Born  York, England, 17 June 1800

Died  Monkstown, Co. Dublin, Ireland, 31 October 1867

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William Parsons, Third Earl of Rosse, a skilled engineer, ingenious scientist, and dedicated astronomer, constructed a reflecting telescope larger than any previously made, the largest in the world for seven decades. With it he discovered the spiral nature of many nebulae.

William was the eldest son of Sir Lawrence Parsons, Second Earl of Rosse and Alice (née Lloyd). The Parsons family came to Ireland from England at the end of the 16th century and settled in Birr, King's County (county offaly) (Parsonstown, King's County), in 1620. The Second Earl had been a prominent member of the Irish parliament since 1782, representing the University of Dublin and then his own county. With the passing of the Act of Union in 1800, his interest in politics waned and he devoted his time to the development of the town of Birr and the education of his family. William, with his brothers and his sisters, was educated at home by a series of tutors and governesses and with the active involvement of his parents.

When the title of Earl of Rosse passed to his father in 1807, William as eldest son assumed the courtesy title of Lord Oxmantown. He entered Trinity College, Dublin, in 1818 and then, with his brother John, transferred to Magdalen College, Oxford, in 1821. The following year he graduated with first class honors in mathematics. In 1823, Parsons was elected to represent King's County in the Westminster parliament; he held his seat until 1834, when he resigned in order to concentrate on his scientific interests.

Parsons became a member of the Astronomical Society of London (soon to become the Royal Astronomical Society) in 1824, just 4 years after its establishment. Through such a connection, he could have met John Herschel , whose father William Herschel had pioneered the building of large reflecting telescopes and their use in scanning the skies; in any event, Parsons and John Herschel exchanged numerous letters on astronomical matters. Parsons resolved to make a reflecting telescope as large as existing resources would allow. He could not benefit from the entire scope of knowledge of previous telescope makers, including the Herschels, for they took pains to keep some of their methods secret.

Parsons established a workshop and foundry at Birr Castle and trained his estate workers in all the practical skills that were required. After a long series of experiments to determine the optimum mixture of copper and tin for a speculum mirror, he settled on an alloy of four atoms of copper to one of tin, which he took to be in the ratio of 2.15:1 by weight. Parsons first tried making large mirrors from small segments of speculum soldered to brass plates. He invented a special machine (since widely adopted) for grinding, polishing, and parabolizing mirrors in a systematic way. The machine was driven by a small rotary steam engine of his design and made under his direction at Birr in 1827. On the strength of these achievements Parsons was elected to membership in the Royal Society in 1831. The same year he was appointed Lord Lieutenant for King's County.

In 1834, Parsons married Mary Wilmer‐Field (1813–1885), the eldest daughter of a wealthy landowner who lived near Bradford, England. Mary inherited estates that were valued at £88,000 as well as a cash settlement of £8,700. Among her many interests, she became a pioneer photographer and one of the founders of the Photographic Society of Ireland.

After building a new forge, foundry, and workshop, Parsons resumed his experiments by constructing a 36‐in.‐diameter segmented mirror. After many trials and tribulations, in 1839 he succeeded in casting a perfect 36‐in. speculum disk in one piece; it weighed 1¼ tons. A key factor in his success was the design of the casting mold, which had a base of closely packed steel strips through which gases could escape. The performance of this mirror encouraged him to attempt the casting of a 6‐ft. monolithic mirror. When his father died in February 1841, William assumed the title of Third Earl of Rosse.

After many failures, two 6‐ft. mirrors were successfully cast in 1842 and 1843, each weighing more than 3 tons. To avoid distortion when it was pointed in different directions, the mirror was supported by a system of 81 “equilibrated levers” suggested by Thomas Grubb .

The telescope tube resembled a giant barrel, 54 ft. long and 7 ft. in diameter, bulging to 8 ft. in the middle. The tube was pivoted about a huge universal joint at its base and slung with chains from two massive masonry walls, 23 ft. apart and parallel to the meridian. Horizontal movement was limited to 10° on either side of the meridian; a vertical range of nearly 110° was possible. The cost of the entire telescope was estimated between £20,000 and £30,000.

In February 1845, Thomas Romney Robinson of the Armagh Observatory and James South of London were present for the initial observations. Despite unfavorable weather, they caught a glimpse of a magnificent double star and numerous faint stars shining in M67. In April 1845, Parsons made a pencil drawing of the M51 nebula that caused a sensation when it was displayed at a meeting of the British Association for the Advancement of Science in Cambridge in June 1845. As the spiral arms suggested some sort of motion, the nebula was called “The Whirlpool.” Parsons went on to discover 15 other spiral nebulae.

In autumn 1845 observational work was brought to a halt by the failure of the Irish potato crop and the resulting Great Famine. The Earl and Countess of Rosse devoted all their time and most of their income to alleviating the terrible effects of the famine. By 1848, when observations were resumed, there were many other demands on Parsons's time, so he employed a succession of able assistants, most notably George Stoney and Robert Ball . As the fame of the Great Telescope spread, visitors came from all over the world to see it and, if the weather allowed, to view the heavens.

Parsons received many honors. He was president of the Royal Society from 1848 to 1854 and was awarded its Gold Medal in 1851. He was made a knight of Saint Patrick in 1845; Napoleon III created him a knight of the Legion of Honor in 1855. Parsons was a member of the Royal Irish Academy (1822) and a member of the Imperial Academy of Science of Saint Petersburg (1852). He received honorary degrees from Cambridge (1842) and Dublin (1863). Parsons was chancellor of the University of Dublin from 1862 until his death. From 1845, he was a representative peer for Ireland in the Westminster parliament. The International Astronomical Union named the lunar crater at 17°.  9 S and 35°. 0 E in his honor.

In 1867, the Third Earl's health declined, and he took a house by the sea, just south of Dublin. He died after an operation to remove a tumor on his knee, and was buried in the old church of Saint Brendan in Birr; some 4,000 of his tenants attended his funeral.

Of the 11 children born to the Earl and Countess of Rosse, only 4 survived to adulthood. The eldest, Laurence Parsons , followed his father's interest in astronomy; Randal became a canon in the Church of England; Richard Clere became a successful railway engineer; and the youngest, Charles Algernon, became world famous as the inventor of the steam turbine.

After the death of the Fourth Earl in 1908, the Leviathan was partially dismantled, and one 6‐ft. speculum was sent to the Science Museum in London. The great tube lay recumbent for many years until, as a result of the efforts of the sixth and seventh Earls, funding for restoration was secured in 1994. The telescope was completely reconstructed in 1996 to form the centerpiece of a historic science museum at Birr Castle.