Leviathan of Parsonstown

1845 · Reflecting Telescope · Work 202 of 1,871

Leviathan of Parsonstown, 1845, Reflecting Telescope

The seventy-two inch Rosse reflector at Birr Castle: a huge wooden tube about fifty-four feet long standing steeply tilted between two tall parallel masonry walls, its lower end resting on a universal joint on the ground at the foot of the walls and its open upper end raised high above the wall tops toward the evening sky, the tube leaning like a great cannon aimed upward, held by chains running over pulleys on the walls to counterweights, a wooden observing gallery spanning between the walls near the top, the walls about forty feet high with the tube rising past them, castle parkland around the mounting.

What it was used for
William Parsons, third Earl of Rosse, built the telescope at his seat, Birr Castle in Parsonstown, today's Birr in Ireland. He cast the first 72-inch mirror in 1842; the instrument was finished in 1845. Its task was to settle what the nebulae in the sky were: distant star clusters or clouds of gas. The tube hung between two masonry walls and could move only in a limited way. Barely in service, it showed a spiral in the nebula M51 in spring 1845. In summer 1845 the Great Famine began. According to the astrophysicist Trevor Weekes, it seriously curtailed observing just when the telescope was in prime condition; the Parsons family carried out famine relief in those years. The Dictionary of Scientific Biography places the important nebula observations between 1848 and 1878.
What was new
Its speculum-metal mirror measured 72 inches (1.83 m), weighed about four tonnes and was one and a half times the width of Herschel's 48-inch mirror. With it Rosse and his assistants saw detail in hundreds of nebulae that no one could make out before, above all that some are spiral-shaped; earlier observers had seen only round forms in M51. Who was at the eyepiece in spring 1845 when the spiral was noticed is unclear: no observing records survive (Weekes 2010). How long it remained the world's largest telescope is given differently: until 1917 (Irish Times), the year the 100-inch Hooker telescope on Mount Wilson saw first light, or until 1914 (Birr Castle).

Further reading