Image: Uranus
Space.com / Starry Night Software
Uranus can be found high in the southern sky after midnight, floating just off the corner of Aquarius.
By Skywatching columnist
updated 10/3/2008 7:17:11 PM ET 2008-10-03T23:17:11

Here is a trivia question: How many planets are visible without a telescope? Most will answer "five" (Mercury, Venus, Mars, Jupiter and Saturn). Some might answer "six" and include Earth in the mix. Six, in fact, is the correct number. If you exclude our own world, there is indeed one other planet that can be spied without optical aid: the planet Uranus.

This week will be a fine time to seek it out, especially since Uranus is now favorably placed for viewing in our evening sky and the waxing moon is not overly bright.

Of course, you'll have to know exactly where to look for it. Barely discernable by a keen naked eye on very dark, clear nights, Uranus — currently shining at magnitude +5.7 — is now visible during the evening hours among the stars of Aquarius the Water Carrier. On this scale, larger numbers are dimmer, with magnitude 6.0 being about the dimmest object visible under ideal, dark-sky conditions.

Tilted giant
It is best to study a sky chart first, and then scan that region with binoculars. Using a magnification of 150 power with a telescope of at least 3-inch (75mm) aperture, you should be able to resolve it into a tiny, pale-green featureless disk.

Uranus, which currently lies 1.868 billion miles (3.005 billion kilometers) from the sun, has a diameter of about 31,700 miles (51,120 kilometers) and, according to flyby magnetic data gathered by Voyager 2 in 1986, has a rotation period of 17.4 hours.

At last count, Uranus has 27 moons, all lying in orbits around the planet's equator, in which there is also a complex of nine narrow, nearly opaque rings that were discovered in 1978.

Uranus likely has a rocky core, surrounded by a liquid mantle of water, methane and ammonia, encased in an atmosphere of hydrogen and helium. A bizarre feature is how far over Uranus is tipped. Its north pole lies 98 degrees from being directly perpendicular to its orbit plane. Thus, its seasons are extreme: When the sun rises at its north pole, it stays up for 42 Earth-years; then it sets and the north pole is in darkness for 42 Earth-years.

Fortuitous find
The British astronomer Sir William Herschel discovered Uranus on March 13, 1781, noting that it was moving slowly through the constellation Gemini. Initially, Herschel thought he had discovered a new comet.

Soon the name of Herschel became known over all of Europe together with the news of his discovery. King George III, who loved the sciences, had the astronomer presented to him, and bestowed upon him a life pension and a residence at Slough, in the neighborhood of Windsor Castle.

Eventually it was determined that Herschel's "comet" was, in fact, a new planet. For a while, it actually bore Herschel's name. Herschel himself proposed the name Georgium Sidus — "The Star of George," after his benefactor. However, the custom of using mythological names ultimately prevailed and the new planet was finally christened Uranus. Prior to its discovery, the outermost planet was considered to be Saturn, named for the ancient god of time and destiny, but Uranus was the father of Saturn and considered the most ancient deity of all.

It probably was for all for the best. After all, if Herschel's request was granted, just think of how we might have listed the planets in order from the sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn and ... George?

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.

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