Celestial spheres

Concentric celestial spheres; Peter Apian's Cosmographia (Antwerp, 1539)

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.

The celestial spheres, or celestial orbs, are the fundamental element of Earth-centered (geocentric) astronomies and cosmologies developed by Plato, Aristotle, Ptolemy, and others. In these geocentric models the stars and planets are carried around the Earth on spheres or circles. Note that the spheres carry the planets and thus are not to be confused with the modern concept of the spherical planets themselves.

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 on. Plato and his followers ordered them Moon, Sun, Mercury, Venus, and then followed the standard model for the upper spheres; there were further disagreements on the relative place of the spheres of Mercury and Venus.

History

Antiquity

One of the earliest intimations of celestial spheres appears in Plato's "Myth of Er," a section of the Republic, which describes the cosmos as the Spindle of Necessity, attended by the Sirens and turned by the three Fates.

In his Metaphysics, Aristotle developed a philosophical cosmology of spheres, based on the mathematical astronomy of Eudoxus and Callippus. In the fully developed Aristotelian system, the spherical Earth is at the center of the universe. 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.

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 ealier 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.

Middle Ages

Christian and Muslim philosophers modified Ptolemy's system to include an unmoved outermost sphere, which was the dwelling place of God and all the elect. Each of the lower spheres was moved by a subordinate spiritual mover (a replacement for Aristotle's multiple divine movers), called an intelligence. The outermost moving sphere, whose movement affected all others, was referred to as the Primum Mobile ("first moved").

Near the end of the twelfth century, the Spanish Muslim al-Bitrūjī 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.

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.

Renaissance

Early in the sixteenth century Nicolaus Copernicus drastically reformed the model of astronomy by displacing the Earth from its central place, 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.

Tycho Brahe's observation of the comet of 1577, which passed through the planetary orbs, 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.

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.

Literary and symbolic expressions

Dante and Beatrice gaze upon the highest Heaven; from Gustave Doré's illustrations to the Divine Comedy Paradiso Canto 31

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.

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.. 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. Below this figure Oresme quotes the Psalms that "The heavens declare the Glory of God and the firmament showeth his handiwork."

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
• 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.

See also

• Firmament
• Primum Mobile

External links

Dennis Duke, Animated Ptolemaic model of the nested spheres