Uranus reached opposition on August 27, bringing it to a point opposite the sun in sky and marking the Uranus observing season. At that time it was 20.1 AU from the Sun and 19 AU from the earth (1 AU is the mean distance between the Earth and Sun). This month it shines at magnitude 5.7, making it visible in binoculars and even to the naked eye of a keen observer. The two outer moons of Uranus, Titania and Oberon, will be ~14th magnitude; bright enough to observe in an eight-inch telescope. The two inner moons, Ariel and Umbriel, are both fainter and closer to the glare of the planet; requiring at least a 16-inch instrument.
Uranus 2004 Sep 15
| Right Ascension | Declination | Magnitude | Diameter | Con |
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3.7" | Aqr |
What You Will Need To Observe Uranus And It's Moons
For best results use as much magnification as the conditions will permit to see the tiny satellites.
Darker skies and/or an eight-inch or larger telescope will be required to spot the two outer moons, 13.9 magnitude Titania and 14.1 magnitude Oberon. Dark skies and at least a 16-inch scope will be required to observe the inner two major moons.
Finder
Charts for Uranus in September.
What You Will See
Use
as much magnification as the conditions will bear to see Uranus's tiny
blue-green disk. Although it is currently twice the apparent size
of Neptune, it is still quite small. At right is a simulation of
the appearance of Uranus and its two outer moons in a 8-inch f/8 scope
under the best conditions. Unfortunately, Uranus will show no detail
even in the largest instruments.
The moons will appear as faint stars near the planet. Their orbits are currently tilted such that they trace a wide, egg-shaped path. They are best observed when farthest from the planet, which occurs when they are to the north or south of it, although they can often be observed at other times as well.
This month, of the two brighter outer moons, Titania ranges in apparent separation from the planet by about 7" to 31" and the outermost major moon, Oberon, ranges from about 10" to 42".
Of the closer moons, Ariel is nearer to the glare of the planet, ranging from about 3" to 13", but shining at magnitude 14.4 it is the brighter and easier of the two. Umbriel ranges from about 5" to 19" from the planet and shines at a dim 15.0 magnitude. In the diagram below the moons orbit in a counter clockwise direction. North is up and east is to the left.
The best time to observe
a particular moon is when it is farthest from the planet. These satellite
elongations occur twice per orbit for each satellite, once to the North,
and once to the South.
What Are You Seeing?
Uranus
was encountered by Voyager 2 in 1986, leaving us a wealth of information
regarding the planet and its satellites. At a distance from the Sun
of over 19 AU this gas giant takes over 84 years to orbit the sun.
The first planet discovered in modern times, it was found by the great
18th century observer William Herschel in 1781. Herschel, a musician
by trade, spent his nights surveying the sky. He wrote, "In examining
the small stars in the neighborhood of H Geminorum I perceived one that
appeared visibly larger than the rest..." Herschel's discovery was by no
means an accident as is often stated. He and his brother Alexander
built the largest telescopes of the day--the first practical application
of the reflecting telescope invented by Newton. Assisted by his sister
Caroline, Herschel set out to observe every star in the sky. As he
put it, "In a regular manner I examined every star in the heavens and that
night it was its turn to be discovered." Biographies are full of
stories that reflect his great zeal for observing, including slipping out
between the acts of a play.
Unlike the other planets, Uranus is very highly tilted with respect to its orbit; it lies nearly on its side, tilted by some 98o. It seems unlikely that a planet would form with such a large tilt. Did Uranus collide or interact with another planet-sized body sometime in the distant past?
Uranus is composed of gas above ice above a small rocky core. The cloud tops we observe in the telescope are composed primarily of methane in a clear atmosphere of hydrogen.
Uranus has 21 moons discovered to date, five of which can be observed from earth. Of those five, the four largest are visible in amateur instruments. The two outer moons, Titania and Oberon are each about 1600 km in diameter, about half that of Earth's Moon. The two inner satellites are smaller, about 1200 km in diameter.
| Finder
Charts for Uranus in September.
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