Tychonic system

A 17th century illustration of the *Hypothesis Tychonica* from Hevelius' Selenographia, 1647 page 163, whereby the Sun, Moon, and sphere of stars orbit the Earth, while the five known planets (Mercury, Venus, Mars, Jupiter, and Saturn) orbit the Sun.

The Tychonic system (or Tychonian system) is a model of the universe published by Tycho Brahe in 1588^[1], which combines what he saw as the mathematical benefits of the Copernican system with the philosophical and "physical" benefits of the Ptolemaic system. The model may have been inspired by Valentin Naboth^[2] and Paul Wittich, a Silesian mathematician and astronomer.^[3] A similar cosmological model was independently proposed in the Hindu astronomical treatise Tantrasamgraha (c. 1500 CE) by Nilakantha Somayaji of the Kerala school of astronomy and mathematics.^[4]

It is conceptually a geocentric model, or more precisely geoheliocentric: the Earth is at the centre of the universe, the Sun and Moon and the stars revolve around the Earth, and the other five planets revolve around the Sun. At the same time, the motions of the planets are mathematically equivalent to the motions in Copernicus' heliocentric system under a simple coordinate transformation, so that, as long as no force law is postulated to explain why the planets move as described, there is no mathematical reason to prefer either the Tychonic or the Copernican system.^[5]

^ Cite error: The named reference retrying was invoked but never defined (see the help page).
^ Westman, Robert S. (1975). The Copernican achievement. University of California Press. p. 322. ISBN 978-0-520-02877-7. OCLC 164221945 – via Google Books.
^ Owen Gingerich, The Book Nobody Read: Chasing the Revolutions of Nicolaus Copernicus, Penguin, ISBN 0-14-303476-6
^ Ramasubramanian, K.; Sriram, M. S.; Somayajī, Nīlakaṇṭha (2011). Tantrasaṅgraha of Nīlakaṇṭha Somayājī. Sources and studies in the history of mathematics and physical sciences. Dordrecht: Springer. p. 521. ISBN 978-0-85729-035-9.
^ "The Tychonic system is, in fact, precisely equivalent mathematically to Copernicus' system." (p. 202) and "[T]he Tychonic system is transformed to the Copernican system simply by holding the sun fixed instead of the earth. The relative motions of the planets are the same in both systems ..." (p. 204), Kuhn, Thomas S. , The Copernican Revolution (Harvard University Press, 1957).

[retrying-1] Cite error: The named reference retrying was invoked but never defined (see the help page).

[2] Westman, Robert S. (1975). The Copernican achievement. University of California Press. p. 322. ISBN 978-0-520-02877-7. OCLC 164221945 – via Google Books.

[3] Owen Gingerich, The Book Nobody Read: Chasing the Revolutions of Nicolaus Copernicus, Penguin, ISBN 0-14-303476-6

[4] Ramasubramanian, K.; Sriram, M. S.; Somayajī, Nīlakaṇṭha (2011). Tantrasaṅgraha of Nīlakaṇṭha Somayājī. Sources and studies in the history of mathematics and physical sciences. Dordrecht: Springer. p. 521. ISBN 978-0-85729-035-9.

[5] "The Tychonic system is, in fact, precisely equivalent mathematically to Copernicus' system." (p. 202) and "[T]he Tychonic system is transformed to the Copernican system simply by holding the sun fixed instead of the earth. The relative motions of the planets are the same in both systems ..." (p. 204), Kuhn, Thomas S. , The Copernican Revolution (Harvard University Press, 1957).

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