LYING in the outer depths of the Solar System, URANUS is the seventh of the eight known planets from the Sun. This giant gas planet was the first ever telescopically discovered planet, being found within the constellation of Taurus by Sir William Herschel (1738-1822) on 13th March 1781. Uranus orbits some 2.72 billion kilometres or 19.1 A.U. from the Sun, and travels at the mean velocity of 6.8 km.s-1 or roughly one-fifth that of Earth. From Uranus, the Sun would appear as a very bright star, shining at about -20 magnitude — six magnitudes fainter than the Sun as seen from Earth. Subtended size to the same Uranian observer finds the Sun is around 1.5 arcmin, and this varies only by some 2% or 3%.
Although in dark skies this outer planet is just visible to the naked-eye, Uranus remains mostly indistinguishable from the many of its nearby background stars. Although the brightness may only vary by 0.6 magnitudes, it is often better to search for several months before or after opposition, where the maximum visual magnitude reaches about 5.2. When approaching solar conjunction, the brightness often diminishes to about 5.8 or 5.9 magnitude, which makes it harder to spot. A maximum apparent diameter of the disk is merely 4.3 arcsec, and to see this small disk does require apertures at least 7.5cm (3-inch) to 10.5cm. (4-inch) under good seeing conditions.
Uranus has being in the constellation of Pisces in the last few years, but will move into Cetus for short time during mid-2012. Then between the end of 2012, it will return into Pisces until 2017. The given side table shows in which constellation Uranus can be found until 2060. For southern observers, the planet continues to be less favourably placed as it moves into the northern portion of the zodiac. This trend will not be reversed until about 2035 AD, and will be best placed again 2077 AD. Best placement in Sagittarius last occurred in 1993 AD.
Small variations in magnitude occurs depending on the position of Earth in its orbit and the observed true distance to Uranus itself. This also is caused by slight differences between successive oppositions, varying the small size between 3.4 and 3.7 arcsec (3.55±0.15″). Other variations are caused by the small elliptical orbit of the planet, whose almost circular or orbital eccentricity, ‘e’, is 0.0467 — 0.000 being a circle. True heliocentric distances throughout the 83¾ year-long orbit varies between 18.327 AU and 20.171 AU (4.7% difference), averaging 19.218±0.953 AU. When Uranus was closest to the Sun, or periapsis, was last on 23rd March 1967, and this will not occur again until 2nd December 2050. Furthest distance from the Sun, or apapsis, happened on 26th January 2009. At the moment, Uranus is therefore not at its ideal largest apparent diameter for the visual or telescopic observer.
Finding Uranus is certainly helped by always being located within all the twelve familiar zodiac constellations, though to be sure you have identified it correctly really requires several successive nights of visual observations to detect some real movement. Using the telescope is more of an advantage, because the planet is not quite stellar, but instead, will display its usual small bluish or bluish-green discernible disk.
DISCOVERY of URANUS
Uranus was first new planet to be discovered since antiquity. It was accidentally found by William Herschel using his own constructed telescope by observing the changes in sky position over several nights of an uncatalogued ‘star’ that appeared to move against several background field stars. This unusual object was found one night from his yard from Bath in Somerset, England, which he initially thought was either some undiscovered deep-sky hazy star or comet. Once the object showed some movement, Herschel decided it was a new comet. After checking his results, he then announced the discovery of this puzzling new object on 26th April 1781 to the Royal Society of London. Soon it was realised it was not a comet but a new planet of the Solar Sytem.
This very unique discovery proved to be surprising and totally unexpected revelation to astronomers at the time. For William Herschel finding Uranus gained him much prestige and fame and towards his English patron, King George III. It is extraordinary to think that although this sizeable planet was visible by the naked-eye, it still was completely missed by other earlier observers. This is especially poignant because the planet both lies so close to the ecliptic and also regularly passes the other planets, the Moon and other bright catalogued stars. It is likely no one had seriously considered that there were any additional planets to be found, especially held that the ‘classical’ five known planets (Mercury, Venus, Mars, Jupiter and Saturn) had were known from prehistorical times. Once known, observers went back through their own records where they found that the presumed “star” was really Uranus.
First of these astrometric positional observations were made in 1690 by the English astronomer John Flamsteed (1646-1719) at Greenwich, who amazingly catalogued the planet as the star 34 Tauri. Even Pierre Le Monnier (1715-1799) observations between 1750 and 1769 at Paris Observatory in France, amazingly saw Uranus no less than twelve times, but totally failed to recognise its importance even when observing four nights in a row without even suspecting or noting any movement!
Uranus soon lead to new and sometimes frantic searches for other planetary bodies in the Solar System, which later result in the discovery of the eighth and final planet of the known Solar System, Neptune. Between these two outer planet discoveries, there were the finding of the four first asteriods; Ceres in 1801, now classed as a dwarf planet, Pallas, Juno and lastly Vesta. All four were quickly ruled out as planets, as they were only small bodies, and showed no discernable sized disks.
Today, Uranus has now moved into its third observed orbit since discovery, which was reached on 9th September 1991. Uranus’ true fourth birthday will next be in 15th October 2033. A small attached table shows dates of whole or half of orbits.
MYTHOLOGY of URANUS
Uranus was named after the first ruling Greek deity of the whole Universe and the Heavens. He is one of the oldest and original gods in Greek mythology, who was also known to the Romans as Coelus or Cœlus. Uranus was born from the foremost deity named Chaos, shared with his sister Gaia also known as Roman Tellus (Earth). These two principal gods ruled over universe, whose domains were related to Earth and sky. The famous twelve children of these parents, were six males and six females, who became to be known collectively as the Titans. Uranus also the father to Cronos (Greek) who is also now known as the Roman god, Saturn.
Uranus was portrayed as being a cruel and overbearing father, whose eventual supremacy was final toppled by his Titan child, Zeus (Jupiter). His first-born son lead an open rebellion against his throne, punishing Uranus with castration so he could bear no more children as possible rivals, ostracising from his offspring, then banishing him for eternity from his beloved heaven. Zeus, the strongest son, soon usurped the throne for himself, and ruled his domain from Mount Olympus with the Pantheon of gods over the whole Earth, its people and all living creatures.
[See the much fuller discussion of these related mythologies in Saturn Pt.2.]
Relation With Astronomy
Uranus namesake is still used in modern language. For example, it is associated directly with the branch of astronomical science known as uranography or stellar cartography, which is the process of mapping and generally describing the starry heavens. The original word relates to the heavens, coming from the Greek word, ourmas, meaning for heaven or sky. Such formal studies were importantly made by several 16th and 17th century astronomers, known as uranographers, who aimed to accurately map the sky constellations with the precise star positions within them. Then the main purpose was to find more about the distances and proper motion of stars. Some of the published and often embellished works of these maps were akin to skilled artists portraying the heavens for general consumption by the populous.
Uranography is now considered a antiquated term, who modern usage is now more associated astrometry, using telescopes and instrumental techniques to position star very accurately, including their brightnesses, distances and proper motions. It is also associated with Johann Bayer’s famous late 17th Century star atlas and catalogue called Uranometria.
Another association is with the prescribed phobia of uranophobia whose individuals having a fear of heaven.
The Roman god, Coelus or Cœlus also appears in English, even though its common usage is on the slow decline. A Cœlestat or coelostat is an optical design with two flat mirrors, which follows the Sun across the sky. Another flat mirror reflects the solar image of the sun that does not rotate as the Sun moves. Optics of the heliostat can be either one or two mirrored systems.
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Last Update : 25th September 2012
Southern Astronomical Delights © (2012)
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