Andrew C. D. Crommelin, as he styled himself (there are several versions of his name; this appears in a signed photograph), was a highly distinguished world figure in astronomy, with a special interest in comets; he was perhaps most famous for his study of Halley's Comet, whose 1910 return to the solar system he predicted with almost perfect accuracy.

Crommelin was born in Cushendun, County Antrim, the third son of Nicholas de la Cherois Crommelin (1819-1900), a descendant of Louis Crommelin, a Huguenot who was a founder of the linen trade in Ulster, and Nicolas De La Cherois. He was educated at Marlborough College where he won a scholarship to Trinity College, Cambridge. He graduated BA in 1886 as 27th "Wrangler" in the part II Mathematics tripos: that is, he obtained a first-class honours degree in Mathematics. After a brief period reading for Holy Orders in the Anglican Church, (he ultimately converted to Roman Catholicism) he was an Assistant Master at Lancing College, 1889-91.

In 1891 he took up a post at the Royal Observatory, Greenwich, where he began making routine observations with the Sheepshanks Equatorial (a 6.7-inch refracting telescope at Greenwich then used regularly for the observation of comets, occultations, double-stars and planetary measurement). He was also put in charge of the special "altazimuth" instrument (that is, a mounting on perpendicular axes used for particularly large telescopes) for observing the Moon. Observing the occultations of stars by the Moon, and comets was also put in his care. Crommelin's work was extensive at Greenwich and he was an expert researcher, whether as observer or computer (as such people were then called). In 1911 he made an accurate determination of the Lunar parallax and prepared the ephemerides (tables of values that gives the positions of astronomical objects in the sky at a given time or times) of both the Moon and outer planets including the path of Jupiter's eighth satellite.

Crommelin took part over his career in numerous expeditions throughout the world to study solar eclipses. In 1919 he participated in the solar eclipse expedition to Brazil which aimed to determine the amount of the deflection of light caused by the gravitational field of the Sun. The results from these observations were crucial in providing confirmation of the General Theory of Relativity, which Albert Einstein had proposed in 1916.

Crommelin, together with Philip Herbert Cowell, a specialist in the mathematics of motion, simplified the method of calculating the perturbations (distortions) in the orbits of long-period comets. In their essay on the return of Halley's Comet, they traced historical references to the comet's return as far back as one in China in 240 BC, and with some probability to 625 BC. They predicted the comet's return for the April 1910 to an accuracy of just over 3 days; their suggestion that the error was due mainly to non-gravitational forces was later confirmed. This prediction had appeared in 1909 under the title "A corrected ephemeris for Halley's comet"; the piece, "Essay On The Return Of Halley's Comet" appeared in Publikation der Astronomischen Gesellschaft the following year, written with Cowell. For their work the authors were awarded the Lindemann Prize of the Astronomische Gesellschaft (Astronomical Society) of the University of Jena, and each of them received an honorary Doctorate of Science from Oxford University.

His calculation of orbits of what were then called Comet Forbes 1928 III, Comet Coggia-Winnecke 1873 VII, and Comet Pons 1818 II, in 1929, showed that these comets were one and the same periodic comet. The comet thus received the rather maladroit name "Comet Pons-Coggia-Winnecke-Forbes". In 1956, however, when the comet reappeared according to his prediction, he was posthumously honoured when the comet was renamed after him alone by the International Astronomical Union (today it is designated 27P/Crommelin and known more popularly as Comet Crommelin). A modern convention has grown up that a periodic comet is named after the person determining the orbit rather than the discoverers at each apparition, though this was only the fourth-ever renaming of a comet for this reason. In 1956 the comet returned to perihelion (its closest point to the sun) just four days later than Crommelin had predicted.

As well as numerous contributions to such publications as the Encyclopaedia Britannica, Crommelin prepared astronomical almanacs for observations of Mars, Jupiter, Saturn, and the Moon. He also contributed observations of comets and minor planets to the Journal of the British Astronomical Association for over 40 years, and was the author of Comet Catalogue (1925), an updating of an 1894 work by the famous German astronomer Johann Gottfried Galle (the discoverer of the planet Neptune; his work, Verzeichniss Der Elemente Der Bisher Berechneten Cometenbahnen Nebst Anmerkungen Und Literatur-Nachweisen, is usually known in English simply under the easier title "Galle's Cometenbahn"). One modern astronomer said of this work: "Crommelin's sequel to Galle's Cometenbahnen certainly advanced cometry science in his day". He also published, assisted by Mary Proctor Comets (1937) and was the author of two other books The Star World (1914), The Story of the Stars (1930) and Diamonds in the Sky (1940).

Crommelin was a cousin of Maria Henrietta de la Cherois Crommelin. His maternal uncle John Mulholland was created 1st Baron Dunleath in 1892; a sister, Constance, taught mathematics at Roedean, the famous girls' school in Brighton, until her marriage to the (rather younger) John Masefield, the poet. In 1897 Crommelin married Laetitia Noble, who predeceased him, as did two of their four children: Claude de la Cherois Crommelin, an electrical engineer, and Philomena Mary de la Cherois Crommelin, a teacher were killed in July 1933 in a climbing accident at Pillar Rock, Ennerdale Valley, Cumberland, aged respectively 24 and 27; their father had warned them against undertaking this dangerous climb, even though they had climbing experience.

Crommelin, described by one leading astronomer of today as "The Comet Man", is widely acknowledged by astronomers quite simply as "great". He was President of the British Astronomical Association, 1904-6, and was awarded their Walter Goodacre Medal and Gift for contribution to the progress of astronomy over the course of his career. He was Secretary to the Royal Astronomical Society, 1917-23 and their President, 1929-1930.

Four objects of astronomic significance are named for him: Comet 27P/Crommelin; the Crommelin crater on the Moon, the Crommelin crater on Mars; and Asteroid 1899 Crommelin.

Born:	6 February 1865
Died:	20 September 1939