Why did Herschel name his planet Uranus?
Herschel did not name the planet Uranus, he called it "the Georgium Sidus" (the Georgian Planet) in honor of King George III of England. The name "Uranus" was first proposed by German astronomer Johann Elert Bode in order for it to be in conformity with the other planetary names - which are from classical mythology. Uranus is the ancient Greek deity of the Heavens, the earliest supreme god. This name did not achieve common use until 1850.
Astronomy is very old science - with at least 4,000 years of history. Many of the names of objects that have been known for a long time are historic in nature. The planets and their moons were given names which came from Greek or Roman mythology. This seemed sensible long ago when the objects were named. These days, so many objects are known that names tend to come from the satellite or observatory which discovered (and catalogued) them and a series of numbers which tell astronomers something about where they are located in the sky. The International Astronomical Union (http://www.iau.org)(IAU) is officially in charge of assigning astronomical names. Note that they do not "sell" names to any objects, be they planets or stars.
A lengthy dicsussion of astronomical names and how they are assigned can be found at: http://nineplanets.org/names.html
An excellent list of the names of planets in various languages can be found at http://nineplanets.org/days.html
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