Celestial spheres

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This article is about material celestial spheres from Antiquity to the Renaissance. For modern uses of the celestial sphere in astronomy and navigation, see Celestial sphere.
Geocentric celestial spheres; Peter Apian's Cosmographia (Antwerp, 1539)
Geocentric celestial spheres; Peter Apian's Cosmographia (Antwerp, 1539)
Thomas Digges' 1576 Copernican heliocentric model of the celestial orbs
Thomas Digges' 1576 Copernican heliocentric model of the celestial orbs

The celestial spheres, or celestial orbs, were the fundamental celestial entities of the cosmological celestial mechanics first invented by Eudoxus, adopted by Aristotle and developed by Ptolemy, Copernicus and others.[1] In this celestial model the stars and planets are carried around by being embedded in rotating spheres made of an aetherial transparent fifth element (quintessence), like jewels set in orbs.

In geocentric models the spheres were most commonly arranged outwards from the center in this order: the sphere of the Moon, the sphere of Mercury, the sphere of Venus, the sphere of the Sun, the sphere of Mars, the sphere of Jupiter, the sphere of Saturn, the starry firmament, and sometimes one or two additional spheres. The order of the lower planets was not universally agreed: Plato and his followers ordered them Moon, Sun, Mercury, Venus, and then followed the standard model for the upper spheres;[2] there were other disagreements about the relative place of the spheres of Mercury and Venus.

Contents

[edit] History

[edit] Antiquity

In his Metaphysics, Aristotle adopted a celestial physics of geo-concentric rotating nested spheres first devised and developed by the astronomers Eudoxus and Callippus.[3] In the fully developed Aristotelian celestial physics, the spherical Earth is at the center of the universe and the planets are attached to anywhere from 47 to 55 concentric spheres that rotate around the Earth. Aristotle considers that these spheres are made of an unchanging fifth element, the aether, and each of these concentric spheres is moved by a god — an unchanging divine mover. Aristotle says that to determine the exact number of spheres and the number of divine movers, one should consult the astronomers.[4][5]

Ptolemaic model of the spheres for Venus, Mars, Jupiter, and Saturn with epicycle, eccentric deferent and equant point. Georg von Peuerbach, Theoricae novae planetarum, 1474.
Ptolemaic model of the spheres for Venus, Mars, Jupiter, and Saturn with epicycle, eccentric deferent and equant point. Georg von Peuerbach, Theoricae novae planetarum, 1474.

The astronomer Ptolemy (fl. ca. 150 AD) defined a geometrical model of the universe in his Almagest and extended it to a physical model of the cosmos in his Planetary hypotheses. In doing so, he added mathematical detail and predictive accuracy that had been lacking in earlier spherical models of the cosmos. In Ptolemy's model, each planet is moved by two or more spheres (or strictly speaking, by thick equatorial slices of spheres): one sphere is the deferent, with a center offset somewhat from the Earth; the other sphere is an epicycle embedded in the deferent, with the planet embedded in the spherical epicycle. Through the use of the epicycle, eccentric, and equant, this model of compound circular motions could account for all the irregularities of a planet's apparent movements in the sky.[6]

[edit] Middle Ages

Christian and Muslim philosophers modified Ptolemy's system to include an unmoved outermost region, which was the dwelling place of God and all the elect. The outermost moving sphere, which moved with the daily motion affecting all subordinate spheres, was moved by a fixed unmoved mover, the Prime Mover, who was identified with God. Each of the lower spheres was moved by a subordinate spiritual mover (a replacement for Aristotle's multiple divine movers), called an intelligence.

