William Parsons (1800-1867), Irish astronomer and 3rd Earl of Rosse. Parsons is most famous for the large telescope (the Leviathan of Parsonstown) that he had built on his estate at what is now Birr, Ireland. This 72-inch (183 cm) telescope was the largest in the world from 1847 until 1917, and it enabled the spiral nature of 'nebulae' like the M51 galaxy to be discerned. He inherited his father's Irish title in 1841. He was also a Member of Parliament (1821-1834), and a member of the House of Lords from 1845. He was President of the Royal Society (1848-1854), and chancellor of the University of Dublin from 1862. A crater on the Moon is named after him.