Get a rare look at the sixth visible planet

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Here is a trivia question: How many planets are visible without a telescope? Most people will answer “five” (Mercury, Venus, Mars, Jupiter and Saturn). But if you answered “six” — congratulations, you can go to the head of the class! That sixth world that can be spied without optical aid is the planet Uranus. This week will be a fine time to try and seek it out, especially since it is now favorably placed for viewing in our evening sky and the bright moon is out of the way.

OF COURSE, you’ll have to know exactly where to look. Barely visible 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. Conveniently, Mars serves as a great guidepost, being just below the more distant world in our sky.

It is best to study the accompanying map first, then scan that region with binoculars. Using a magnification of 150 power with a telescope of at least 3-inch aperture, you should be able to resolve it into a tiny, pale green, featureless disk.

Uranus is on average 1.785 billion miles (3.009 billion kilometers) from the sun. Only Neptune and Pluto are farther away. With a diameter of about 32,000 miles (51,000 kilometers), the greenish world has a rotation period of 17.4 hours.

At last count, Uranus has 24 moons, all in orbits lying in the planet’s equator. There is also a complex of nine narrow, nearly opaque rings, which 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 up and down to its orbit plane. Seasons are therefore extreme: When the sun rises at Uranus’ north pole, it stays up for 42 Earth years; then it sets, and the north pole is in darkness for 42 Earth years.

The British astronomer Sir William Herschel discovered Uranus on March 13, 1781, noting that it was moving slowly through the constellation Gemini. Initially, however, 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 II, who loved the sciences, had the astronomer presented to him and gave 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 for a mythological name 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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