Rosse, Third Earl of (William Parsons) (1800–1867)

William Parsons, the third Earl of Rosse, was an Irish astronomer who, in 1845, built by far the largest telescope in the world at the time, a 72-in (1.8-m) reflector, in the grounds of Birr Castle, Parsonstown. Its mirror was made of speculum and its wood and metal tube, 17 m long, was supported between two parallel masonry walls. The so-called Leviathan of Parsonstown was used mainly for the study of nebulae and (what we know today to be) galaxies, its most notable discovery being the spiral nature of M51 (the Whirlpool Galaxy). Although a great milestone in the development of large telescopes, the Leviathan was hampered by its cumbersome mounting, the limited area of sky to which it had access – and the southern Irish weather.