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The Tychonic system (or Tychonian system) was a model of the solar system published by Tycho Brahe in the late 16th century which combined 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[1] and Paul Wittich, a Silesian mathematician and astronomer.[2] A similar geoheliocentric model was also earlier proposed by Nilakantha Somayaji of the Kerala school of astronomy and mathematics.[3][4]

It is essentially a geocentric model; the Earth is at the center of the universe. The Sun and Moon and the stars revolve around the Earth, and the other five planets revolve around the Sun. It can be shown that the motions of the planets and the Sun relative to the Earth in the Tychonic system are equivalent to the motions in a heliocentric system.

Motivation for the Tychonic system

Tycho admired aspects of Copernicus's heliocentric model of the solar system, but felt that it had problems as concerned physics, astronomical observations of stars, and religion. Regarding the Copernican system Tycho wrote,

This innovation expertly and completely circumvents all that is superfluous or discordant in the system of Ptolemy. On no point does it offend the principle of mathematics. Yet it ascribes to the Earth, that hulking, lazy body, unfit for motion, a motion as quick as that of the aethereal torches, and a triple motion at that.[5]

In regard to physics, Tycho held that the Earth was just too sluggish and heavy to be continuously in motion. According to the accepted Aristotelian physics of the time, the heavens (whose motions and cycles were continuous and unending) were made of "Aether" or "Quintessence"; this substance, not found on Earth, was light, strong, unchanging, and its natural state was circular motion. By contrast, the Earth (where objects seem to have motion only when moved) and things on it were composed of substances that were heavy and whose natural state was rest—thus the Earth was a "lazy" body that was not readily moved.[6] Thus while Tycho acknowledged that the daily rising and setting of the sun and stars could be explained by the Earth's rotation, as Copernicus had said, still

such a fast motion could not belong to the earth, a body very heavy and dense and opaque, but rather belongs to the sky itself whose form and subtle and constant matter are better suited to a perpetual motion, however fast.[7]

In regards to the stars, Tycho also believed that if the Earth orbited the Sun annually there should be an observable stellar parallax over any period of six months, during which the angular orientation of a given star would change thanks to Earth's changing position (this parallax does exist, but is so small it was not detected until 1838, when Friedrich Bessel discovered a parallax of 0.314 arcseconds of the star 61 Cygni in 1838[8]). The Copernican explanation for this lack of parallax was that the stars were such a great distance from Earth that Earth's orbit was almost insignificant by comparison. However, Tycho noted that this explanation introduced another problem: Stars as seen by the naked eye appear small, but of some size, with more prominent stars such as Vega appearing larger than lesser stars such as Polaris, which in turn appear larger than many others. Tycho had determined that a typical star measured approximately a minute of arc in size, with more prominent ones being two or three times as large.[9] In writing to Christoph Rothmann, a Copernican astronomer, Tycho used basic geometry to show that, assuming a small parallax that just escaped detection, the distance to the stars in the Copernican system would have to be 700 times greater than the distance from the sun to Saturn. Moreover, the only way the stars could be so distant and still appear the sizes they do in the sky would be if even average stars were gigantic — at least as big as the orbit of the Earth, and of course vastly larger than the sun. And, Tycho said, the more prominent stars would have to be even larger still. And what if the parallax was even smaller than anyone thought, so the stars were yet more distant? Then they would all have to be even larger still.[10] Tycho said

Deduce these things geometrically if you like, and you will see how many absurdities (not to mention others) accompany this assumption [of the motion of the earth] by inference.[11]

Copernicans offered a religious response to Tycho's geometry: titanic, distant stars might seem unreasonable, but they were not, for the Creator could make his creations that large if He wanted.[12]

Religion played a role in Tycho's geocentrism also – he cited the authority of scripture in portraying the Earth as being at rest. He rarely used Biblical arguments alone (to him they were a secondary objection to the idea of Earth's motion) and over time he came to focus on scientific arguments, but he did take Biblical arguments seriously.[13]

Tycho advocated as an alternative to the Ptolemaic geocentric system a "geo-heliocentric" system (now known as the Tychonic system), which he developed in the late 1570s. In such a system, the sun, moon, and stars circle a central Earth, while the five planets orbit the Sun.[14] The essential difference between the heavens (including the planets) and the Earth remained: Motion stayed in the aethereal heavens; immobility stayed with the heavy sluggish Earth. It was a system that Tycho said violated neither the laws of physics nor sacred scripture — with stars located just beyond Saturn and of reasonable size.[15]
History and development of the Tychonic system

Tycho's system was foreshadowed, in part, by that of Martianus Capella, who described a system in which Mercury and Venus are placed on epicycles around the Sun, which circles the Earth. Copernicus, who cited Capella's theory, even mentioned the possibility of an extension in which the other three of the six known planets would also circle the Sun.[16] In the 15th century, his work was anticipated by Nilakantha Somayaji, an Indian astronomer of the Kerala school of astronomy and mathematics, who first presented a geoheliocentric system where all the planets (Mercury, Venus, Mars, Jupiter and Saturn) orbit the Sun, which in turn orbits the Earth.[3][4][17]

