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Geocentrism

Pietro Daniel Omodeo History of astronomy and philosophy, Max Planck Institute for the History of Science, Berlin, Germany

Abstract

“Geocentrism” refers to a cosmological and plan- etary theory, in which the Earth occupies the cen- tral position of the world system. In antiquity and in the Middle Ages, geocentrism was the most common cosmological view, although some astronomers and philosophical schools embraced alternative visions about worldly order. During the Renaissance, debates following Nicolaus Copernicus’s proposal of a heliocentric planetary theory prompted a reexamination of traditional geocentric (and geostatic) arguments (see “▶Copernicanism” and “▶Astronomy”). This also led to their reworking and expansion. Aristotle’s and Ptolemy’s arguments were atten- tively reconsidered. Many scholars deemed them conclusive and therefore stuck to terrestrial cen- trality even after parallax computation (in the 1580s, especially Tycho Brahe) and telescopic evidence (after 1610, especially Galileo Galilei) demonstrated the impossibility of geocentric paths for Mars and the inferior planets. Geo-heliocentrism thus emerged as the only via- ble solution. It was a planetary theory according to

which all or some of the planets rotate around the Sun, while the Sun remained Earth centered along with the Moon and the fixed stars. The Inquisition prohibition of Copernican astronomy in 1616 gave new impetus to geocentrism, in its geoheliocentric form, among Catholics.

The Aristotelian Legacy

Aristotle’s arguments in favor of terrestrial cen- trality, especially those inDe coelo II 12–13, were particularly influential during the Middle Ages and the Renaissance. They was especially due to Aristotle’s acknowledged authority and to their integration in a systematic vision of nature. In particular, geocentrism was compatible with the elemental theory according to which the four sub- lunary elements – earth, water, air, and fire – are ordered according to a concentric scheme beneath the sphere of the Moon (the first of the celestial bodies). They were thought to have innate tenden- cies to move toward their “natural places.” According to this doctrine, the earth, as the “heaviest” element, strives toward the cosmolog- ical center, which coincides with the center of gravity.

Against this philosophical backdrop an ad hoc explanation had to account for the fact that the orbis terrarum – the three continents, Africa, Asia, and Europe – was not submerged in the water. During the Middle Ages, this explanation was sought in the providential intervention of God

# Springer International Publishing Switzerland 2015 M. Sgarbi (ed.), Encyclopedia of Renaissance Philosophy, DOI 10.1007/978-3-319-02848-4_67-1

at the moment of the Creation. In the sixteenth century, the oceanic explorations and the discov- ery of continents thus far unknown to the Europeans – especially the so-called antipodes – undermined traditional cosmography and proved the fundamental unity of the globus terracquaeus (the earthly watery globe). This new evidence and its theoretical consequences for geography were a mortal blow to medieval cos- mology, as astronomical innovators such as Copernicus stressed. The latter mentioned the epi- stemic discontinuity in geographical knowledge in his first book of De revolutionibus orbium coelestium (I 3) in order to make his heliocentric theory acceptable in spite of its novelty. Coperni- cus claimed that astronomy ought to be emended just like geography.

In spite of Copernicus’s claim, the Aristotelian tradition lived long after him, as witnessed by the countless editions and commentaries of Sacrobosco’s standard textbook, which began with reference to the theory of the elements and the “two-physics” distinction between the sublu- nary realm of corruption and the quintessentially incorruptible heavenly realm. The most authorita- tive Renaissance commentary on this book was composed by the Jesuit mathematician Christoph Clavius. It was widely circulated during the Renaissance, especially as a textbook for the teaching of spherical astronomy at Jesuit colleges.

The Ptolemaic Legacy

By far the most important astronomical defense of geocentrism from antiquity is Ptolemy’s. The Hel- lenistic astronomer, who was still regarded as the “prince of astronomy” during the Renaissance, refutes the eccentricity of the Earth in Almagest I 5 (standard modern numbering). He bases his objection on considerations of how a hypothetical displacement of the Earth in different directions would alter celestial phenomena. He considers in particular the cases of terrestrial eccentricity on the equatorial plane or along the rotational axis. A “third” case, that the Earth is neither on the axis of the daily rotation nor on the equatorial plane, cumulates the disadvantages of both dislocations.

Therefore Ptolemy only mentions it but does not treat it extensively.

Concerning the first case, Ptolemy argues,

If we imagined [the Earth] removed towards the zenith or the nadir of some observer then, if he were at sphaera recta [at the equator], he would never experience equinox, since the horizon would always divide the heavens into two unequal parts, one above and one below the Earth; if he were at sphaera obliqua [at an arbitrary latitude], either, again, equinox would never occur at all, or [if it did occur], it would not be at a position halfway between summer and winter solstices, since these intervals would necessarily be unequal, because the equator, which is the greatest of all parallel circles drawn about the poles of the [daily] motion, would no longer be bisected by the horizon; instead [the horizon would bisect] one of the circles parallel to the equator, either to the north or to the south of it. (Toomer 1984, p. 41)

Ptolemy additionally observes that, if the Earth were “removed towards the east or west of some observer,” the sizes and the distances of the stars would be different at eastern and western hori- zons. Moreover, the time intervals between rising and culmination and between culmination and setting would be unequal.

In the second case – that the Earth is displaced along the axis toward the north or the south – the plane of the horizon would divide the heavens into unequal parts for any observer (at the equator or at any other latitude). Furthermore, the shadows of sundials would be altered. At equinoxes the shadows of a gnomon at sunrise and at sunset would not form a straight line in a plane parallel to the horizon.

These were the main Ptolemaic arguments for geocentrism based on geometrical reasoning and empirical evidence. According to Ptolemy a dis- placement of the Earth from the center would not be compatible with the heavenly phenomena. Copernicus and the supporters of the heliocentric system solved these inconveniences by simply assuming that the dimensions of the annual “orbit” of the Earth around the Sun are negligible relative to the distance of the starry heaven, as stated in De revolutionibus I 5.

Copernicus’s claim about the immensitas of the heavens persuaded only a few among his contem- porary and immediate followers. Particularly

2 Geocentrism

revealing of the post-Copernican endurance of geocentrism is Erasmus Reinhold’s edition w