Uranus and Moons - Astronomy Matters

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Uranus and Moons . . .

Uranus, pronounced YUR uh nuhs, was the first god of the sky in Greek mythology. According to the ancient Greek poet Hesiod, Uranus was the child of Gaea, or Ge, who was the earth. Uranus had no father.

Uranus was the first planet discovered since ancient times. British astronomer William Herschel discovered it in 1781. Johann E. Bode, a German astronomer, named it Uranus after a sky god in Greek mythology. Most of our information about Uranus comes from the flight of the United States spacecraft Voyager 2. In 1986, that craft flew within about 50,000 miles (80,000 kilometers) of the planet's cloud tops.

Uranus is a giant ball of gas and liquid. Its diameter at the equator is 31,763 miles (51,118 kilometers), over four times that of Earth. The surface of Uranus consists of blue-green clouds made up of tiny crystals of methane. The crystals have frozen out of the planet's atmosphere. Far below the visible clouds are probably thicker cloud layers made up of liquid water and crystals of ammonia ice. Deeper still-about 4,700 miles (7,500 kilometers) below the visible cloud tops-may be an ocean of liquid water containing dissolved ammonia. At the very center of the planet may be a rocky core about the size of Earth. Scientists doubt Uranus has any form of life.

Mass and density: Uranus has a mass (quantity of matter) 14 ½ times larger than that of Earth. However, the mass of Uranus is only about 1/20 as large as that of the largest planet, Jupiter.

Orbit and rotation: Uranus travels around the sun in an elliptical (oval-shaped) orbit, which it completes in 30,685 Earth days, or just over 84 Earth years. As it orbits the sun, Uranus also rotates on its axis, an imaginary line through its center. The planet's interior (ocean and core) takes 17 hours 14 minutes to spin around once on its axis. However, much of the atmosphere rotates faster than that. The fastest winds on Uranus, measured about two-thirds of the way from the equator to the south pole, blow at about 450 miles per hour (720 kilometers per hour). Thus, this area toward the south pole makes one complete rotation every 14 hours.

Uranus is tilted so far on its side that its axis lies nearly level with its path around the sun. Scientists measure the tilt of a planet relative to a line at a right angle to the orbital plane, an imaginary surface touching all points of the orbit. Most planets' axes tilt less than 30°. For example, the tilt of Earth's axis is about 23 ½. But Uranus's axis tilts 98°, so that the axis lies almost in the orbital plane. Many astronomers think that a collision with an Earth-sized planet may have knocked Uranus on its side soon after it was formed.

This false-color image taken by the Hubble Space Telescope using infrared light shows Uranus's rings and clouds. The different colors in the image represent different atmospheric conditions.