Planet Uranus

BY SHANE Blok

Astronomy and Space Science Writer Member, Sri Lanka Astronomical Association

URANUS is the seventh planet from the Sun. Uranus was discovered in 1781 by William Herschel. It is a large ball of gas, four times as big as Earth. The outer layers of gas are hydrogen and helium. Scientists do not know yet what may lie beneath.

Some scientists think there is a small, rocky core, surrounded by a mantle of frozen water, ammonia and methane. Voyager 2 flew by Uranus in 1986. The axis of Uranus is so tilted that the planet seems to be lying on side. The rings of Uranus cannot be seen with a telescope.

Uranus has a pale blue appearance. Nothing much seems to have happened in the atmosphere of this work. At some time in the past probably when the planets were forming, Uranus was hit by a very large asteroid which caused it to tilt on its side.

It now rolls over and over instead of spinning around and round like the other planets. This means that its equator is from top to bottom instead of side to side. The twenty one or so moons of Uranus orbit around its equator so they, too, go over the top of the planet and round underneath.

Uranus, like Jupiter and Saturn, has a banded appearance, which is barely visible from Earth. The cloud motions reveal a predominantly zonal circulation, where the winds are blowing in a east - west direction rather than from north to south.

This circulation resembles the flow on Jupiter and Saturn and, to a lesser extent, the motions on every planet in the Solar System.

Structure and composition

Uranus has the least interesting appearance of any of the planets. Like Jupiter and Saturn it is covered in clouds, but its clouds are almost featureless, apart from a few brighter streaks that were photographed by the Voyager 2 space probe.

The clouds of Uranus consist of methane ice crystals and appear blue-green because methane gas in the atmosphere above filters out other colours.

Rings and moons

Uranus has 11 thin rings and 21 known moons. The rings are too faint to be seen with the size of telescope used by amateur observers. The outermost ring, called the Epsilon ring, is 100 km (60 miles) wide and has "shepherd" moons, Cordelia and Ophelia, one on each side.

The moons too are faint; even the biggest and brightest, Titania, is of only 14th magnitude and hence cannot be seen without a large telescope.

Ten of Uranus' twenty one moons were discovered by the Voyager 2 space probe in 1986. The moons are named after characters in the writings of William Shakespeare and Alexander Pope.

The largest, Titania, is less than half the size of Earth's moon. The two more distant moons were spotted from Earth in 1997. They are 10 times farther out than Oberon, and orbit Uranus in the opposite direction from all other moons.

Vital statistics

The diameter of Uranus is 50,800 km. The distance from the Sun is 2,866,90,000 km (Two billion eight hundred and sixty six million ninety thousand).

The nearest distance to Earth is 2,720,000,000 km (Two billion seven hundred and twenty million). Uranus orbits the Sun in 84 Earth years, and its extreme tilt gives it unusually long seasons.

As the planet follows its 84-year orbit around the Sun, each pole has 42 years of continuous sunlight, followed by 42 years of darkness. Uranus rotates on its axis once in 17 hours 54 minutes.

The temperature is -215 degrees centigrade, at cloud tops. The atmosphere in Uranus is a thick one including hydrogen and helium.

Seeing Uranus

At its brightest, Uranus can be glimpsed with the naked eye if you know just where to look. Like the other planets, though, it appears to change its position among the stars. It looks like a star, although it does not twinkle.

References:

Stars and planets
Journey to the stars
Atlas of the universe
The big book of space
Astronomy Encyclopedia
Book of astronomy and space.