Basic Astronomical Data

At Uranus's distance from the Sun, the planet takes slightly more than 84 Earth years, essentially an entire human life span, to complete one orbit. The eccentricity of its orbit is low—that is, its orbit deviates little from a perfect circle—and the inclination of the orbit to the eclip-tic—the plane of Earth's orbit and nearly the plane of the solar system in general— is less than 1°. Low orbital eccentricity and inclination are characteristic of the planets of the solar system, with the notable exceptions of Mercury and Pluto. Scientists believe that collisions and gaseous drag removed energy from the orbits while the planets were forming and so reduced the eccentricities and inclinations to their present values. Thus, Uranus formed with the other planets soon after the birth of the Sun nearly 4.6 billion years ago.

Uranus and its neighbour Neptune, the next planet outward from the Sun, are nearly twins in size. Measured at the level of the atmosphere at which the pressure is one bar (equivalent to Earth's sea-level pressure), Uranus's equatorial radius of 25,559 km (15,882 miles) is 3.2 percent greater than that of Neptune. But Uranus has only 85 percent the mass of Neptune and thus is significantly less dense. The difference in their bulk den-sities—1.285 and 1.64 grams per cubic cm, respectively—reveals a fundamental difference in composition and internal structure. Although Uranus and Neptune are significantly larger than the terrestrial planets, their radii are less than half those of the largest planets, Jupiter and Saturn.

Because Uranus's spin axis is not perfectly parallel to the ecliptic, one of its poles is directed above the ecliptic and the other below it. (The terms above and below refer to the same sides of the ecliptic as Earth's North and South poles, respectively.) According to international convention, the north pole of a planet is defined as the pole that is above the ecliptic regardless of the direction in which the planet is spinning. In terms of this definition, Uranus spins clockwise, or in a retrograde fashion, about its north pole, which is opposite to the prograde spin of Earth and most of the other planets. When Voyager 2 flew by Uranus in 1986, the north pole was in darkness, and the Sun was almost directly overhead at the south pole. In 42 years, or one-half the Uranian year, the Sun will have moved to a position nearly overhead at the north pole. The prevailing theory is that the severe tilt arose during the final stages of planetary accretion when bodies comparable in size to the present planets collided in a series of violent events that knocked Uranus on its side. An alternate theory is that a Mars-sized moon, orbiting Uranus in a direction opposite to the planet's spin, eventually crashed into the planet and knocked it on its side.

Uranus's rotation period of 17.24 hours was inferred when Voyager 2 detected radio wave emissions with that period coming from charged particles trapped in the planet's magnetic field. Subsequent direct measurements of the field showed that it is tilted at an angle of

58.6° relative to the rotation axis and that it turns with the same 17.24-hour period. Because the field is thought to be generated in the electrically conducting interior of the planet, the 17.24-hour period is assumed to be that of the interior. The relatively fast rotation causes an oblateness, or flattening of the planet's poles, such that the polar radius is about 2.3 percent smaller than the equatorial radius. Winds in the atmosphere cause cloud markings on the visible surface to rotate around the planet with periods ranging from 18 hours near the equator to slightly more than 14 hours at higher latitudes.

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