk of Uranus. Before the Voyager mission,
five satellites were known – Miranda, Ariel, Umbriel,
Titania and Oberon; Voyager added ten more, all close
to the planet.
On 10 March 1977, Uranus passed in front of a star,
and hid or occulted it. This gave astronomers an excellent
chance of measuring Uranus’ apparent diameter – which
is not easy by sheer visual observation, because the edge
of the disk is not sharp, and the slightest error in measure-
ment will make a tremendous difference to the final value.
Therefore the phenomenon was carefully observed, with
surprising results. Both before and after the actual occul-
tation the star ‘winked’ several times, and this could be
due only to a system of rings surrounding the planet.
Subsequently D. A. Allen, at Siding Spring in Australia,
managed to photograph the rings in infra-red light.
However, our knowledge of Uranus and its system
remained decidedly meagre, and a detailed survey had to
await the fly-by of Voyager 2 in January 1986. The best Earth-based
views have been obtained
with the Hubble Space
Telescope and the Very
Large Telescope in Chile.This photograph of Uranus
and some of its satellites was
taken in near-infra-red light
by the Antu Telescope, one
unit of the VLT.▲ Axial inclination of
Uranus. The planets’
inclinations have a wide
range; 2° (Mercury),
178° (Venus), 24° (Mars),
3° (Jupiter), 26.5° (Saturn),
98° (Uranus), 29° (Neptune)
and 122° (Pluto). Uranus
thus differs from all the other
planets – discounting Pluto.Earth 23.5 ̊Uranus 98 ̊TitaniaUmbrielPortiaMirandaPuckArielOberonD108- 151 UNIVERSE UK 2003CB 7/4/03 5:15 pm Page 122