Uranus the Blue Green Ice Giant: Sideways Seventh Planet From the Sun

Summary: Blue green ice giant Uranus has a sideways rotation and is visible to the naked eye despite its orbital placement as the seventh planet from the sun.

view of Uranus in 1986 from Voyager 2, first spacecraft to visit the sideways planet's neighborhood: NASA/JPL-Caltech, CC BY 2.0, via NASA Photojournal

German-born English astronomer and musician Sir William Herschel (Nov. 15, 1738-Aug. 25, 1822) is credited with the official discovery of the planet Uranus on March 13, 1781. William noticed the planet, which he at first suspected to be either a comet or a star, while scanning the skies through his customized, homemade 7-foot reflecting telescope in his back yard in Bath, in South West England.

By 1783, William had designated his puzzling discovery as a planet and named it Georgium Sidus ("George's Star") to honor the reigning king, George III (June 4, 1738-Jan. 9, 1848). Other suggestions for renaming the Georgian Planet were proposed until Uranus gained universal acceptance in 1850.

Uranus holds the honor of being the first planet to be discovered by a telescope and, therefore, the first planet to be discovered in modern times.

German astronomer Johann Elert Bode (Jan. 19, 1747-Nov. 23, 1826) proposed Uranus as the planet's name. He was impassioned by Uranus' discovery to uncover 17th century sightings of Uranus, mistakenly recorded as a star, by poring through old star charts.

English astronomer John Flamsteed (Aug. 19, 1646-Dec. 31, 1719) is credited, thanks to Johann Bode's diligent research, with the first official recorded sighting of Uranus. On Dec. 23, 1690, the blue-green ice giant was mistakenly noted and named as a star, 34 Tauri, in the constellation Taurus the Bull. Flamsteed was appointed as the first Astronomer Royal in 1675 with the founding of the Royal Observatory Greenwich.

Actually, Uranus is visible without optical aids against a backdrop of darkened skies. The dimly-lit planet is not as easy to spot as the five brightest planets, known as the naked eye planets: Mercury, Venus, Mars, Jupiter and Saturn. The earliest naked eye sighting of Uranus, mistaken as a star in the constellation Virgo, may have been made by Greek astronomer and mathematician Hipparchus of Nicaea (ca. 190-ca. 120 BCE) in 128 BCE.

Uranus orbits the sun from an average distance of 1.8 billion (1,800,000,000) miles, which equates to 2.9 billion (2,900,000,000) kilometers. In Uranian time, one year equals about 84 Earth years, the amount of time for Uranus to complete a full orbit around the sun.

One day lasts about 17 hours, the amount of time required for Uranus to make a complete rotation on its axis. Uranus joins Venus as the only two planets in the solar system with a retrograde, or backward, rotation. While the other planets rotate from west to east, Uranus and Venus spin from east to west.

Uranus' east-to-west rotation actually is described better as a south-to-north spin. Uranus is known as the sideways planet because its geographic and magnetic north and south poles display such dramatic misalignment that Uranus virtually spins on its sides. Uranus' axial tilt, the angle between its rotational axis and its orbital axis, registers at a lopsided 98 degrees, in comparison to the Earth's tilt of 23.5 degrees.

The reason for Uranus' uniquely skewered magnetic field geography is unknown but is routinely explained as stemming from an early interplanetary collision with an earth-sized protoplanet. An alternative hypothesis proffered by the Observatoire de Paris in 2009 suggests a collision-free scenario prompted by ejection of a larger moon, responsible for the tilt early in Uranus' formation, during planetary migrations as the planets settled into their current orbits.

Uranus joins its neighbor, Neptune, as the solar system's only ice giants, giant planets primarily composed of elements heavier than helium and hydrogen and with ices as their constituent compounds during planetary formation. Methane gas in Uranus' atmosphere causes the planet's brilliant blue-green color.

