The Medieval Period

The span of time between the breakup of the Roman Empire in the west (476 A.D.) to the beginning of the Renaissance (c. 1400 A.D.) is usually referred to as the Middle Ages or the Medieval Period. Near anarchy reigned throughout much of Western Europe in the early years of this epoch (500 AD – 1000 AD). The continent divided into small principalities constantly at war with one another, and daily life for most of the population became much more a struggle for survival than had been the case under the Romans. Under these conditions very little effort could be devoted to intellectual pursuits. The only continent - wide institution was the Catholic Church, and scholarly activity, conducted in Latin, focused almost exclusively on religion.

However, Western Europe was not the world. While Rome had collapsed in the west, the eastern half of the empire, centered at Constantinople, continued to thrive. Although much intellectual life in the Eastern Roman Empire revolved around religion, just as in the west, the continuity of civil order at least provided safe haven for ancient manuscripts, insuring that ancient learning would be preserved. In other parts of the world during this same period Mayan civilization reached its peak in Yucatan, China was flourishing under a succession of imperial dynasties, and learning was being carried to new heights throughout the Islamic world.

The contributions to astronomy by Islamic civilization was particularly significant. By 1000 A.D. the new religion of Islam proclaimed by the prophet Mohammad in 622 AD had spread from its point of origin on the Arabian peninsula to cover a large fraction of the world's surface from Spain and North Africa on the west to India in the East. Thus, virtually all of the centers of ancient learning were unified under a single social system. The Arabs and their newly converted Muslim compatriots synthesized the ancient wisdom of many lands and added their own unique perspectives to advance astronomy beyond the cosmology of the Greeks and Romans. The most significant advances during this epoch are summarized below:

Arabic/Islamic Civilization (622 – 1400)

· Star names

The most obvious impact of the Arab world on astronomy is in the names of the stars. Virtually all naked eye stars now bears an Arabic name (or at least a name whose origin is Arabic). Examples of prominent bright stars include:

Altair

Algol

Betelgeuse

Deneb

Rigel

Vega

Arabic names are prominent because the Arabs made precise maps, beginning with translations of Hipparchus and then extending these to include southern stars not visible to the Greeks.

Other astronomical/scientific terms of Arabic origin

algebra

algorithm

altitude

azimuth

cipher

nadir

zenith

zero

· Advanced Mathematics: Arabic numerals, Zero and Algebra

o Our familiar counting system 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 came to the west via the Arabs, although its point of origin is now thought to be India.

o zero (0) is an extremely important concept which allows large (and using decimal notation, small) numbers to be constructed using relatively few symbols. This, too, originated in India and was transmitted to Europe via the Arabs. It was a great improvement over the Roman numeral system which had no zero.

o Algebra (from the Arabic al-jebr, to calculate) which allows formulas to be written abstractly is the basis for all mathematical science. Omar Khayyam (1048 – 1131), better known to English majors as a poet, was an Iranian astronomer who made a number of contributions to the development of algebra.

· Optics

The Arab scholar Alhazen (965-1040) working in Basra, Iraq, performed experiments and wrote a number of books on optics, mathematics and astronomy. He made particularly significant contributions to the understanding of the reflection and refraction of light rays. He is credited with inventing the camera obscura, a darkened room in which light from the outside enters through a small hole in one wall and projects an image of the outside scene on the wall opposite this opening. This is the principle of the modern camera. All you need is a way to make the image permanent and you have photography.

· Star catalogues

The first major revision of Ptolemy's star catalogue (Almagest 150 AD) was published by al-Sufi (903 – 986), an Arab astronomer working in Baghdad. Al-Sufi's book was entitled Book on the Constellations of Fixed Stars. Here he gave Arabic names to the stars and constellations and improved the accuracy of star magnitudes. His work was the basis of a later catalogue, the Alfonsine Tables, thought to have been prepared in 1327 by a committee of 50 Islamic scholars in Toledo, Spain for King Alfonso X. These tables became highly influential in Europe in the late middle ages and were still being used as a reference by Copernicus and Kepler at the beginning of the Renaissance.

· Translations of and commentaries on Greek philosophers

The works of ancient Greek and Roman philosophers were rediscovered by Latin Europe in the later middle ages through contact with the Islamic world. The Arabs translated Greek manuscripts into Arabic and sometimes into the local languages of newly acquired Islamic lands. The Arabs did more than simply translate ancient texts. They wrote commentaries on these works, corrected errors, and expanded concepts to achieve, among other things, greater accuracy in predicting celestial events. The usual route of this knowledge into Western Europe was through Spain which was under Islamic domination for 700 years. Particularly significant was a complete translation of Aristotle which became available in the west around 1000 AD. This reinvigorated intellectual life in the Latin speaking world and eventually led to the Renaissance in the fifteenth century.

· Improved Astrolabe

The astrolabe was the most widely used astronomical instrument during the middle ages. It is basically a flat version of the celestial sphere and can be use to determine the time of day based on a single observation of the sun during daylight hours or the stars at night. It can also predict the times of sunrise and sunset as well as the rise and set time of other astronomical objects. A rudimentary version of this instrument was invented by the Greeks, but it was perfected by the Arabs during the Medieval period. The modern star wheel which shows the appearance of the sky for any hour of the night is a direct descendent of the astrolabe.

The Later Middle Ages in Western Europe

From about 1000 AD on, intellectual life began to improve throughout Western Europe. Well known scholars of the period include Thomas Aquinas, Dante Alighieri, and Roger Bacon. Two scholars in particular are of interest in the history of astronomy: Nicole Oresme and William of Ockham.

Nicole Oresme (1323 – 1382)

Nicole Oresme was a French philosopher born in Normandy. He wrote extensively in mathematics, physics and philosophy. Of most interest to astronomers, however, is his remarkable treatise concerning Aristotle's On the Heavens. Here he argues, contrary to Aristotle, that it is impossible to tell whether it is the Earth or the celestial sphere which is turning. He asks us to consider the case of a boat moving in still water. The passengers in the boat feel no sensation of motion. Thus, if this boat is passed by a second boat, it is not possible to tell from observation which of the two is moving. Similarly, we cannot tell whether it is the motion of the Earth or of the heavens which produces night and day. However he concludes the essay by proclaiming that he himself believes it is the Earth which is stationary, but this belief is based on faith (it says so in the Bible), not reason. The point here is to illustrate that the idea of a non-stationary Earth had been seriously considered several centuries before it was proposed by Copernicus.

William of Ockham (1288 – 1348)

William of Ockham was born in England but spent most of his adult life in Germany. He is best known for his dictum known as Ochkham's Razor:

"It is vain to do with more what can be done with less"

The is usually interpreted to mean that the simplest explanation of a phenomenon is most likely to be correct. This remains something of a guiding principle in science, although it should be remembered that this is philosophy, not a research finding. It is also tricky. The obvious explanation for night and day is that the sun moves around the Earth. However, this is a reasonable explanation only for the most elementary observation. If other features of the sky are included such as the annual motion of the sun on the celestial sphere, retrograde motion of the planets, and precession of the equinoxes, retaining the idea that the earth is stationary results in a very complex mathematical model. In fact, Ockham's Razor was invoked by Copernicus as a strong point in favor of his heliocentric cosmos, arguing that it resulted in a simpler system.