Hansen, Peter Andreas

(b. Tondern, Schleswig, Germany, 8 December 1795; d. Seeberg, Germany, 28 March 1874)

astronomy.

Hansen, a leading German theoretical astronomer of the mid—nineteenth century, was the son of a goldsmith. The straitened circumstances m which the family found itself after the Napoleonic Wars prevented him from embarking on a higher scholastic career in his youth and, like his older contemporary Friedrich Bessel, he arrived at it by a detour through trade. He was apprenticed to a clockmaker at Flensburg and eventually qualified as a master craftsman of this art, for a time becoming a clockmaker in his native town. During his spare time he privately studied French and Latin as well as mathematics, in which he had shown particular proficiency since early childhood.

Hansen’s chance to turn his intellectual gifts to better account came in 1820, when Heinrich Christian Schumacher, then a leading astronomer in Denmark, was temporarily in need of a computing assistant. Hansen applied for the position; and even though unsuccessful, he volunteered to accompany Schumacher in his measurements of an arc of the meridian in Holstein. At the beginning of 1821 he worked in Tondern on the calculations connected with these measurements; later Schumacher summoned him to Copenhagen under an appointment sanctioned by the Danish government.

Between 1821 and 1825 Hansen continued to serve as assistant to Schumacher—mainly at Altona, where a small observatory was set up for him by the Danish king. In 1823 Schumacher started publishing the Astronomische Nachrichten, the oldest astronomical journal still appearing today, and Hansen became its editorial assistant and frequent contributor. In the summer of 1824 Schumacher and Hansen traveled to Helgoland to determine, together with a parallel expedition sent out by the British Admiralty, the accurate geographical position of that island.

Hansen’s connection with Schumacher ended in 1825, when he was invited to succeed Johann Franz Encke as director of the private observatory of the duke of Mecklenburg at Seeberg, near Gotha; he remained its head until his death almost half a century later.

During this time Hansen’s contributions to astronomy were so numerous and enriched so many branches of that field that he was considered among the foremost astronomers of his time. He was above all a theoretician, concerned with the representation of the motion of the moon and the planets in the sky in terms of Newtonian celestial mechanics. His first major work was an extensive study of the mutual perturbations of Jupiter and Saturn (1831), for which he received a prize from the Royal Academy of Sciences in Berlin in 1831 and a gold medal from the Royal Astronomical Society in 1842. Several of his important papers were devoted to the theory of the motion of comets or minor planets (1859), but his main work was concerned with the motion of the moon (1838).

The latter work became the basis of extensive tables of lunar motion, published in 1857 at the expense of the British government. These tables proved to be so accurate that, in the words of George Biddell Airy, then astronomer royal, “Probably in no recorded instance has practical science ever advanced so far by a singles stride.” The theoretical investigations on which these tables were founded were published later in two parts (1862-1864). In recognition of this work Hansen received a prize of £1,000 from the British Admiralty in 1860, and in the same year the Royal Astronomical Society awarded him their gold medal for the second time.

Hansen’s work in celestial mechanics constitutes the main part of, but does not exhaust, his contributions to astronomy. Another lifelong interest was the theory of astronomical instruments, to which he contributed a number of studies concerning the theory and use of heliometers, of the astronomical equatorial, and of the transit instrument. Moreover, his early work with Schumacher led Hansen to many refinements of theoretical geodesy, and he also made several contributions to the calculus of probabilities.

The high regard in which Hansen was held by his contemporaries as a result of these contributions is reflected in the opinion recorded by Simon Newcomb, a leading American astronomer of his time, in his Reminiscences of an Astronomer (New York, 1903):

Modest as was the public position that Hansen held, he may now fairly be considered the greatest master of celestial mechanics since Laplace. In what order Leverier, Delaunay, Adams, and Hill should follow him, it is not necessary to decide. To many readers it will seem singular to place any name ahead of that of the master who pointed out the position of Neptune before a human eye had ever recognized it. But this achievement, great as it was, was more remarkable for its boldness and brilliancy than for its inherent difficulty. If the work had to be done over again today, there are a number of young men who would be as successful as Leverrier; but there are none who would attempt to reinvent the methods of Hansen, or even to improve radically upon them [p. 315],

BIBLIOGRAPHY

Hansen’s writings include Untersuchung über die Gegenseitigen Störungen des jupiters und saturns (Berlin. 1831); Fundamenta nova investigationis orbitae verae quam luna perlustrat (Goth a. 1838): Tables de la lune construites d’aprés Ie principe newtonien de la gravitation universelle (London. 1857): “Auseinandersetzung einer zweckmässigen Methode zur Berechnung der absoluten Störungen der kleinen Planeten,” in Abhandlungen der. K. Sächsischen Gesellschaft der Wissenchaften, 5 (I859), 1-148; and “Darlegung der theoretischen Berechnung der in den Mondtafeln angewandeten Störungen,” ibid 6 (1862). 91-498, and 7 (1864), 1-399.

ZdenÈk Kopal