June 17: William Parsons, 3rd Earl of Rosse

William Parsons, 3rd Earl of Rosse

June 17, 1800 – October 31, 1867

William Parsons, 3rd Earl of Rosse, built several telescopes including the world's largest telescope in 1845 and it remained the world's largest for the rest of the century. Using this telescope he saw and cataloged a large number of galaxies. His 72-inch (6 feet/1.8 m) Leviathan was the first to see the spiral structure of what was later known as the Whirlpool Galaxy, then called M51.

He was born in York in Yorkshire, England. He was educated at Trinity College, Dublin, and Magdalen College, Oxford, graduating with first-class honors in mathematics in 1822. He inherited an earldom and a large estate in King's County (now County Offaly) in Ireland when his father died in 1841.

In addition to his astronomical pursuits, Rosse served as an Member of Parliament (MP) for King's County from 1821 to 1834, an Irish representative peer after 1845, president of the Royal Society (1848–1854), and chancellor of Trinity College, Dublin (1862–1867).

In the 1840s, he built the Leviathan of Parsonstown, a 72-inch (183-cm) telescope at Birr Castle, Parsonstown, County Offaly. He had to invent many of the techniques he used in constructing this telescope, both because its size was without precedent and because earlier telescope builders had guarded their secrets or had simply failed to publish their methods. Rosse's telescope was considered a marvelous technical and architectural achievement, and images of it were circulated widely within the British commonwealth. Building of the telescope began in 1845 and in 1847 it was put into service. The 72-inch (1.8 m) telescope replaced a 36-inch (910 mm) one that he had built previously.

Lord Rosse carried out pioneering astronomical studies and discovered the spiral nature of some nebulas, today known to be spiral galaxies. The first spiral galaxy he detected was M51, and his drawings of it closely resemble modern photographs (today it is known as the Whirlpool Galaxy).

Rosse named the Crab Nebula, based on an earlier drawing made with his older 36-inch (91cm) telescope in which it resembled a crab. A few years later, when the 72-inch (183cm) telescope was in service, he produced an improved drawing of considerably different appearance, but the original name stuck.

A main component of Rosse's nebular research was attempting to resolve the nebular hypothesis, which posited that planets and stars were formed by gravity acting on gaseous nebulae. Rosse himself did not believe that nebulas were truly gaseous, but rather that they were made of such an amount of fine stars that most telescopes could not resolve them individually (that is, he considered nebulas to be stellar in nature). Rosse and his technicians claimed to resolve the Orion nebula into its individual stars, which would have both political and cosmological implications, as at the time there was considerable debate over whether or not the universe was "evolved" (in a pre-Darwinian sense), a concept Rosse disagreed with strongly. Rosse's primary opponent in this was John Herschel, who used his own instruments to claim that the Orion nebula was a "true" nebula, and discounted Rosse's instruments as flawed (an insult Rosse returned about Herschel's own). In the end, neither man (nor telescope) could establish sufficient scientific authority in its results to solve the question by themselves (the convincing evidence for the gaseous nature of the nebula would come later from spectroscopic evidence, though it would not resolve the philosophical issues).

One of Rosse's telescope admirers was Thomas Langlois Lefroy, a fellow Irish MP, who said, "The planet Jupiter, which through an ordinary glass is no larger than a good star, is seen twice as large as the moon appears to the naked eye/.../But the genius displayed in all the contrivances for wielding this mighty monster even surpasses the design and execution of it. The telescope weighs sixteen tons, and yet Lord Rosse raised it single-handed off its resting place, and two men with ease raised it to any height."

Lord Rosse's son published his father's findings, including the discovery of 226 NGC objects in Observations of Nebulae and Clusters of Stars Made With the Six-foot and Three-foot Reflectors at Birr Castle From the Year 1848 up to the Year 1878, Scientific Transactions of the Royal Dublin Society Vol. II, 1878.

Lord Rosse built a variety of optical reflecting telescopes. His telescopes used cast speculum metal parabolicly grounded and polished.

The Lunar crater Rosse is named in his honor.