Henry Ussher (1741–1790) was an Irish mathematician and astronomer, best known as the inaugural Andrews Professor of Astronomy at

Trinity College Dublin Trinity College Dublin (), officially titled The College of the Holy and Undivided Trinity of Queen Elizabeth near Dublin, and legally incorporated as Trinity College, the University of Dublin (TCD), is the sole constituent college of the Unive ...

(TCD), a title later accompanied with the designation Royal Astronomer of Ireland. Ussher was a key player in the setting up of

Dunsink Observatory The Dunsink Observatory is an astronomical observatory established in 1785 in the townland of Dunsink in the outskirts of the city of Dublin, Ireland. Alexander Thom''Irish Almanac and Official Directory''7th ed., 1850 p. 258. Retrieved: 2011-0 ...

outside the city of Dublin and was its first director.

Life

Henry was the fourth son of Samuel Ussher, rector of Dunganstown,

County Wicklow County Wicklow ( ; ) is a Counties of Ireland, county in Republic of Ireland, Ireland. The last of the traditional 32 counties, having been formed as late as 1606 in Ireland, 1606, it is part of the Eastern and Midland Region and the Provinces ...

, by his wife Frances Walsh. Elected a Scholar of Trinity College Dublin in 1759, he graduated B.A. in 1761, M.A. in 1764, B.D. and D.D. in 1779. He was elected to a fellowship in 1764, and co-opted senior fellow in 1781. He was Donegall Lecturer in Mathematics at TCD 1769-1770. Appointed on 22 Jan 1783, the first Andrews Professor of Astronomy, Ussher went to London to order instruments for the planned Dunsink Observatory from

Jesse Ramsden Jesse Ramsden Fellow of the Royal Society, FRS FRSE (6 October 1735 – 5 November 1800) was a British mathematician, astronomy, astronomical and scientific instrument maker. His reputation was built on the engraving and design of dividing engine ...

. These included: a small

achromatic lens An achromatic lens or achromat is a lens (optics), lens that is designed to limit the effects of chromatic aberration, chromatic and spherical aberration. Achromatic lenses are corrected to bring two wavelengths (typically red and blue) into ...

telescope, mounted on a polar axis, and carried by a heliostatic movement; an equatorial machine with circles five feet in diameter; a transit of six feet focal length, and a ten-foot vertical circle executed, after delays, on a reduced scale. Ussher chose the site for the observatory at Dunsink near Dublin, planned the building, and supervised its construction. Ussher's election as a fellow of the

Royal Society of London The Royal Society, formally The Royal Society of London for Improving Natural Knowledge, is a learned society and the United Kingdom's national academy of sciences. The society fulfils a number of roles: promoting science and its benefits, r ...

on 24 November 1785 followed shortly after the incorporation of the

Royal Irish Academy The Royal Irish Academy (RIA; ), based in Dublin, is an academic body that promotes study in the natural sciences, arts, literature, and social sciences. It is Ireland's premier List of Irish learned societies, learned society and one of its le ...

, of which he was an original member. He died at his house in Harcourt Street, Dublin, on 8 May 1790, and was buried in the college chapel.

Works

Papers contributed by Ussher to the ''Transactions'' of the Royal Irish Academy included "Observations on the Disappearance and Reappearance of Saturn's Rings in the Year 1789". From the compression of the globe of

Saturn Saturn is the sixth planet from the Sun and the second largest in the Solar System, after Jupiter. It is a gas giant, with an average radius of about 9 times that of Earth. It has an eighth the average density of Earth, but is over 95 tim ...

, he deduced a rotation period for the planet of 10 hours 12½ minutes.

Family

Ussher married Mary Burne, and left three sons and five daughters. The eldest son was Admiral Thomas Ussher.

Notes

Attribution {{DEFAULTSORT:Ussher, Henry 1741 births 1790 deaths Directors of Dunsink Observatory Donegall Lecturers of Mathematics at Trinity College Dublin Fellows of the Royal Society Fellows of Trinity College Dublin 18th-century Irish astronomers Scholars of Trinity College Dublin Members of the Royal Irish Academy 18th-century Irish mathematicians