Rosse, William Parsons, 3rd earl of

born June 17, 1800, York, Eng.

died Oct. 31, 1867, Monkstown, County Cork, Ire.

Irish astronomer.

His "Leviathan," 54 ft (16.5 m) long, was the largest reflecting telescope of the 19th century, and its mirror had a diameter of 72 in. (183 cm). With it Rosse discovered the spiral shape of many objects then classed as nebulae, now recognized as galaxies, and he studied and named the Crab Nebula. He was also the first to discover binary and triple stars. As Lord Oxmantown, he sat in the House of Commons (1821–34); on inheriting his father's earldom in 1841, he joined the House of Lords.

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▪ Irish astronomer

also called (1807–41) Lord Oxmantown

born June 17, 1800, York, Eng.

died Oct. 31, 1867, Monkstown, County Cork, Ire.

      Irish astronomer and builder of the largest reflecting telescope, the “Leviathan,” of the 19th century.

      In 1821 Rosse was elected to the House of Commons as Lord Oxmantown. He resigned his seat in 1834 but in 1841 inherited his father's earldom and served as one of the Irish peers in the House of Lords.

      Lord Rosse was obsessed with the idea of constructing a truly large telescope. He worked for five years to find an alloy suitable for the mirror. His final choice was composed of four atoms of copper to each atom of tin. This alloy was quite brittle, however, and his first 36-inch-diameter mirror was composed of 16 thin plates soldered to a brass framework.

      The moderate success of this telescope encouraged Lord Rosse to try to cast a solid 36-inch mirror. After much experimentation he succeeded in casting and cooling the mirror without cracking it, a serious problem in the construction of all large telescopic mirrors. In 1842 he began work on a mirror of 72-inch diameter. Three years later the four-ton disk was mounted and the installation completed. Fifty-four feet in length, Lord Rosse's telescope was used primarily to observe nebulae on those rare occasions when weather conditions around his estate permitted.

      With his telescope, however, he discovered the remarkable spiral shape of many objects then classed as “nebulae,” which are now recognized as individual galaxies. He studied and named the Crab Nebula. He also made extensive, detailed observations of the Great Nebula in Orion. Though his telescope was dismantled in 1908, it was not until the 100-inch reflector was installed in 1917 at the Mt. Wilson Observatory, California, that a larger telescope was used.

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