Hansen became director of the Seeberg Observatory, near Gotha, in 1825, and in 1857 a new observatory was built for him. He worked on theoretical geodesy, optics, and the theory of probability. His most important books on the theory of the motion of the Moon are the Fundamenta (1838; “Foundation”) and the Darlegung (1862–64; “Explanation”). The tables based on his theory were printed in Great Britain in 1857 and were used until 1923. From his theory of the Moon, Hansen deduced a value close to that now accepted for the distance between the Earth and the Sun. Assisted by the astronomer Christian Olufsen, Hansen in 1853 compiled new tables of the Sun's positions at various times.