Peter Andreas Hansen (December 8, 1795 – March 28, 1874) was a Danish astronomer, was born at Tønder, Schleswig.
The son of a goldsmith, he learned the trade of a watchmaker at Flensburg, and exercised it at Berlin and Tønder, 1818–1820. He had, however, long been a student of science; and Dr Dircks, a physician practising at Tønder, prevailed with his father to send him in 1820 to Copenhagen, where he won the patronage of H.C. Schumacher and attracted the personal notice of King Frederick VI. The Danish survey was then in progress, and he acted as Schumacher's assistant in work connected with it, chiefly at the new observatory of Altona, 1821–1825.
Thence he passed on to Gotha as director of the Seeberg observatory; nor could he be tempted to relinquish the post by successive invitations to replace F.G.W. Struve at Dorpat in 1829, and F.W. Bessel at Königsberg in 1847. The problems of gravitational astronomy engaged the chief part of Hansen's attention. A research into the mutual perturbations of Jupiter and Saturn secured for him the prize of the Berlin Academy in 1830, and a memoir on cometary disturbances was crowned by the Paris Academy in 1850.
In 1838 he published a revision of the lunar theory, entitled Fundamenta nova investigationis, &c., and the improved Tables of the Moon ("Hansen's Lunar Tables") based upon it were printed in 1857, at the expense of the British government, their merit being further recognized by a grant of £1000, and by their immediate adoption in the Nautical Almanac, and other Ephemerides. A theoretical discussion of the disturbances embodied in them (long familiarly known to lunar experts as the Darlegung) appeared in the Abhandlungen of the Saxon Academy of Sciences in 1862–1864.
Hansen twice visited England and was twice (in 1842 and 1860) the recipient of the Gold Medal of the Royal Astronomical Society. He communicated to that society in 1847 an able paper on a long-period lunar inequality (Memoirs Roy. Astr. Society, xvi. 465), and in 1854 one on the moon's figure, advocating the mistaken hypothesis of its deformation by a huge elevation directed towards the earth (ib. xxiv. 29). He was awarded the Copley Medal by the Royal Society in 1850, and his Solar Tables, compiled with the assistance of Christian Olufsen, appeared in 1854. Hansen gave in 1854 the first intimation that the accepted distance of the sun was too great by some millions of miles (Month. Notices Roy. Astr. Soc. xv. 9), the error of J.F. Encke's result having been rendered evident through his investigation of a lunar inequality. He died on 28 March 1874, at the new observatory in the town of Gotha, erected under his care in 1857.
See Vierteljahrsschrift astr. Gesellschaft, x. 133; Month. Notices Roy. Astr. Society, xxxv. 168; Proc. Roy. Society, xxv. p. V.; R Wolf, Geschichte der Astronomie, p. 526; Wochenschrift für Astronomie, xvi. 207 (account of early years by E Heis); Allgemeine deutsche Biographie (C Bruhns).