Parsons, Sir Laurence (1840–1908), 4th earl of Rosse , astronomer, was born 17 November 1840 at Birr castle, Parsonstown, King's Co. (Offaly), the eldest surviving son of William Parsons (qv), 3rd earl of Rosse, astronomer, and his wife Mary (née Field). His youngest brother was Sir Charles Algernon Parsons (qv), inventor of the compound steam turbine. Educated at home in Birr by T. T. Gray (1833–1924), a fellow of TCD, and John Purser (1835–1903), later professor of mathematics at QCB (1863–1901), he entered TCD in 1860 and graduated BA (1864). In 1867 he succeeded as 4th earl, becoming a representative peer for Ireland, and was made sheriff of King's Co.
Parsons divided his time between the management of the family estates and the study of astronomy. While he was not as gifted as his father, he made some important contributions. He devised and built a water clock to drive the 36 inch (0.92m) and 72 inch (1.83m) telescopes at Birr and developed an independent observer's bucket for the 36 inch telescope, an innovation which later became standard. He carried out a series of prolonged observations which resulted in his ‘An account of observations on the Great Nebula in Orion, made at Birr Castle’ in the Philosophical Transactions of the Royal Society, clviii (1868). Best remembered for his researches into the radiant heat of the moon, he developed an attachment to the 36 inch telescope at Birr that could focus lunar radiation and produced a series of accurate temperature graphs. Active in the leading scientific societies, he was made a fellow of the Royal Society and the Royal Astronomical Society in 1867, later serving on the council of both bodies. In 1870 he was elected MRIA and received an honorary doctorate from Oxford University. He outlined his researches on lunar radiation at the Royal Society's Bakerian lecture of 1873, publishing his lecture in the Proceedings of the Royal Society (1869). He was awarded an honorary doctorate by TCD in 1879.
In 1885 Rosse became chancellor of TCD, an office he held until his death. Conscientious in his duties, he was a leading figure in efforts to raise funds for new scientific laboratories in the college in the early 1900s, and himself made a substantial donation to the project. He also served as president of the RDS (1887–92) and the RIA (1896–1901). Created a knight of the Order of St. Patrick (1890), he was appointed lord lieutenant of King's Co. (1892) and awarded an honorary doctorate by Cambridge University in 1900. He was a keen supporter of the Valentia magnetic observatory scheme and also gave funds to this project. After some years of ill-health, he died 30 August 1908 at Birr castle and was buried in the local churchyard. He left a bequest to the Royal Society to establish the ‘Rosse Fund’, and a further £1,000 to the science school fund at TCD.
In 1870 he married Frances Cassandra Hawke, the only child of Edward William, 4th Baron Hawke of Towton; they had two sons and one daughter. He was succeeded as 5th earl by his eldest son, William Edward Parsons (1873–1918), a major in the Irish Guards who died of wounds received during the Great War. A portrait by an unknown artist of the 4th earl in his chancellor's robes was given to TCD by the 6th earl, Lawrence Michael Harvey Parsons (1906–79).