Encyclopedia
Uranus is the seventh
planet from the
Sun. It is a
gas giant, the third largest by
diameter and fourth largest by
mass. It is named after
Uranus, the Greek god of the sky and progenitor of the other gods. Its symbol is either
or
. The first symbol derives from the name of its discoverer,
William Herschel. The second symbol is a combination of the devices for the Sun and Mars, as Uranus was the personification of heaven in Greek mythology, dominated by the light of the Sun and the power of Mars. It is also the
alchemical symbol of
platinum.
NASA's
Voyager 2 is an unmanned [i] interplanetary [i] spacecraft [i]. ...
is the only spacecraft to have visited the planet and no other visits are currently planned. Launched in 1977,
Voyager 2 made its closest approach to Uranus on January 24, 1986, before continuing its journey to
Neptune.
Uranus is the first planet discovered in modern times. Sir
William Herschel formally discovered the planet on March 13, 1781; the other planets have been known since ancient times, and Uranus' discovery expanded the boundaries of the solar system for the first time in modern human history. It was also the first planet discovered using
technology rather than the naked eye.
Discovery and naming
Uranus was the first planet to be discovered that was not known in ancient times; although it had been observed on many previous occasions, it was always mistakenly identified as a star. The earliest recorded sighting was in 1690 when
John Flamsteed catalogued Uranus as 34 Tauri. Flamsteed observed Uranus at least six more times. The record belongs to a French astronomer, Pierre Lemonnier, who observed Uranus at least twelve times between 1750 and 1771, including on four consecutive nights.
Sir
William Herschel discovered the planet on March 13, 1781, but reported it on April 26, 1781, as a "
comet."
- « On the 13th of March, 1781, between ten and eleven o'clock at night, while Herschel was examining the small stars near H Geminorum with a seven-foot telescope, bearing a magnifying power of two hundred and twenty-seven times, one of these stars seemed to have an unusual diameter; and it was, therefore, thought to be a comet. It was under this denomination that it was discussed at the Royal Society of London. But the researches of Herschel and of Laplace showed later that the orbit of the new body was nearly circular, and Uranus was consequently elevated to the rank of a planet. »
Herschel originally named it
Georgium Sidus in honour of King
George III of
Great Britain. When it was pointed out that
sidus means star and not planet, he rebaptised it the
Georgian Planet. This name was not acceptable outside of Britain. Lalande proposed in 1784 to name it
Herschel, at the same time that he created the planet's symbol ; his proposal was readily adopted by French astronomers. Prosperin, of
Uppsala, proposed the names
Astraea,
Cybele, and
Neptune . Lexell, of
St. Petersburg, compromised with
George III's Neptune and
Great-Britain's Neptune.
Bernoulli, from
Berlin, suggested
Hypercronius and
Transaturnis.
Lichtenberg, from
Göttingen, chimed in with
Austräa, a goddess mentioned by
Ovid . The name
Minerva was also proposed. Finally,
Bode, as editor of the
Berliner Astronomisches Jahrbuch, opted for
Uranus, after Latinized version of the
Greek god of the sky,
Ouranos;
Maximilian Hell followed suit by using it in the first ephemeris, published in
Vienna and computed by the Benedictine priest Placido Fixlmillner. The earliest publication to include Uranus in its title, according to NASA's , was in 1823 . The name was in use in Germany at least as far back as 1791, however . Examination of earliest issues of
Monthly Notices of the Royal Astronomical Society from 1827 shows that the name
Uranus was already the most common name used even by British astronomers by then, and probably earlier. The name
Georgium Sidus or "the Georgian" were still used infrequently thereafter. The final holdout was HM Nautical Almanac Office, which did not switch to
Uranus until 1850.
The stressed syllable in the name is properly the first, antepenultimate syllable, since in Latin the penultimate vowel
a is short ' and in an open syllable, and such syllables are never stressed in Latin. The historically correct pronunciation of the name by English-speakers is therefore
['jur?n?s] or
['jur?n?s]. The historically incorrect pronunciations
[ju're?n?s] or
[j?'re?n?s], with stress on the second syllable and a "long a" ' have become very common, however, perhaps through the influence of the related adjective "Uranian" or the similarly-pronounced name of the element
uranium.
In the
Chinese,
Japanese,
Korean, and
Vietnamese languages, the planet's name is literally translated as the
sky king star .
Physical characteristics
Composition
Uranus is composed primarily of gas and various ices. The atmosphere is about 83%
hydrogen, 15%
helium and 2%
methane and traces of
acetylene, while the interior is richer in heavier elements, most likely compounds of oxygen, carbon, and nitrogen, as well as rocky materials. This is in contrast to
Jupiter and
Saturn which are mostly hydrogen and helium. Uranus is very much similar to the cores of Jupiter and Saturn without the massive fluid metallic hydrogen envelope. Uranus'
cyan color is due to the absorption of
red light by atmospheric
methane. Surface temperature on Uranus's cloud cover is approximately 55
K .
Axial tilt
One of the most distinctive features of Uranus is its axial tilt of ninety-eight degrees. Consequently, for part of its orbit one pole faces the
Sun continually while the other pole faces away. At the other side of Uranus' orbit the orientation of the poles towards the Sun is reversed. Between these two extremes of its orbit the Sun rises and sets around the equator normally.
