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Uranus

2007 Schools Wikipedia Selection. Related subjects: The Planets

   CAPTION: Uranus Astronomical symbol of Uranus

                        The planet Uranus

   Click image for description
                            Discovery
   Discovered by         William Herschel
   Discovered on         March 13, 1781
             Orbital characteristics ( Epoch J2000)
   Semi-major axis       2,870,972,220 km
                         19.191 263 93 AU
   Orbital circumference 18.029 T m
                         120.515 AU
   Eccentricity          0.047 167 71
   Perihelion            2,735,555,035 km
                         18.286 055 96 AU
   Aphelion              3,006,389,405 km
                         20.096 471 90 AU
   Orbital period        30,707.4896 d
                         (84.07 a)
   Synodic period        369.65 d
   Orbital speed         6.795 km/s
   Max. orbital speed    7.128 km/s
   Min. orbital speed    6.486 km/s
   Inclination           0.769 86°
                         (6.48° to Sun's equator)
   Longitude of the
   ascending node        74.229 88°
   Argument of the
   perihelion            96.734 36°
   Number of satellites  27
                    Physical characteristics
   Equatorial diameter   51,118 km
                         (4.007 Earths)
   Polar diameter        49,946 km
                         (3.929 Earths)
   Oblateness            0.0229
   Surface area          8.084×10^9 km^2
                         (15.849 Earths)
   Volume                6.834×10^13 km^3
                         (63.086 Earths)
   Mass                  8.6832×10^25 kg
                         (14.536 Earths)
   Mean density          1.318 g/cm^3
   Equatorial gravity    8.69 m/s^2
                         (0.886 g)
   Escape velocity       21.29 km/s
   Rotation period       −0.718 33 d (17 h 14 min 24 s
                         by convention) ^1
   Rotation velocity     2.59 km/s = 9320 km/h (at the equator)
   Axial tilt            97.77°
   Right ascension
   of North pole         77.31° (5 h 9 min 15 s)
   Declination           +15.175°
   Albedo                0.51
   Cloudtop avg. temp.   55 K
   Surface temp.
                         min  mean  max
                         59 K 68 K N/A K
   Adjective             Uranian
                   Atmospheric characteristics
   Atmospheric pressure  120 kPa (at the cloud level)
   Hydrogen              83%
   Helium                15%
   Methane               1.99%
   Ammonia               0.01%
   Ethane                0.00025%
   Acetylene             0.00001%
   Carbon monoxide
   Hydrogen sulfide      trace

          Note: This article contains special characters.

   Uranus ( IPA: /jəˈreɪnəs/ or /ˈjurənəs/) 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 (astrological) or
   Astronomical symbol for Uranus (astronomical). 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 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 (from Mercury out to Saturn) 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 (a telescope) 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. (Lemonnier is often called careless or even
   "sloppy" for this, but it is important to know that he realized 9 of
   these within a short time of Herschel's discovery and most of his
   observations occurred at the stationary point in Uranus' orbit.)

   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 (George's Star) in honour
   of King George III of Great Britain (c.f. American poet Elizabeth
   Graeme Fergusson's "Upon the Discovery of the Planet..." about the
   event). When it was pointed out that sidus means star and not planet,
   Herschel 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 (astrological)
   symbol ("a globe surmounted by your initial"); his proposal was readily
   adopted by French astronomers. Prosperin, of Uppsala, proposed the
   names Astraea, Cybele, and Neptune (now borne by two asteroids and
   another planet). 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 (but who is
   traditionally associated with Virgo). 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 ADS, was in 1823 ( Schwerd,
   Opposition des Uranus 1821, Astronomische Nachrichten, Vol. 1, pp.
   18-21). The name was in use in Germany at least as far back as 1791,
   however ( Fixlmillner, Acta Astronomica Cremifanensia, Steyr, AT: Franz
   Josef Medter, 1791). 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 (by the British alone) 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 (ūrănŭs) 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 [ˈjurənəs] or [ˈjurənʌs]. The
   historically incorrect pronunciations [juˈreɪnəs] or [jəˈreɪnəs], with
   stress on the second syllable and a "long a" (ūrānŭs) have become very
   common, however, perhaps through the influence of the related adjective
   "Uranian" (always pronounced [juˈreɪniən] or [jəˈreɪniən]) 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 (天王星).