Around the turn of the millennium, the Arabian astronomer and polymath Ibn al-Haytham (Alhacen) presented a development of Ptolemy's geocentric epicyclic models in terms of nested spheres. Despite the similarity of this concept to that of Ptolemy's Planetary Hypotheses, al-Haytham's presentation differs in sufficient detail that it has been argued that it reflects an independent development of the concept.[7] In chapters 15-16 of his Book of Optics, Ibn al-Haytham also discovered that the celestial spheres do not consist of solid matter.[8]

Near the end of the twelfth century, the Spanish-Arabian Muslim astronomer al-Bitrūjī (Alpetragius) sought to explain the complex motions of the planets using purely concentric spheres, which moved with differing speeds from east to west. This model was an attempt to restore the concentric spheres of Aristotle without Ptolemy's epicycles and eccentrics, but it was much less accurate as a predictive astronomical model.[9][10]

In the thirteenth century, scholars in European universities dealt with the implications of the rediscovered philosophy of Aristotle and astronomy of Ptolemy. One issue that arose concerned the nature of the celestial spheres. Through an extensive examination of a wide range of scholastic texts, Edward Grant has demonstrated that scholastic philosophers generally considered the celestial spheres to be solid in the sense of three-dimensional or continuous, but most did not consider them solid in the sense of hard. The consensus was that the celestial spheres were made of some kind of continuous fluid.[11]

[edit] Renaissance

Paul Wittich's 1578 Capellan geoheliocentric planetary model in which the Martian and Solar orbits do not intersect
Paul Wittich's 1578 Capellan geoheliocentric planetary model in which the Martian and Solar orbits do not intersect

Early in the sixteenth century Nicolaus Copernicus drastically reformed the model of astronomy by displacing the Earth from its central place in favour of the sun, yet he called his great work De revolutionibus orbium coelestium (On the Revolutions of the Celestial Spheres). Although Copernicus does not treat the physical nature of the spheres in detail, his few allusions make it clear that, like many of his predecessors, he accepted non-solid celestial spheres.[12] However, it seems a crucial physical reason for his heliocentrism in order to save the celestial spheres may have been that he rejected the possibility of interpenetrating spheres, but for some reason thought Martian parallax at opposition is greater than solar parallax,[13] whereby Mars must then be nearer the Earth than the sun is, but also whereby the Martian and solar spheres must intersect on all geocentric and geoheliocentric planetary models. They can only be non-intersecting with Mars less than 1 AU away at opposition in the pure heliocentric model. As Copernicus's pupil and herald Rheticus expressed this in his 1540 Copernican Narratio Prima published 3 years before Copernicus's De Revolutionibus, "Mars unquestionably shows a parallax sometimes greater than the sun's, and therefore it seems impossible that the earth should occupy the centre of the universe.".[14] But this is only an impossibility for a spherist cosmology in which different planetary spheres cannot intersect, but not for non-spherist astronomy, as illustrated by the non-spherist Tychonic geocentric model, for example, in which the Martian and solar orbits intersect.

Tycho Brahe's 1587 geoheliocentric planetary model in which the Martian and Solar orbits intersect
Tycho Brahe's 1587 geoheliocentric planetary model in which the Martian and Solar orbits intersect

But although Martian parallax at its maximum of some 23 arcseconds is indeed greater than the sun's at some 9 arcseconds, such differences are thought to have been instrumentally observationally indiscernible at that time before telescopes and micrometers, when the maximum discernible resolution by human naked eye observation is reckoned to be no more than some 30 arcseconds. Moreover at the time the traditionally accepted value for solar parallax, even by Tycho Brahe, was some 3 arcminutes. This all raises the question of the basis on which astronomers compared Martian and solar parallax and what the consensus in the 16th century was, if any, on which is greater. The (geoheliocentric) planetary models of such as Paul Wittich and Nicolaus Reimers(aka Ursus) supposed that of Mars was never greater, whereas those of Copernicus and Tycho supposed it was greater at opposition.[15]] This all seems to imply disagreement in the 16th century about the observational facts of Martian parallax, but about which crucial issue the history of science literature is silent. Yet it seems it was a firm belief in the greater oppositional parallax of Mars within geocentrism that undermined belief in the solid celestial spheres as physically possible because of the intersecting spheres problem, to which the only pro-spherist solution was pure heliocentrism, but which was itself observationally 'refuted' by the apparent lack of any annual stellar parallax. Thus Tycho's view that heliocentrism was observationally refuted by the fact of no discernible stellar parallax enforced his rejection of solid spheres to sustain his observationally unjustified belief that Mars was less than 1 AU from the Earth at opposition, But his rejection of the spheres was at least observationally buttressed by his observations of the 1577 comet.

