Sir Frederick William Herschel FRS ( 15 November 1738 â `` 25 August 1822 ) was a German-born British astronomer , the greatest observational astronomer of the day . Sir Frederick William Herschel , KH , FRS , German : Friedrich Wilhelm Herschel ( 15 November 1738 â `` 25 August 1822 ) was a German-born British astronomer , technical expert , and a composer .