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Christian Sørensen Longomontanus (or Longberg) (October 4, 1562 – October 8, 1647), was a Danish astronomer.
The name Longomontanus was a Latinized form of the name of the village of Longberg, Jutland, Denmark, where he was born. His father, a laborer called Søren, or Severin, died when he was eight years old. An uncle took charge of the child, and had him educated at Lemvig; but after three years sent him back to his mother, who needed his help to work the fields. She agreed that he could study during the winter months with the clergyman of the parish; this arrangement continued until 1577, when the ill-will of some of his relatives and his own desire for knowledge caused him to run away to Viborg.
There he attended the grammar school, working as a labourer to pay his expenses, and in 1588 went to Copenhagen with a high reputation for learning and ability. 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.
He visited Frauenburg, where Copernicus had made his observations, took a masters degree at Rostock, and at Copenhagen found a patron in Christian Friis, chancellor of Denmark, who employed him in his household. Appointed in 1603 rector of the school of Viborg, he was elected two years later to a professorship in the University of Copenhagen, and his promotion to the chair of mathematics ensued in 1607. This post he held till his death.
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 found that the circle whose diameter is 43 has for its circumference the square root of 18252 which gives 3.14185 ... for the value of π. John Pell and others tried in vain to convince him of his error. 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.
His major works in mathematics and astronomy were:
- Systemalis Mathematici, etc. (1611)
- Cyclometria e Lunulis reciproce demonstrata, etc. (1612)
- Disputatio de Eclipsibus (1616)
- Astronomia Danica, etc. (1622)
- Disputationes quatuor Astrologicae (1622)
- Pentas Problematum Philosophae (1623)
- De Chronoiabio Historico, seu de Tempore Dispulatsones tres (1627)
- Geometriae quaesita XIII. de Cyclometria rabionali et vera (1631)
- Inventio Quadraturae Circuli (1634)
- Disputatio de Matheseos Indole (1636)
- Coronis Problematica ex Mysteriis trium Numerorum (1637)
- Problemata duo Goemetrica (1638)
- Problema contra Paulum Guidinum de Circuli Mensura (1638)
- Introductio in Theatrum Astronomicum (1639)
- Rotundi in Piano, etc. (1644)
- Admiranda Operatio trium Numerorum 6, 7, 8, etc. (1645)
- Caput tertium Libri primi de absoluta Mensura Rotundi plani, etc. (1646)