At the time of
Voyager 2s passage in 1986, Uranus' south pole was pointed almost directly at the Sun. Note that the labelling of this pole as "south" is actually in some dispute. Uranus can either be described as having an axial tilt of slightly more than 90°, or it can be described as having an axial tilt of slightly less than 90° and rotating in a
retrograde direction; these two descriptions are exactly equivalent as physical descriptions of the planet but result in different definitions of which pole is the north pole and which is the south pole.
One result of this orientation is that the polar regions of Uranus receive a greater energy input from the Sun than its equatorial regions. Uranus is nevertheless hotter at its equator than at its poles, although the underlying mechanism which causes this is unknown. The reason for Uranus' extreme axial tilt is also not known. It is speculated that perhaps during the formation of the planet it collided with an enormous protoplanet, resulting in the skewed orientation.
It appears that Uranus' extreme axial tilt also results in extreme seasonal variations in its weather. During the Voyager 2 flyby, Uranus' banded cloud patterns were extremely bland and faint. Recent
Hubble Space Telescope observations, however, show a more strongly banded appearance now that the Sun is approaching Uranus' equator. By 2007 the Sun will be directly over Uranus's equator.
Magnetic field
Uranus'
magnetic field is peculiar since it is not originating from the geometric center of the planet and is tilted almost 60° from the axis of rotation. It is probably generated by motion at relatively shallow depths within Uranus. Neptune has a similarly displaced magnetic field, which suggests the magnetic field is not necessarily a consequence of Uranus' axial tilt. The
magnetotail is twisted by the planet's rotation into a long corkscrew shape behind the planet. The magnetic field's source is unknown.
Explanation for bland atmosphere
The internal heat of Uranus is lower than that of
Jupiter and
Saturn. Both Jupiter and Saturn radiate more energy than they receive from the Sun.
This causes many powerful convection currents to form in the atmosphere. On Uranus that heat source is much lower due to its lower mass, with the temperature of its core roughly 7000K compared to 30000K at Jupiter's core and 18000K at Saturn. The convection currents formed in the Uranian atmosphere are not as strong and hence it lacks the atmosphere banding of the larger gas giants.
Planetary rings
Uranus has a faint
planetary ring system, composed of dark particulate matter up to ten meters in diameter. This ring system was discovered in March 1977 by James L. Elliot, Edward W. Dunham, and Douglas J. Mink using the
Kuiper Airborne Observatory. The discovery was
serendipitous; they planned to use the
occultation of a star by Uranus to study the planet's
atmosphere. However, when their observations were analyzed, they found that the star had disappeared briefly from view five times both before and after it disappeared behind the planet. They concluded that there must be a ring system around the planet; it was directly detected when
Voyager 2 is an unmanned [i] interplanetary [i] spacecraft [i]. ...
passed Uranus in 1986.
As of 2005, 13 rings had been identified. In December 2005, the
Hubble Space Telescope photographed a pair of previously unknown rings. The largest is twice the diameter of the planet's previously known rings. The new rings are so far from the planet that they are being called Uranus's "second ring system." Hubble also spotted two small satellites. One shares its orbit with one of the newly discovered rings. The new data reveals that the orbits of Uranus's family of inner moons have changed significantly in the last decade.
In April 2006, information about two more rings was published, one of them appearing blue and the other red. The rest of the planet's rings appear grey. The blue ring is thought to get its color from being swept by a moon, which may draw away all large debris, leaving only fine dust which refracts light in much the same way the Earth's atmosphere does.
Natural satellites
Uranus has 27 known
moons. The names for these moons are chosen from characters from the works of
Shakespeare and
Alexander Pope. The five main satellites are Miranda, Ariel, Umbriel, Titania and Oberon.
The main Uranian moons
|
|---|
|
Name
| Diameter
| Mass
| Orbital radius
| Orbital period
|
|---|
| Miranda | m?-ran'-d?
| 470
| | 129,000
| 1.4
|
| Ariel | arr'-ee-?l
| 1160
| | 191,000
| 2.5
|
| Umbriel | um'-bree-?l
| 1170
| | 266,000
| 4.1
|
| Titania | t?-taan'-y?
| 1580
| | 436,000
| 8.7
|
| Oberon | oe'-b?r-on
| 1520
| | 584,000
| 13.5
|
- For a timeline of discovery dates, see Timeline of discovery of Solar System planets and their natural satellites
Visibility
The brightness of Uranus is between magnitude +5.5 and +6.0, so it can be seen with the naked eye as a faint star under dark sky conditions. It can be easily found with binoculars. From Earth, it has a diameter of four arc-seconds. In larger amateur telescopes with an objective diameter greater than 12" the planet appears as a pale blue disc with distinct limb shading, and two of the larger satellites, Titania and Oberon, may be visible. Even in large professional instruments no details can be seen on its disc. However,
infrared studies of its atmosphere using
adaptive optics have yielded interesting data in the years since the Voyager flyby.
- For more details, see Aspects of Uranus
See also
References
External links
-
- - Press release with some photographs showing rings, satellites and clouds
- News reports of 22 December 2005 rings and moons discovery
- A kid's guide to Uranus.
- at Jet Propulsion Laboratory's planetary photojournal.
Special characters