Uranus belongs to a quartet of giant planets encircled by rings. Along with Uranus, the Jovian ringed planets are composed of Neptune and the two gas giants, Jupiter and Saturn.

Uranus remains a tantalizing mystery, with many secrets to glean and few facts to know with certainty. Launched Sept. 5, 1977, with a focus on Jupiter and Saturn, Voyager 1 was the first spacecraft to photograph Uranus. Its planetary images were taken through a narrow angle camera with 1500 mm focal length and with violet, blue and green filters. Earthlings received their first close-up view of Uranus with Voyager 2's flyby visit Jan. 24, 1986, at a closest approach of 50,600 miles (81,500 kilometers).

NASA Hubble Space Telescope (HST) has been sending intriguing images of the seventh planet since its first photographs taken Aug. 14, 1998. Hubble images taken in 2005 surprised scientists by revealing a pair of hitherto unknown dusty rings.

view of Uranus from Voyager 1, first spacecraft to photograph the seventh planet from the Sun: February 14, 1990 image by NASA/JPL-Caltech ~ NASA Planetary Photojournal, CC BY 2.0, via NASA

Acknowledgment

My special thanks to talented artists and photographers/concerned organizations who make their fine images available on the internet.

Image credits:

solar system family portrait - Uranus, taken Feb. 14, 1990, by Voyager 1 at a distance of more than 4 billion miles from Earth: NASA Planetary Photojournal, CC BY 2.0, via NASA @ http://solarsystem.nasa.gov/multimedia/display.cfm?Category=Planets&IM_ID=187

edge-on views of Uranus' rings 2003, 2005, 2007: NASA, ESA and M. Showalter (SETI Institute). Aug. 23, 2007. Available via Hubblesite @ http://hubblesite.org/newscenter/archive/releases/2007/32/image/a/format/web_print/

PIA18182 Uranus as seen by NASA's Voyager 2 in 1986: NASA/JPL-Caltech, CC BY 2.0, via NASA Photojournal @ http://photojournal.jpl.nasa.gov/catalog/PIA18182

For further information:

Boué, Gwenaël, and Jacques Laskar. "A Collisionless Scenario for Uranus Tilting." arXiv.org > Abstracts > Astrophysics > Earth and Planetary Astrophysics.  Version 2: Feb. 9, 2010.
Available via arXiv @ http://arxiv.org/pdf/0912.0181v2.pdf

Brown, Dwayne, George Deutsch, and Ray Villard. "NASA's Hubble Discovers New Rings and Moons Around Uranus." Hubblesite > Newscenter > News Release Archive. Dec. 22, 2005.
Available @ http://hubblesite.org/newscenter/archive/releases/2005/33/text/

Forbes, Eric G. "Early Astronomical Researches of John Flamsteed." Journal of the History of Astronomy, Vol. 7, No. 2 (June 1976): 124 - 138.
Available via SAO/NASA Astrophysics Data System @ http://adsabs.harvard.edu/full/1976JHA.....7..124F

Hunt, Garry E., and Patrick Moore. Atlas of Uranus. Cambridge UK, New York NY, Melbourne Australia: Cambridge University Press, 1989.

Marriner, Derdriu. "Uranus Discovery 234 Years Ago on March 13, 1781, by Sir William Herschel." Earth and Space News. Friday, March 13, 2015.
Available @ https://earth-and-space-news.blogspot.com/2015/03/uranus-discovery-234-years-ago-on-march.html

"Target is Uranus." Jet Propulsion Laboratory > Photojournal.
Available via NASA @ http://photojournal.jpl.nasa.gov/target/Uranus

"Uranus and Neptune." NASA's Cosmos > Planets and Moons.
Available via Tufts University @ http://ase.tufts.edu/cosmos/print_chapter.asp?id=11

"Uranus: Overview." Solar System Exploration > Planets > Uranus > Overview.
Available via NASA @ http://solarsystem.nasa.gov/planets/profile.cfm?object=Uranus