The Leviathan of Parsonstown was a 72-inch telescope and the largest telescope in the world from 1847 until the construction of the 100-inch Hooker Telescope at Mount Wilson Observatory in 1917. The telescope was built by William Parsons, 3rd Earl of Rosse on his estate, Birr Castle, at Parsonstown (now called Birr) in Ireland. The 72-inch telescope replaced a 36-inch telescope that had been at the same site.
The telescope mirror was made from speculum metal. Because the mirror tarnished so rapidly in the damp climate, the mirror had to be repolished every six months. Lord Rosse had two mirrors built so that one could be used while the other was being repolished. The light-gathering ability exceeded expectations, enabling 18th magnitude stars to be observed for the first time. The telescope was also the first one able to detect the spiral structure of some galaxies. The Irish sky was not kind to astronomy, and due to weather conditions there were only about 60 good nights of viewing available per year. The telescope itself was dismantled in 1914 but has since been reassembled as a tourist attraction, though for insurance reasons, the general public cannot use the telescope.
