Uranus Help (page 3)
Introduction to Uranus
Uranus (pronounced “YOU-run-us”) is approximately twice as far from the Sun as is Saturn: 2,871 million kilometers (1,784 million miles). This is 19.22 AU, so Uranus receives only about (1/19.22) 2 , or 1/369, as much sunlight per unit area as does our planet.
Finding And Observing Uranus
Uranus is so far from the Sun that its brightness does not change very much as the Earth revolves. However, observation is still the best when Uranus is at or near opposition (Fig. 7-6). If you were to travel to Uranus, the Sun would be as dim as it is during the darkest part of an annular solar eclipse as seen from Earth.
Uranus can be seen with the unaided eye, but just barely. It is better if you have a pair of strong binoculars or a small telescope. You must know exactly where to look if you are to find it. A good Web-based astronomical map can be found at the Weather Underground Web site:
Click on the “Astronomy” link and proceed according to the instructions to get a sky map for your area. Celestron also publishes a CD-ROM called The Sky that works in a similar way. When you find Uranus, don’t expect much. If you have a large telescope, you should be able to resolve it into a bluish green disk, and you also might see one or two of its moons.
Year, Day, Seasons
In several respects, Uranus is unique. It is tilted on its axis so much that the axis is only 8 degrees away from lying in the planet’s orbital plane. Some texts say the axis is tilted by 98 degrees; however, because the north pole of a planet is generally defined as the pole that lies “above” the planet’s orbital plane (generally toward the star Polaris), it is more precise to say that Uranus’ axis is tilted by 82 degrees and its rotation is retrograde.
Uranus has dramatic, exaggerated seasons of variable daylight and darkness. When Voyager 2 visited the planet in 1986, one pole was facing almost directly toward the Sun. Despite the fact that half the planet remains in total darkness for many Earth years at a time, however, the temperature on the dark side is just about the same as that on the daylight side.
The Uranian year is 84 Earth years long. The period of rotation is a little less than 18 hours, about three-quarters of an Earth day. We must be careful about how we define a “day” on Uranus. The best scheme is to use the planet’s rotational period. If someone lived on Uranus (not likely for the same reasons as Jupiter and Saturn are unlivable), they would want to use a 24-hour system where each Uranian “hour” lasts about 45 Earth minutes.
Because the axis of Uranus is almost parallel to the plane of the planet’s orbit around the Sun, daylight over much of the planet lasts for many Earth years, followed by an interval of Earthlike daylight and darkness hours, followed by many Earth years of continuous darkness, followed by another interval of Earthlike daylight and darkness hours, followed by continuous daylight for many years, and so on. Only in the immediate equatorial region do Earthlike daylight and darkness occur in sync with the planet’s rotation all the Uranian year round.
A Lifetime On Uranus
Imagine what it would be like to live your whole life on Uranus, born on January 1, 1986. (Suspend your disbelief for a moment and pretend that Uranus has a surface on which humans can live; astronomers believe that it does not.) Let us say you dwelt at 45 degrees latitude, the equivalent of Minneapolis, Minnesota.
When Voyager 2 made its visit in 1986, you would have just been born into the middle of a long night. You would never have seen daylight. The Sun would never come anywhere near rising above the horizon. It would not be until you were in grade school that, every 18 Earth hours, you would see a glimmer of twilight. As winter’s dark grip (though no colder than any other season) came to an end, the Sun would give you a peek. The days would grow longer, and the Sun’s course across the sky would get higher.
At the Uranian vernal equinox, when you were 21 Earth years old, the Sun would travel in an Earthlike way across the sky, behaving like it does in Minneapolis around the end of March or September, but with daylight lasting only 9 Earth hours and darkness another 9 Earth hours. Perhaps you would read in books (or on computer screens) about the changes of seasons in places like Minnesota and be thankful that no such fluctuations in temperature occurred on Uranus. The season would evolve, the Sun would move further toward the celestial pole, and the hours of daylight would grow long. For a while, the Sun would follow a path similar to that in midsummer in Minneapolis, but it would still be early spring on Uranus. The daylight hours would grow longer yet and the darkness hours shorter. One night darkness would never fall. After that, you would have a full 18 hours of daylight every day, and as the Earth years continued to progress, the Sun would describe a smaller and smaller circle in the sky around the celestial pole.
