The Day the Universe Changed - James Burke [60]
Pope Sixtus IV had asked the German astronomer Regiomontanus to tackle calendar reform, but without adequate observational tables there was little he could do. Insufficient data had been gathered for two reasons. The first was due to the fact that prior to the oceanic crossings, navigators who relied on prevailing winds and coastal waters were relatively unconcerned about what went on in the sky. According to Aristotle, whose view of the cosmos was dominant, there was little celestial activity to observe.
The difficulty with the calendar: solar and lunar months do not fit. The outer ring shows the lunar months, each of 28 days, giving a lunar year of 336 days. This is 29¼ days shorter than the inner, solar year.
Aristotle’s cosmological system, which had survived almost intact for two thousand years, was based on a common-sense view of the universe. To the ordinary observer the sky seems to move. The stars, whose positions never change, wheel past every night. At the Pole they never disappear, circling round the North Star. Among the fixed stars five wandering lights, called planets from the Greek word for ‘wanderer’, can be seen. The moon circles the earth, as does the sun.
Aristotle explained these phenomena by means of a cosmological system made up of eight crystalline spheres on which the sun, moon, planets and stars were each fixed. These spheres rolled eternally round the earth, which did not move. Moreover, while the sky was evidently perfect and unchanging, earth was not. Terrestrial things decayed and died. All motion on earth was straight-line motion, manifested by the vertical manner in which objects sought their ‘preferred’ position, the lowest they could find.
In the sky the perfect, eternal motion of the stars was circular. The spheres were composed of ether, a substance which could neither be destroyed nor changed into anything else. This was the fifth element. The other, terrestrial, elements were earth, water, air and fire. The heavens were incorruptible because their motion was circular and so they never suffered ‘forced’ movement. On earth, any natural movement occurred in straight lines, although the natural state of things was at rest. Any movement on earth was, therefore, forced movement, except for that of the four elements. Air and fire, being light, rose. Earth and water, being heavy, sank. All things were made up of these four elements and decayed because they were frequently subjected to forced movement.
The earth was a sphere, because that was a perfect shape and because its shadow could be seen on the moon. he earth stood still because had it moved this would have been due to either natural or forced movement. Forced movement destroyed things and the earth still existed, so whatever movement there might be would have to be natural. The only natural movement possible on earth, however, was movement straight to the centre of the earth. If the earth turned or moved in any way this would presuppose two natural movements. The only possible explanation was that the earth stood still. This view was, of course, supported by the Bible.
As for how the heavenly spheres moved, either there were a number of things which were self-propelling, like the planets, or, more likely, God had played the role of original un-moved mover who could initiate eternal movement.
There were obvious anomalies in this system, which made things difficult for the Church. Over the centuries it had explained them in various ways. The principal irregularity was visible to the naked eye. There were occasions when the planets changed course: Mars, for instance, would sometimes stop and go backwards. Given a system in which the celestial spheres could not change direction the only convincing explanation was that put