William Parsons Rosse, earl of, a British astronomer, born in York, June 17, 1800, died at Parsonstown, Ireland, Oct. 31, 1867. He graduated at Magdalen college, Oxford, in 1822. From 1821 to 1834, under the title of Lord Oxmantown, he represented King's county, Ireland, in parliament. At the death of his father in 1841 he succeeded to the peerage, and in 1845 was elected one of the representative peers for Ireland. He voted with the liberal party. In 1826 he erected upon the grounds of his residence, Birr castle, near Parsonstown, an observatory for which instruments were made under his special direction. The most important was the enormous reflecting telescope, finished about 1844 at a cost of about $60,000; it has an aperture of 6 ft. and a focus of 53 ft., and is now the most powerful reflector in the world as far as great space-penetrating capacity is concerned. It has been of especial use in resolving nebulae, for which it was in great measure designed. In 1843 Lord Rosse was made president of the British association. He was elected to the astronomical society in 1824, and to the royal society in 1831. From 1849 to 1854 he was president of the royal society.

During the last six years of his life he was chancellor of Trinity college, Dublin.