Uranus Help (page 2)
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.
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