HANSEN, PETER, ANDREAS, 179r-1874,; a Danish' astronomer .; when young apprenticed to a watchmaker ; afterwards employed by Schumacher, professor of astronomy at Copenhagen, to assist in the measurement of an arc of meridian In Hol stein. This led to his appointment as assistant to Schumacher, at the observatory of Altona. Hansen's reputation as a mathematician had by this time become generally known, and in 1825 he was selected to succeed professor Encke as director of the obser vatory of :iceberg, near Gotha. There he remained for the rest of his life, devoting his talents to the development of the highest branches of mathematical astronomy, with an originality of conception which was acknowledged by the English royal astronomical society on two occasions, by the award of their gold medal for his researches in phys ical astronomy and his lunar tables. His Tables de in Lune appeared in 1857, published at the expense of the British government, which awarded him a prize of 1:1000; have been adopted for use hi the calculations of the _Nautical Almanac. In addition to
this important volume containing the full details of the formulae explanatory of his lunar theory, Hansen was the author of a large number of miscellaneous papers, prin cipally relating to the orbits of comets and pftinets or to perturbational astronomy. In one of these he was the first to point out that Encke's value of the horizontal equatorial solar parallax required to be increased to reconcile the lunar theory with modern obser vations—an opinion which was subsequently confirmed by Le Verrier from his planetary researches and by the observations of Mars and the transit of Venus of 1874. Hansen was a foreign member of the royal society, and an associate of the royal astronomical society.