The Tychonic system became a major competitor with the Copernican system as an alternative to the Ptolemaic. After Galileo's observation of the phases of Venus in 1610, most cosmological controversy then settled on variations of the Tychonic and Copernican systems. In a number of ways, the Tychonic system proved philosophically more intuitive than the Copernican system, as it reinforced commonsense notions of how the Sun and the planets are mobile while the Earth is not. Additionally, a Copernican system would suggest the ability to observe stellar parallax, which could not be observed until the 19th century. On the other hand, because of the intersecting deferents of Mars and the Sun (see diagram), it went against the Ptolemaic and Aristotelian notion that the planets were placed within nested spheres. Tycho and his followers revived the ancient Stoic philosophy instead, since it used fluid heavens which could accommodate intersecting circles.[citation needed]
Legacy of the Tychonic system

After Tycho's death, Johannes Kepler used the observations of Tycho himself to demonstrate that the orbits of the planets are ellipses and not circles, creating the modified Copernican system that ultimately displaced both the Tychonic and Ptolemaic systems. However, the Tychonic system was very influential in the late 16th and 17th centuries. After the Galileo affair, which transpired early in the 17th century, Copernicanism was never officially forbidden to astronomers in the Roman Catholic Church but the Tychonic system was an acceptable alternative that matched available observations, the Copernican system having more epicycles even than the Ptolemaic. Jesuit astronomers in China used it extensively, as did a number of European scholars. Jesuits (such as Clavius, Christoph Grienberger, Christoph Scheiner, Odo van Maelcote) were the most efficient agent for the diffusion of the Tychonic system. It was chiefly through the influence of the Jesuit scientists that the Roman Catholic Church adopted the Tychonic system, over a period of nine years (from 1611 to 1620), in a process directly prompted by the Galilean telescopic discoveries.[18]

The discovery of stellar aberration in the early 18th century by James Bradley convinced people that the Earth did in fact move around the Sun, although, in reality, it is not a proof. Nevertheless, after that Tycho's system fell out of use among scientists. The theory of relativity in the early 20th century taught that motion is relative and that different frames of reference are valid, including non-inertial reference frames. In the modern era, some of the modern geocentrists use a modified Tychonic system with elliptical orbits.

^ Westman, Robert S. (1975). The Copernican achievement. University of California Press. p. 322. ISBN 978-0-520-02877-7. OCLC 164221945.
^ Owen Gingerich, The Book Nobody Read: Chasing the Revolutions of Nicolaus Copernicus, Penguin, ISBN 0-14-303476-6
^ a b Ramasubramanian, K. (1994). "Modification of the earlier Indian planetary theory by the Kerala astronomers (c. 1500 AD) and the implied heliocentric picture of planetary motion". Current Science 66: 784–90.
^ a b Joseph, George G. (2000), The Crest of the Peacock: Non-European Roots of Mathematics, p. 408, Princeton University Press, ISBN 978-0-691-00659-8
^ Owen Gingerich, The eye of heaven: Ptolemy, Copernicus, Kepler, New York: American Institute of Physics, 1993, 181, ISBN 0-88318-863-5
^ Blair, Ann, "Tycho Brahe's critique of Copernicus and the Copernican system", Journal of the History of Ideas, 51, 1990: 355-377, doi:10.2307/2709620, pages 361-362. Moesgaard, Kristian Peder, "Copernican Influence on Tycho Brahe", The Reception of Copernicus' Heliocentric Theory (Jerzy Dobrzycki, ed.) Dordrecht & Boston: D. Reidel Pub. Co. 1972. ISBN 90-277-0311-6, page 40. Gingerich, Owen, "Copernicus and Tycho", Scientific American 173, 1973: 86 – 101, page 87.
^ Blair, 1990, 361.
^ J J O'Connor and E F Robertson. Bessel biography. University of St Andrews. Retrieved 2008-09-28
^ The sizes Tycho measured turned out to be illusory -- an effect of optics, the atmosphere, and the limitations of the eye (see Airy disk or Astronomical seeing for details). By 1617, Galileo estimated with the use of his telescope that the largest component of Mizar measured 3 seconds of arc, but even that turned out to be illusory -- again an effect of optics, the atmosphere, and the limitations of the eye [see L. Ondra (July 2004). "A New View of Mizar". Sky & Telescope: 72–75.]. Estimates of the apparent sizes of stars continued to be revised downwards, and, today, the star with the largest apparent size is believed to be R Doradus, no larger than 0.057 ± 0.005 seconds of arc.
^ Blair, 1990, 364. Moesgaard, 1972, 51.
^ Blair, 1990, 364.
^ Moesgaard, 1972, 52. Vermij R., "Putting the Earth in Heaven: Philips Lansbergen, the early Dutch Copernicans and the Mechanization of the World Picture", Mechanics and Cosmology in the Medieval and Early Modern Period (M. Bucciantini, M. Camerota, S. Roux., eds.) Firenze: Olski 2007: 121-141, pages 124-125.
^ Blair, 1990,362-364
^ Gingerich, 1973. Moesgaard, 1972, 40-43.
^ Moesgaard 40, 44
^ [1]
^ Ramasubramanian, K., "Model of planetary motion in the works of Kerala astronomers", Bulletin of the Astronomical Society of India 26: 11–31 [23–4], retrieved 2010-03-05
^ Pantin, Isabelle (1999). "New Philosophy and Old Prejudices: Aspects of the Reception of Copernicanism in a Divided Europe". Stud. Hist. Phil. Sci. 30 (237–262): 247.

External links

Animated Copernican and Tychonian orreries; click on "Copernican" or "Tychonian" in the lower left corner

Astronomy Encyclopedia

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