HANSEN, Peter Andreas (1795-1874), astronomer, was born on December 8, 1795, at Tondern, in the duchy of Schleswig. When a youth he served an apprenticeship to a clock and watchmaker at Flensburg, with whom he remained several years. During this time he exhibited an intense interest fur mathematics and the physical sciences, which was the means of his introduction in 1820 to Schumacher, professor of astronomy at Copenhagen, who, appreciating Hansen’s abilities, engaged him to assist in the measurement of an arc of meridian in Holstein. This led to his appointment as assistant to Schumacher at the observatory of Altona, on its foundation in 1823. Hansen’s reputation as an excellent mathematician had by this time become generally known, and in 1825 he was selected to succeed Professor Encke as director.of the observatory of Seeberg, 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 physical astronomy and his lunar tables. His Tables de la Lune appeared in 1857, published at the expense of the British Government; they have been adopted for use in the calculations of the Nautical Almanac. In addition to his important volume containing the full details of the formulae explanatory of his lunar theory, Hansen was the author of a large number of miscellaneous astronomical papers, principally relating to the orbits of comets and planets 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 ...