William Parsons, 3rd Earl of Rosse KP PRS HFRSE (17 June 1800 – 31 October 1867), was an Anglo-Irish astronomer, naturalist, and engineer. He was President of the Royal Society (UK), the most important association of naturalists in the world in the nineteenth century. He built several giant telescopes.His 72-inch telescope, built in 1845 and colloquially known as the "Leviathan of Parsonstown", was the world's largest telescope, in terms of aperture size, until the early 20th century. From April 1807 until February 1841, he was styled as Baron Oxmantown.