Leviathan of Parsonstown, Newton telescope at Birr Castle, Ireland

The Leviathan of Parsonstown is a reflecting telescope of 72 inches aperture on the grounds of Birr Castle in County Offaly, Ireland. Its tube and mirror box measure about 54 feet and weigh some 12 tons, mounted at an incline between two stone walls 40 feet high, 23 feet apart and 71 feet long. The tube rests on a universal joint at the mirror end, with a chain and counterweight for balance and a rack and pinion beam for azimuth.

William Parsons built this instrument between 1842 and 1845 to study nebulae and distant galaxies. It remained the largest telescope in the world for over 70 years and allowed astronomers to see spiral structures in space for the first time.

The name Leviathan comes from the biblical sea monster, an informal nickname for the 72-inch instrument. William Parsons used it to resolve Messier 51 into stars and to record spiral structure in 14 galaxies, describing 224 previously unknown nebulae. From 1874 to 1878 J. L. E. Dreyer observed here, laying the basis for his New General Catalogue of 1888.

The telescope stands on the grounds of Birr Castle, where visitors can view the structure and surrounding walls. On clear days, the system of cables, weights, and pulleys that once controlled its movement can be seen.

The metal mirror required regular polishing because it tarnished quickly through oxidation and lost its reflective power. Astronomers climbed platforms to reach the eyepiece, which moved to different positions depending on where the instrument was pointed.