On the day of the summer solstice when you were 42 Earth years old, the Sun would follow a circle only 8 angular degrees in radius around the celestial pole. You would have become used to continuous daylight. The Sun would shine down through the Uranian haze at a favorable angle, casting a shadow just about as long as you were tall. As summer progressed, the circle would widen; your shadows would change length and orientation more noticeably with the time of day. After a few more Earth years, the Sun would rise to near the zenith at noon and dip to near the horizon at midnight. Then one midnight, the Sun would plunge below the horizon beneath the celestial pole. Twilight would once again become a familiar sight. The twilights would become longer and darker until one midnight darkness would become total for a while. Imagine the emotional effects of the first total darkness you had seen in a quarter of a lifetime!
At the autumnal equinox, when you were 63 Earth years old, the daylight and darkness hours would be of equal length. The days would grow shorter. You might remember the first time you saw the Sun as a child. As an elderly adult, you would see the Sun for the last time. In the days that followed, the noon twilight would grow dimmer and shorter until once again continuous darkness would reign. You would never see the Sun again, unless, of course, you moved to a southerly latitude.
Composition, Atmosphere, And Weather
Uranus is about 51,000 kilometers (32,000 miles) in diameter, which is almost exactly four times the diameter of Earth (Fig. 7-7). When the Voyager space probe approached this planet in 1986, Earth-based scientists were disappointed. The face of Uranus is featureless, light aquamarine in color, and reminiscent of a greenish blue–tinted, enlarged version of Venus. Beneath that bland face, however, Uranus is nothing like Venus. Uranus consists of hydrogen, helium, methane, ammonia, and perhaps water ice mixed in with rocky material.
Uranus is cold in its upper layers but is believed to have a hot interior. Just how hot is a mystery, though, because Uranus, unlike the other three “gas giants” Jupiter, Saturn, and Neptune, does not radiate more energy toward the Sun than it receives. Perhaps Uranus is cooler inside than the other “gas giants,” or maybe Uranus is better insulated. By examining the extent to which Uranus is flattened by its rotation, and by measuring the relative proportions of hydrogen, helium, methane, and other substances in its visible outer layer, astronomers have begun to suspect that Uranus might be a gigantic ball of dirty slush. Such a planet would have no defined boundaries in its depths. It would be gaseous on the outside, then liquid, then slush, and then goo at the core.
Further missions to Uranus will be necessary to get a better idea of what, exactly, this planet is made of. One thing is certain, however: It manages very well to maintain a constant temperature in its outer layers despite the exaggerated seasonal changes in solar irradiation.
The climatic system that governs the temperatures on Uranus might be called the great thermal equalizer . If Earth were tilted on its axis as much as is Uranus, the weather on our planet would be incredibly severe. Winters would be brutal everywhere except at the very lowest latitudes. The prevailing winds would be fierce as they attempted to equalize the radical annual seesaw of solar energy received at most points on the surface. Hurricanes of unimaginable size would prowl the seas and slam into land masses. The whole course of the evolution of life on our planet would be much different from what it was. We cannot be certain that intelligent life would have evolved at all.
When the Voyager probe visited Uranus in January of 1986, the rings, which exist in the plane of the planet’s equator, were seen in detail for the first time. Astronomers were not surprised to find them; their existence was known already because, as Uranus passed near distant stars, those stars seemed to blink several times. The only possible cause for such blinking was the existence of thin, nearly opaque rings around the planet. The fact that Uranus is tipped on its side so that the rings sometimes appear as pronounced ellipses (almost circles when either pole is nearly facing us) was an assist in their discovery prior to the Voyager “grand tour.”
The rings of Uranus are much different than those of Saturn; they more resemble the faint rings around Jupiter. The albedo (proportion of light reflected) of the rings is only about 1 percent, similar to that of charcoal. If you were to visit the Uranian system in a space ship, you would have a hard time seeing the rings even if you passed right through their plane. Only if you actually struck one would you notice it easily; this would not be likely because the rings are exceedingly narrow. However, the rings consist of good-sized rocks, generally on the order of 70 centimeters (28 inches) or larger in diameter. You would not want to navigate your ship through them.
The narrowness of the Uranian rings seems to run contrary to dynamics. The natural tendency, over time, is for the rings to spread out and become more flattened, like those of Saturn (although much less prominent). However, small moons orbit near the rings, and the gravitational fields from these moons tends to force and keep the rings into narrow circles. Moons of this type have been observed inside the system of Saturn, too, accounting for some of the narrow rings there. Because these moons act to confine the ring particles and hold them within specific orbits, they have been termed shepherd moons .
Practice problems of this concept can be found at: The Outer Planets Practice Problems
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