Ursus's 1588 geoheliocentric planetary model in which the Martian and Solar orbits do not intersect
Ursus's 1588 geoheliocentric planetary model in which the Martian and Solar orbits do not intersect

Tycho Brahe's observations that the comet of 1577 displayed less daily parallax than the Moon implied it was superlunary and so, impossibly, must pass through some planetary orbs in its transit. This led him to conclude that "the structure of the heavens was very fluid and simple." Tycho opposed his view to that of "very many modern philosophers" who divided the heavens into "various orbs made of hard and impervious matter." Since Grant has been unable to identify such a large number of believers in hard celestial spheres before Copernicus, he concludes that the idea first became dominant sometime after the publication of Copernicus's De revolutionibus in 1542 and either before, or possibly somewhat after, Tycho Brahe's publication of his cometary observations in 1588.[16][17]

Although in his early works Johannes Kepler made use of the notion of celestial spheres, by the Epitome of Copernican Astronomy (1621) Kepler was questioning the existence of solid spheres and consequently the need for intelligences to guide the motions of the heavens. An immobile sphere of the fixed stars, however, was a lasting remnant of the celestial spheres in Kepler's thought.[18]

[edit] Literary and symbolic expressions

Dante and Beatrice gaze upon the highest Heaven; from Gustave Doré's illustrations to the Divine Comedy, Paradiso Canto 28, lines 16–39
Dante and Beatrice gaze upon the highest Heaven; from Gustave Doré's illustrations to the Divine Comedy, Paradiso Canto 28, lines 16–39

In Cicero's Dream of Scipio, the elder Scipio Africanus describes an ascent through the celestial spheres, compared to which the Earth and the Roman Empire dwindle into insignificance. A commentary on the Dream of Scipio by the late Roman writer Macrobius, which included a discussion of the various schools of thought on the order of the spheres, did much to spread the idea of the celestial spheres through the Early Middle Ages.[19]

Nicole Oresme, Le livre du Ciel et du Monde, Paris, BnF, Manuscrits, Fr. 565, f. 69, (1377)
Nicole Oresme, Le livre du Ciel et du Monde, Paris, BnF, Manuscrits, Fr. 565, f. 69, (1377)

Some late medieval figures inverted the model of the celestial spheres to place God at the center and the Earth at the periphery. Near the beginning of the fourteenth century Dante, in the Paradiso of his Divine Comedy, described God as a light at the center of the cosmos.[20] Here the poet ascends beyond physical existence to the Empyrean Heaven, where he comes face to face with God himself and is granted understanding of both divine and human nature.

Later in the century, the illuminator of Nicole Oresme's Le livre du Ciel et du Monde, a translation of and commentary on Aristotle's De caelo produced for Oresme's patron, King Charles V, employed the same motif. He drew the spheres in the conventional order, with the Moon closest to the Earth and the stars highest, but the spheres were concave upwards, centered on God, rather than concave downwards, centered on the Earth.[21] Below this figure Oresme quotes the Psalms that "The heavens declare the Glory of God and the firmament showeth his handiwork."[22]

