Hamilton, James Archibald (1747–1815), Church of Ireland clergyman and astronomer, was born at Athlone, Co. Westmeath, son of Col. Gustavus Hamilton and Jane Hamilton (née Giverdot). From 1754 he attended the Royal School at Armagh before entering TCD (November 1764). He distinguished himself in the study of natural sciences and practical astronomy, graduating BA in the spring of 1769. He had been in holy orders before his graduation and held the position of rector of Kildress, Co. Armagh (1766–84). He subsequently served as perpetual curate at Lisnadill (1780–88), rector of Derryloran (1780), prebendary of Dunbin (1784), rector of Creggan and treasurer of St Patrick's cathedral, Armagh (1784–90), prebendary of Tynan (1790), prebendary of Mullaghbrack, and archdeacon of Ross (1780–1804). With the exception of the archdeaconry of Ross, all these positions were in Co. Armagh. While noted for his pluralist tendencies, Hamilton was by all accounts a conscientious and diligent clergyman and graduated BD and DD (Dubl.) in 1784. He was appointed as dean of Cloyne in September 1804 and resigned his position as archdeacon of Ross.
It was in the field of astronomy, however, that Hamilton was to achieve a level of scientific notoriety. In 1780 he acquired a residence at Cookstown, Co. Armagh, and fitted out a private observatory. He devoted himself to the study of perihelia and meteors and made valuable observations on the transit of Mercury. Like most astronomers of his era, he endeavoured to find a viable method of making longitude determinations and based his calculations on the movements of Jupiter's satellites. He was one of the first fifty members appointed to the RIA by royal charter in early 1786, and maintained a high profile in scientific circles in England and Ireland. A friend of Nevil Maskelyne, the astronomer royal, he visited him at Greenwich and maintained a correspondence with him on astronomical matters. When Primate Richard Robinson (qv) began work on Armagh observatory, Maskelyne recommended Hamilton for the position of astronomer and keeper of the observatory and museum. He was duly appointed (July 1790) and oversaw the equipping of the observatory. Retaining this position until his death, he made numerous contributions in the field of astronomy and developed a portable mountain barometer. He published several papers in Transactions of the Royal Irish Academy, including ‘An account of perihelia seen at Cookstown’ (1787), ‘On a new kind of portable barometer for measuring heights’ (1791), and ‘On the method of determining longitude by observations of the meridian passage of the moon and a star at two places’ (1797). He died 21 November 1815 at the observatory, and was buried at Mullaghbrack.
Hamilton married Jane Bunbury; they had two daughters. There are Hamilton letters, including his correspondence with Maskelyne, in the Armagh observatory archive, College Hill, Armagh.