by Duane Dunkerson
Be careful how you say it, find it, and describe it. The planet Uranus has been mispronounced to much accompanying chagrin for those attempting to be correct and inoffensive. It is not easy to find. Twilight time is said to be a help. Nowadays use of a GOTO telescope frees one of most of the frustration in finding it. The disk is certainly noticeable in the size of apertures that are readily available. I have seen Uranus via a sixteen inch CAT with GOTO as a pretty rotund blue-green dot, seen at dusk.
In the description of Uranus, William Herschel erred by seeing it as a tail-less comet since it did move. But it was a dot, using his six inch telescope on March 13, 1781, when he then found it to have appreciable diameter. The orbit was not a cometlike parabola but found to be almost circular. Herschel proposed to call it Georgium Sidus after the King George that American colonists detested. But Bode, of Titius-Bode "law" fame, thought it should be named as Uranus in keeping with familial myth already placed in the planetary heavens by means of names applied to other far planets - Jupiter and Saturn.
The location of Uranus presumably confirmed the Titius-Bode law that had been published a few years earlier. Other then discovered planets had been found at average relative distances from the Sun in geometric progression. Uranus fit in nicely within the progression. Acclaim for William , money from George.
The observance by Uranus of the geometrical progression requires it to be usually 1.787 million miles from the Sun. It takes 84 years to complete a revolution about the Sun. In this slow movement through the constellations, it had been found again and again as a star. Le Monnier spotted it twelve times from 1750-1769. Flamsteed in 1690 had seen it as 34 Tauri, that is, a star in the constellation Taurus. By the time Herschel got the credit for discovery, it was in the constellation Gemini. With lesser magnification and less aperture with a telescope, it has a stellar appearance. It can be seen with the unaided eye at magnitude 5.7 starlike. Small scopes can reveal a small, bland, aquamarine disk or a small, faint, greenish disk. One can have magnifications as low as 40X and see a disk.
It is so small because it is so far from us though it is 32,500 miles in diameter. Its mass is 14.6 times as much as that of the Earth. Such size qualifies it for the status of one among the giant gaseous planets of the Solar System. Others having such a designation are Jupiter, Saturn, and Neptune. The gases of Uranus are hydrogen, helium, and methane. There is also a rocky core. As one would expect from a gaseous composition, it has a low density.
This low density turns upon itself, that is, Uranus rotates in 17.3 hours. An observer on any one of the 22 moons of Uranus would see cloud bands in the atmosphere of Uranus. At the mid-latitudes in this atmosphere are 90 to 360 mile per hour winds. They stir an atmosphere as seen by us to be deep, cold, and remarkably clear. The absorption of red light by the methane yields a blue-green coloration.
An observer on the moons of Uranus may also be able to see the 11 rings around Uranus. For us, the rings are dark having some fairly large particles about 10 miles in diameter and with dust too. The brightest ring is Epsilon. It is shepherded by the moons Coredelia and Ophelia. These moons contain the particles into their ring.
These rings were revealed to be in existence on March 10,1977, when Uranus crossed in front of a star in Libra called SAO 158 687. There was more than a one quick reduction of the star's light before Uranus got to the star. Then the star was covered by Uranus. After that there was another sequence of reductions in the starlight. The rings of Uranus had been found. Narrow and dark, they are seen by us with infrared instrumentation.
Our best look at the rings and newer moons came from the spacecraft Voyager 2 that also surveyed other planets after launch in 1977 and came to the area of Uranus in 1985-1986. Voyager saw the rings and the already known moons of Miranda (seen by Voyager to be a most odd looking moon), Ariel, Umbriel, Titania, and Oberon. Voyager is now gone from the Solar System, it being on a one way journey through the Solar System on a Grand Tour made possible by the fortuitous alignment of the outer planets, excluding Pluto, and to to be offered as a possibility for alignment not again until 2157.
A mention of alignment calls into consideration a most unique feature of Uranus. The planet is tilted far over on its side. The Earth is tilted 23.5 degrees from the vertical. Uranus leans over much more - to 98 degrees. Its north pole gets put 8 degrees under the horizontal, in the plane of orbit about the Sun. At the time of the solstice on Uranus, one of the poles is at the center of the disk. At equinox, one sees the equatorial regions. Winter lasts for 42 years. More oddity derives from a magnetic field offset by 60 degrees from the axis of rotation. The magnetic center is 5,000 miles from the geographic center of Uranus.
Any thing further that is odd? One can lastly consider the
supposed variations in magnitude of Uranus. Uranus is 0.4
magnitude brighter at opposition and 0.2 brighter at perihelion.
Also rotational features vary magnitude by 0.1. If you look down
on a pole, then a 0.3 increase is seen. Amateur astronomers, when
they look at Uranus in its stellar aspect, have seen Uranus as of
magnitude 5.5 though the official magnitude is 5.8
Copyright © 2019
by Duane Dunkerson
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