[edit] See also

[edit] Notes

  1. ^ Before Aristotle, in his Timaeus Plato had previously proposed the planets were transported on rotating bands.
  2. ^ In his De Revolutionibus Bk1.10 Copernicus claimed the empirical reason why Plato's followers put the orbits of Mercury and Venus above the sun's was that if they were sub-solar, then by the sun's reflected light they would only ever appear as hemispheres at most and would also sometimes eclipse the sun, but they do neither. (See p521 Great Books of the Western World 16 Ptolemy-Copernicus-Kepler)
  3. ^ See e.g. The Homocentric Spheres of Eudoxus, Ch 4 of Dreyer's History of Astronomy
  4. ^ G. E. R. Lloyd, Aristotle: The Growth and Structure of his Thought, pp. 133-153, Cambridge: Cambridge Univ. Pr., 1968. ISBN 0-521-09456-9.
  5. ^ G. E. R. Lloyd, "Heavenly aberrations: Aristotle the amateur astronomer," pp.160-183 in his Aristotelian Explorations, Cambridge: Cambridge Univ. Pr., 1996. ISBN 0-521-55619-8.
  6. ^ Andrea Murschel, "The Structure and Function of Ptolemy's Physical Hypotheses of Planetary Motion," Journal for the History of Astronomy, 26(1995): 33-61.
  7. ^ Y. Tzvi Langerman (1990), Ibn al Haytham's On the Configuration of the World, p. 11-25, New York: Garland Publishing.
  8. ^ Edward Rosen (1985), "The Dissolution of the Solid Celestial Spheres", Journal of the History of Ideas 46 (1), p. 13-31 [19-20, 21].
  9. ^ Bernard R. Goldstein, Al-Bitrūjī: On the Principles of Astronomy, New Haven: Yale Univ. Pr., 1971, vol. 1, pp. 6, 44-5
  10. ^ Grant, Planets, Stars, and Orbs, pp. 563-4
  11. ^ Grant, Planets, Stars, and Orbs, pp. 328-30.
  12. ^ Nicholas Jardine, "The Significance of the Copernican Orbs," Journal for the History of Astronomy, 13(1982): 168-194, esp. pp. 177-8.
  13. ^ At least according to his pupil Rheticus
  14. ^ See p136 of Edward Rosen's 1939/59 Three Copernican Treatises.
  15. ^ Albeit Tycho's observations failed to demonstrate any Martian parallax whatever at opposition. But Copernicus and Tycho both put the distance to Mars at opposition at approximately half an AU.
  16. ^ Grant, "Celestial Orbs," 2000, pp. 185-6.
  17. ^ Grant, Planets, Stars, and Orbs, pp. 345-8.
  18. ^ Grant, Planets, Stars, and Orbs, pp. 121, 544-5.
  19. ^ Macrobius, Commentary on the Dream of Scipio, transl. by William Harris Stahl, New York: Columbia Univ. Pr., 1952; on the order of the spheres see pp. 162-5.
  20. ^ C. S. Lewis, The Discarded Image: An Introduction to Medieval and Renaissance Literature, Cambridge: Cambridge Univ. Pr., 1964, p. 116. ISBN 0-521-09450-X
  21. ^ Nicole Oreseme, "Le livre du Ciel et du Monde", 1377, retrieved 2 June 2007.[1]
  22. ^ Ps. 18: 2; quoted in Nicole Oresme, Le livre du ciel et du monde, edited and translated by A, D. Menut and A. J. Denomy, Madison: Univ. of Wisconsin Pr., 1968, pp. 282-3.

[edit] Bibliography

  • Duhem, Pierre, Le Système du Monde: Histoire des doctrines cosmologiques de Platon à Copernic, 10 vols., Paris: Hermann, 1959.
  • Eastwood, Bruce, "Astronomy in Christian Latin Europe c. 500 – c. 1150," Journal for the History of Astronomy, 28(1997): 235–258.
  • Eastwood, Bruce and Gerd Graßhoff, Planetary Diagrams for Roman Astronomy in Medieval Eyrope, ca. 800-1500, Transactions of the American Philosophical Society, vol. 94, pt. 3, Philadelphia, 2004. ISBN 0-87169-943-5
  • Grant, Edward, "Celestial Orbs in the Latin Middle Ages," Isis, 78(1987): 153-73; reprinted in Michael H. Shank, ed., The Scientific Enterprise in Antiquity and the Middle Ages, Chicago: Univ. of Chicago Pr., 2000. ISBN 0-226-74951-7
  • Grant, Edward, Planets, Stars, and Orbs: The Medieval Cosmos, 1200-1687, Cambridge: Cambridge Univ. Pr., 1996. ISBN 0-521-56509-X
  • Lewis, C. S., The Discarded Image: An Introduction to Medieval and Renaissance Literature, Cambridge University Press 1964.
  • McCluskey, Stephen C., Astronomies and Cultures in Early Medieval Europe, Cambridge: Cambridge Univ. Pr., 1998. ISBN 0-521-77852-2
  • Thoren, Victor E., "The Comet of 1577 and Tycho Brahe's System of the World," Archives Internationales d'Histoire des Sciences, 29 (1979): 53-67.

[edit] External links

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