Born in 1865 at Cushendun, Ireland, he was three when he moved 
        with his parents to England. He was educated at Marlborough, whence he 
        secured a scholarship to Trinity College, Cambridge, where he was, for
        what it is worth, 27th Wrangler, and took his BA degree. Subsequently
        he obtained an Oxford DSc. After two years as a schoolmaster at
        Lancing, he was appointed an Assistant at the Royal Observatory,
        Greenwich, a post he held from 1891 until 1927.
             In 1888, when he was twenty-three, he had been elected a Fellow
        of the Royal Astronomical Society; he served for several years as
        Secretary, and from 1904 to 1906 as President, of the British
        Astronomical Association. He "took part in the Eclipse Expeditions of
        that Association of 1896, 1900, and 1905; also in the Expedition to
        Brazil of 1919".
             This last journey was described, on 22 November 1919, in the
        newspaper "The Spere" as follows : "Astronomery: Two eclipse
        expeditions were sent out last May, one to Principle, on the east
        coast of Africa, the observers being Prof. Eddington and Mr.
        Cottingham, and the other to Sobral, in North Brazil, the observers
        being Dr. Crommelin and Mr. Davidson. Their object was to observe the
        eclipse with the special purpose of testing a new theory of
        gravitation put forward by Herr Einstein, a German mathematician."
             Crommelin was a Christian gentleman, modest and unassuming,
        careless of appearance but always courteous. A Protestant in his
        youth, after taking his tripos he stayed on at college reading for
        Holy Orders in the Anglican Church, but about this time became
        unsettled in his religious convictions, ultimately entering the Roman
        Catholic Church in 1891.
             He lived near his parents at Blackheath, handy also for the
        Observatory, and went out of his way to describe himself in 'Who's
        Who' as a Roman Catholic. He had four children, of whom one entered
        the priesthood while two were killed in a climbing accident on Pillar
        Rock, Ennerdale, in 1933.
             With P. H. Cowell, he was the author of "Investigation of the
        Motion of Halley's Comet, from 1759 to 1910". According to the
        "Science News-letter" of 30 October 1926: "Giacobini's Comet, which
        returns to the vicinity of the earth every six and two-thirds year---
        has come back again, according to Dr. Harlow Shapley, director of the
        Harvard College Observatory, and the place where it was found was very
        close to the position predicted a year ago by Dr. A. C. C. Crommelin
        of the Royal Observatory, Greenwich, England. The difference between
        the predicted and the observed place of the comet was about the
        diameter of the full moon."
             In 1929 Crommelin demonstrated that comet Forbes 1928 III was
        identical with comet Coggia-Winnecke 1873 VII and comet Pons 1818 II,
        the revolution period being twenty-eight years. He later showed that a
        comet seen in 1457 was probably (and one in 1625 possibly) the same
        object.
             In 1948 the International Astronomical Union changed the name of
        the comet from Pons-Coggia-Winnecke-Forbes to Crommelin, only the
        fourth occasion on which a comet has been named after the computer of
        its orbit, rather than its discoverer. In 1956 the comet returned to
        perihelion just four days later than Crommelin had predicted.
             He died, aged seventy-four, in 1939. He had a crater on the moon
        named after him.
 

Source:  (C.E.B.Brett, 1997.)