Gas giant Uranus, the 7th planet from the Sun reaches opposition on the 26th September 2011 when it will be at its best for the year. Now is a perfect time to observe, perhaps for the first time, this far distant world.
With an apparent magnitude varying between 5.3 and 5.9, Uranus is a naked eye target. However, it is a dim world, with a small apparent size that moves so slowly it was unknown to ancient observers. For naked eye star watchers with dark skies bagging Uranus is a worthy achievement to cross off the list, but for most suburban observers, binoculars or a small telescope is required. But whatever optical instrument you use or don't use, a good star chart is an essential requirement and we have that chart for you.
Uranus was observed by Sir William Herschel on the 13th March 1781 while in the garden of his house at 19 New King Street, Bath using his 7-foot long (2.1m), 6-inch (150 mm) aperture Newtonian telescope. Herschel initially thought the planet was a comet and duly reported his discovery a few weeks later to the Royal Society. While Herschel continued to cautiously describe his new object as a comet, it soon became obvious that the new object was a planet way beyond Saturn. Herschel himself acknowledged the fact and in recognition of his achievement, King George III gave Herschel an annual stipend of £200. Such a bonus did not come without condition and the condition was that Herschel moved to Windsor with his telescopes so that the Royal Family could also look through them!
It's perhaps a little bit surprising that Uranus was not identified as a planet earlier. On many occasions before Herschel's discovery the planet was observed but it was usually mistaken as a star. John Flamsteed observed the planet on at least six times in 1690 and even catalogued it as 34 Tauri, while the French astronomer Pierre Lemonnier observed Uranus at least twelve times between 1750 and 1769 but failed to notice its true nature.
This year's opposition of Uranus takes place on the 26th September 2011 when the planet is located in the constellation of Pisces "the Fishes". At magnitude 5.7, Uranus is a visible to the naked eye under a dark sky and is an easy object for binocular or small telescope observers.
Pisces is a faint but large zodiac constellation with an area of 889 square degrees. It ranks as the 14th largest constellation. Uranus at opposition is located in a relatively sparse section of Pisces devoid of any bright stars but is relatively easy to identify once you have located the correct region of sky.
Although Pisces is faint there is an excellent starting point nearby…the Great Square of Pegasus. First locate the square and then move about 15 degrees due south to locate the circlet shape that forms one of the two historical Pisces fish. None of the stars that form the circlet are brighter than 4th magnitude, but aim for the SE corner star, mag. 4.49 λ Psc. Once there scan about 7 degrees to the east and 2 degrees south to locate Uranus, which is surrounded by a number of other similar brightness stars.
Despite being the 3rd largest planet by size in the solar system, Uranus is a located at a vast distance of 19.08 AU (2.85 billion kilometers or 1.77 billion miles) from Earth. Hence at opposition the apparent size is only 3.7 arc seconds. A telescope at high magnification will reveal a small obvious non-stellar but blank disk that may appear slightly blue-greenish.
Of Uranus's 27 known satellites four can be seen in moderate to large amateur telescopes. The largest two, Titania and Oberon have been glimpsed with apertures as small as 200mm (8 inches) but usually a larger scope is required. The next largest two, Umbriel and Ariel lie much closer to the planets glare and hence are more difficult requiring much larger aperture telescopes.
Our knowledge of Uranus greatly increased after the flyby of NASA's Voyager 2 space probe in 1986. To date Voyager is the only probe to have visited the planet and provided much of the scientific knowledge about the planets atmosphere, unique weather caused by its axial tilt of 97.77 degrees and its magnetic field. Voyager also photographed in detail and investigated Uranus five largest moons, Titania, Oberon, Umbriel, Ariel and Miranda. In addition, Voyager discovered 10 new moons.
For the next stage of spacecraft exploration of Uranus, NASA has recommended an orbiter and probe for launch sometime between 2020 and 2023. The journey to Uranus will take 13 years.