Physical characteristics

Composition

   Uranus is composed primarily of gas and various ices. The atmosphere is
   about 83 percent hydrogen, 15 percent helium, 2 percent methane and
   traces of acetylene. 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 (like Neptune) is very much similar to the
   cores of Jupiter and Saturn without the massive fluid metallic hydrogen
   envelope. Uranus' cyan colour is due to the absorption of red light by
   atmospheric methane. Surface temperature on Uranus' cloud cover is
   approximately 55  K (−218  °C or −360  °F).

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 2's passage in 1986, Uranus' south pole was
   pointed almost directly at the Sun. The labelling of this pole as
   "south" uses the coordinate definitions currently endorsed by the
   International Astronomical Union, namely that the north pole of a
   planet or satellite shall be the pole which points above the invariable
   plane of the solar system (regardless of the direction the planet is
   spinning) . A different system is sometimes used, defining a body's
   north and south poles according to the right-hand rule in relation to
   the direction of rotation . In terms of this latter coordinate system
   it was Uranus' north pole which was in sunlight in 1986. On page 47 of
   the September 2006 issue of the Sky at Night magazine, Patrick Moore,
   commenting on the issue, sums it up with "take your pick!"

   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 during the formation of the Solar System, an Earth
   sized protoplanet collided with Uranus, causing 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' equator.

Magnetic field

   Uranus' magnetic field is peculiar since it is not originating from the
   geometric centre 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 7000 K compared to 30
   000 K at Jupiter's core and 18 000 K 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. However, as stated
   above, the weather patterns of Uranus do vary with season, being more
   pronounced at the equinoxes than at the solstices.

Cloud Features

   For a short period in Autumn 2004, a number of large clouds appeared in
   the Uranian atmosphere, giving it a Neptune-like appearance.

Planetary rings

   Uranus with its rings in false color
   Enlarge
   Uranus with its rings in false colour

   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 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' "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' family of inner moons have changed significantly in
   the last decade.

   In April 2006, information about the colour of the outer rings was
   published, one of them appearing spectrally blue and the other red. The
   rest of the planet's rings appear grey. The blue ring is thought to get
   its colour 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

   Uranian moon montage
   Enlarge
   Uranian moon montage

   Uranus has 27 known natural satellites. The names for these satellites
   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
   (compared to Earth's Moon)
   Name

   ( Pronunciation key)
   Diameter
   (km) Mass
   (kg) Orbital radius
   (km) Orbital period
   (d)
                            Miranda mə-ran'-də
                             /mɪˈrændə/ 470
                              (14%) 7.0×10^19
                               (0.1%) 129,000
                                  (35%) 1.4
                                    (5%)
                              Ariel arr'-ee-əl
                             /ˈɛəriəl/ 1160
                               (33%) 14×10^20
                               (1.8%) 191,000
                                  (50%) 2.5
                                    (10%)
                            Umbriel um'-bree-əl
                             /ˈʌmbriəl/ 1170
                               (34%) 12×10^20
                               (1.6%) 266,000
                                  (70%) 4.1
                                    (15%)
                            Titania tə-taan'-yə
                   /tɪˈtɑ:njə/ or /tɪˈteɪnjə/ 1580
                               (45%) 35×10^20
                               (4.8%) 436,000
                                 (115%) 8.7
                                    (30%)
                             Oberon oe'-bər-on
                             /ˈoʊbərɒn/ 1520
                               (44%) 30×10^20
                               (4.1%) 584,000
                                 (150%) 13.5
                                    (50%)

Visibility

   Size comparison of Earth and Uranus
   Enlarge
   Size comparison of Earth and Uranus

   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" (30cm) 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.

Special characters

   Retrieved from " http://en.wikipedia.org/wiki/Uranus"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
   of authors and sources) and is available under the GNU Free
   Documentation License. See also our Disclaimer.
