More on Astronomy

More on Astronomy

Cosmology

The accumulated knowledge of the stars gave a cosmology, a picture of the whole universe. Before modern science, cosmology was closely related to theology: as well as describing the physical shape of the universe, a cosmology also explained its meaning, and gave an account of man's nature and purpose. The two central cosmologies in Western history were the geocentric and the heliocentric universe. Both were based on the basic shape of a wheel revolving around a hub.

The geocentric (or Ptolemaic) universe

In the geocentric universe, the earth (geo) is at the centre, with other planets (the Sun being counted as a planet) revolving around it in concentric circles. Looking out from the earth, astronomers noted the Moon, then Mercury, Venus, the Sun, Mars, Jupiter and Saturn. Beyond these came the stars. These were held to be all equidistant from the earth and were placed on a further circle (hence the ‘Fixed Stars').

 

This was the conception of the cosmos held by the ancient Greeks, as described by Aristotle (384-322 BC). It is often referred to as the Ptolemaic universe after the Egyptian scientist Ptolemy (c.90-168). This cosmology persisted throughout the Middle Ages and much of the Renaissance, until the new science started to supplant it from about 1600.

The influence of the geocentric view

Christianity

Though the geocentric universe was originally pre-Christian, it was comfortably Christianised. Aristotle had described a ‘Prime Mover', a force outside the heavens setting them in motion. To Christians, this Prime Mover corresponded to God. They believed that the universe involved human-like intentions: for example, the (presumed) circular orbit of the planets expressed the planets' desire to be close to God.

The music of the spheres

The planets were believed to revolve on invisible but solid crystalline spheres. Their combined movement made the music of the spheres, which expressed divine harmony. The entire design of the universe was seen to reflect the will of a perfect God.

Geocentricity

The central position of the earth reflected the idea that the human race were privileged creatures, and that the cosmos literally revolved around them. But the geocentric universe also put Earth at the furthest possible distance from Heaven (the ‘empyrean'), which lay beyond the Fixed Stars. Thus it also suited ideas of humility, with humans as fallen sinners.

Dual nature

The heavens were held to be perfect and unchanging. Earth, however, was mutable and corrupt. Earthly and heavenly natures were believed to be different, and therefore obeyed two sets of physical laws. Earth corresponded to the body, the heavens to the spirit.

Sub-lunar / super-lunar

Beyond the moon (super-lunar), the heavens existed in the perfect atmosphere of ‘ether', the natural element of angels. Man occupied the sub-lunar sphere. Here the atmosphere was polluted, and matter was composed of the four elements (Earth, Air, Fire and Water). Beneath the moon, all things were liable to corruption and decay.

Finite

The cosmos was seen as being vast but finite. Medieval man looking up at the stars imagined he was looking at the outermost edge of the universe.

Heliocentric

In the heliocentric cosmos, the sun (Greek helios) was believed to be at the centre, and the earth and other planets revolved around it. Copernicus' On the Revolution of the Heavenly Spheres (1543) showed that apparent aberrations in planetary orbits on the old model could be better explained if the sun were put at the centre. Thus Earth spins around its axis and revolves around the sun, giving the appearance that the sun rises in the east and sets in the west.

 

Later astronomers continued this work:

• Kepler (1571-1630) worked out that orbits are elliptical, not circular
• Using the telescope, Galileo (1564-1642) observed sunspots on the sun and irregularities on the moon, showing that the heavens were not perfect.

The response to the heliocentric view

Heretical

The heliocentric cosmos was a matter of great controversy. The Catholic Church authorities condemned the theory, forcing Galileo to recant in 1616, and banning Copernicus' book. The Ptolemaic model had informed Church teachings about God and man, thus the new model presented a challenge to the authority of the Church. Protestant reactions were very mixed; heliocentrism was enthusiastically adopted by some theologians and taught in some Protestant universities.

Culture

The heliocentric picture took centuries to be generally absorbed. Writers continued to use ideas and images drawn from the older picture, as is still common today in the words sunrise and sunset.         

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