Astronomy and Astrology


  • Astronomy studies the movement of the planets and stars. 
  • Astrology deals with the supposed influence of the stars on human life. 

This distinction is a modern one, however: while astrology is regarded as a pseudo-science today, for centuries it was accepted as a way of explaining and predicting terrestrial events. In medieval times, astronomy and astrology were not usually separated, and observation of the ‘heavenly bodies' was accompanied by ideas about their effects on man and his earthly habitat.


A universal study

All settled civilisations have studied astronomy:

  • Knowledge of the movement of the moon and the sun was vital for a pre-industrial society, as a way of telling the time
  • The codification of such knowledge produced a calendar, by which the best periods for sowing, reaping, herding and other activities could be determined
  • Observation of the stars was also necessary for sea navigation and long land journeys
  • The calculations involved in this study made astronomy a main stimulus for mathematics. 


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.  In the Middle ages, the central cosmology was geocentric, based on the basic shape of a wheel revolving around a hub.

The geocentric (or Ptolemaic) universe

Ptolemaic universeIn 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, Jove, and Saturn. Beyond these came the stars. These were held to be 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


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.


The central position of the earth reflected the idea that the human race were privileged creatures, and 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.

Sublunar / superlunar

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


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


The influence of the heavenly bodies

Astrology describes the influence of the stars on human life. It was generally accepted until about 1600. Because the sun is the source of life, and the moon causes tides, it was felt that other heavenly bodies must also influence the earth. In the absence of modern science, this would help explain human behaviour and terrestrial phenomena, and allow for predictions.

The planets

Each planet has an individual influence:

  • Jupiter (Jove) disposes someone to be merry or ‘jovial'
  • Mars and Venus influence man to be warlike (martial) or loving respectively
  • The influence of Mercury is seen in the term ‘mercurial'
  • A ‘lunatic' is affected by the lunar cycle of the moon
  • The influence of Saturn is seen in the adjective ‘saturnine'
  • Planets also dominate particular days of the week (Sun-day etc.).

Planetary influence is affected by the planets' relation to each other (their constellation or aspect). Though planets could influence human behaviour, they could not determine it since, in Christian thinking, man has free will.

The stars

By picturing the geocentric universe as a circle, it can be divided like a cake into twelve equal slices. For about a month (starting on March 21st) each ‘slice' will appear in the east where the sun rises. This segment is said to be ‘in the ascendant'.

Each segment has a distinct grouping of stars, referred to by the signs of the Zodiac: Aries (the Ram), Taurus (Bull) , Gemini (Twins), Cancer (Crab), Leo (Lion), Virgo (Virgin), Libra (Scales), Scorpio (Scorpion), Sagittarius (Archer), Capricorn (Goat), Aquarius (Water-carrier), Pisces (Fishes). The stars in the ascendant were believed to further affect the influence of the planets passing through them.


People from all social ranks turned to astrologers to help them make important life decisions. The mathematics behind it seemed dazzling, and there was no alternative explanation of most events. Astrology also provided a satisfying link between the earth and the rest of the universe. Astrology thus formed part of the general way of thinking until it was displaced by modern science from the seventeenth century.

Scan and go

Scan on your mobile for direct link.