Uranus

Uranus


Saturn is probably best known for its system of planetary rings, which makes it the most visually remarkable object in the solar system.[24] The rings extend from 6,630 km to 120,700 km above Saturn's equator, average approximately 20 meters in thickness and are composed of 93% water ice with a smattering of tholin impurities and 7% amorphous carbon.[63] The particles that make up the rings range in size from specks of dust up to 10 m.[64] There are two main theories regarding the origin of the rings. One theory is that the rings are remnants of a destroyed moon of Saturn. The second theory is that the rings are left over from the original nebular material from which Saturn formed. Some ice in the central rings comes from the moon Enceladus' ice volcanoes.[65]

Beyond the main rings at a distance of 12 million km from the planet is the sparse Phoebe ring, which is tilted at an angle of 27° to the other rings and, like Phoebe, orbits in retrogradefashion.[66] Some of the moons of Saturn, including Pan and Prometheus, act as shepherd moons to keep the planetary rings stable and prevent them from escaping.[67] Pan and Atlas cause weak, linear density waves in Saturn's rings that have yielded more reliable calculations of their masses.[68]

The age of these planetary rings is probably hundreds of millions of years old[69] (in contrast to previous thoughts that the rings formed alongside the planet when it formed billions of years ago)[70] and their fate include spiraling inward towards the planet, or the boulders forming the rings colliding with each other and disappearing.

Discovery
Uranus had been observed on many occasions before its discovery as a planet, but it was generally mistaken for a star. The earliest recorded sighting was in 1690 when John Flamsteed observed the planet at least six times, cataloging it as 34 Tauri. The French astronomer Pierre Lemonnier observed Uranus at least twelve times between 1750 and 1769,[19] including on four consecutive nights. Sir William Herschel observed the planet on March 13, 1781 while in the garden of his house at 19 New King Street in the town of Bath, Somerset, England (now the Herschel Museum of Astronomy),[20] but initially reported it (on April 26, 1781) as a "comet".[21] Herschel "engaged in a series of observations on the parallax of the fixed stars",[22] using a telescope of his own design. He recorded in his journal "In the quartile near ζ Tauri … either [a] Nebulous star or perhaps a comet".[23] On March 17, he noted, "I looked for the Comet or Nebulous Star and found that it is a Comet, for it has changed its place".[24] When he presented his discovery to the Royal Society, he continued to assert that he had found a comet while also implicitly comparing it to a planet:[25] “	The power I had on when I first saw the comet was 227. From experience I know that the diameters of the fixed stars are not proportionally magnified with higher powers, as planets are; therefore I now put the powers at 460 and 932, and found that the diameter of the comet increased in proportion to the power, as it ought to be, on the supposition of its not being a fixed star, while the diameters of the stars to which I compared it were not increased in the same ratio. Moreover, the comet being magnified much beyond what its light would admit of, appeared hazy and ill-defined with these great powers, while the stars preserved that lustre and distinctness which from many thousand observations I knew they would retain. The sequel has shown that my surmises were well-founded, this proving to be the Comet we have lately observed.