October 4: Christen Longomontanus

Christen Sørensen Longomontanus

October 4, 1562 – October 8, 1647

Christen Longomontanus (or Longberg), was a Danish astronomer.

The name Longomontanus was a Latinized form of the name of the village of Lomborg, Jutland, Denmark, where he was born. Engaged by Tycho Brahe in 1589 as his assistant in his great astronomical observatory of Uraniborg, he rendered invaluable service for eight years. Having left the island of Hven with his master, he obtained his discharge at Copenhagen on June 1, 1597, in order to study at some German universities. He rejoined Tycho at Prague in January 1600, and having completed the Tychonic lunar theory, turned homeward again in August.

Longomontanus was not an advanced thinker. He adhered to Tycho's erroneous views about refraction, believed that comets were messengers of evil, and imagined that he had squared the circle. He inaugurated, at Copenhagen in 1632, the erection of a stately astronomical tower, but did not live to witness its completion. King Christian IV of Denmark, to whom he dedicated his Astronomia Danica, an exposition of the Tychonic system of the world, conferred upon him the canonry of Lunden in Schleswig.

However, it was Longomontanus who really developed Tycho's geoheliocentric model empirically and publicly to common acceptance in the 17th century in his 1622 astronomical tables. When Tycho died in 1601, his program for the restoration of astronomy was unfinished. The observational aspects were complete, but two important tasks remained, namely the selection and integration of the data into accounts of the motions of the planets, and the presentation of the results on the entire program in the form of a systematic treatise. Longomontanus, Tycho's sole disciple, assumed the responsibility and fulfilled both tasks in his voluminous Astronomia Danica (1622).

Regarded as the testament of Tycho, the work was eagerly received in seventeenth-century astronomical literature. But unlike Tycho's, his geoheliocentric model gave the Earth a daily rotation as in the models of Ursus and Roslin, and which is sometimes called the 'semi-Tychonic' system. As an indication of his book's popularity and of the semi-Tychonic system, it was reprinted in 1640 and 1663. Having originally worked on calculating the Martian orbit for Tycho with Kepler, he had already modelled its orbit to within 2 arcminutes error in his geoheliocentric model when Kepler had still only achieved 8 arcminutes in his heliocentric system.

The Lunar crater Longomontanus is named in his honor.