Middle Ages - Arabic Astronomy and Astronomical Instruments

Chapter 3 (Cambridge)

http://www-gap.dcs.st-and.ac.uk/~history/HistTopics/Arabic_mathematics.html#72

After the fall of the Roman civilization and the burning of the Alexandrian library, some scholars fled eastward. With the arrival of Islam as a dominating force, the Arabs took up the science and astronomy. In Baghdad, the ruler of the time built the ‘House of Wisdom’, one of the great libraries and learning institutions of the time. From about 800 A.D. – 1200 A.D. Arabic becomes the language of science in the western world. Arabic astronomy was influenced by a number of different sources including India, and the Greek astronomy of Aristotle and Ptolemy.

In this time, there were two main branches of astronomers

1. Islamic - this dealt with the issues of times and dates for religious purposes
2. Arabic – this was the mathematical tradition based mostly on Ptolemy’s almagest that looked at planetary motion and star catalogues.

In the Islamic tradition, astronomy had 3 main purposes

1- Calendar keeping. Islam runs on a strictly lunar calendar. This means that their year is actually 11 days shorter than a solar year. For this reason, holidays like Ramadan mover slowly back through the solar calendar. It takes about 30 years to move completely through the solar calendar. The new month also starts with the sighting of the crescent moon just after sunset. Depending on where you are in the world, this may vary by 2-3 days. Also local land features, or weather can alter when the moon is first spotted again after the new moon.

2- In Islam prayers are said 5 times a day – daybreak, sunset, nightfall, midmorning and midday. For this reason mathematical calculations were necessary to standardize when prayers should be said according to the shadows cast. This would need to be adjusted depending on latitude. To do the astronomical observations and mathematics required for time keeping the position of muwaqqit was developed.

3- The third important tradition was geographical. The tradition is that mosques must be oriented towards Mecca. The Kaaba is actually oriented on the bright star Canopus on one axis and the minor axis is oriented on the summer solstice sunrise. This involved some more sophisticated geometrical calculations, and models were later developed to this purposed.

Arab astronomers (who were not always necessarily Muslims) were also very mathematically oriented. One of their major references would have been Ptolemy’s Almagest. This work seems to have been translated into Arabic around 800- 900 A.D.

Many of the works of these Arabic astronomers would later become available to the European scholars through Toledo Spain.

The Arabs were active in two main areas - compiling tables of planetary motion, know as zij, and in producing stellar catalogues. Main of our modern stellar names are actually Arabic in origin from this time period.

One of the first astronomers around 900 A.D. to produce a set of tables that is probably one of the majors works in astronomy between Ptolemy and Copernicus was al-Battani. His work in particular would be influential on later astronomers such as Copernicus, Kepler and Tycho Brahe. His major work was the Kitab al- Zij, which would later be translated into latin as De Motu Stellarum (on the motion of stars)

In his work he includes a catalogue of 489 stars, refined the existing values for the length of the year, ( to a value of 365 days 5 hours 48 minutes 24 seconds), and of the seasons as well as values for the motion of constellation due to precession and the inclination of the ecliptic (Earth’s axial tilt)

One of the important advances he also makes is in the use of trigonometric functions for his calculations rather than geometric methods.

http://www-gap.dcs.st-and.ac.uk/~history/Mathematicians/Al-Battani.html

Many of his calculations and measurements seem to be more accurate than Ptolemy’s, and he improves on his numbers.

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The work of other Arabic astronomer in producing zij will later on be the model for a set of tables called the Alphonsine Tables, which are based on Ptolemy’s Almagest and the calculations of the Arabic astronomers in Toledo.

In the area of cataloguing 2 major catalogues were produced. These are what give us the Arabic names for the visible stars (like Betelgeuse, Mizar and Alcor). One was by Ulagh Beg – a noble who built his own observatory and compiled a catalogue of over 1000’s stars. The other was Al-Sufi whose

‘Book on the Constellations of Fixed Stars’ was an improvement on the catalogue found in Almagest.

The other work that was more mathematical in nature was the work done on Almagest. The Arabs were excellent mathematicians. They had gotten updates in trigonometry in the form of sines, cosine, and tangents from India, so they could improve Ptolemy’s work. One of their main goals was to remove the equant.

Two individuals in particular contributed to the removal of the equant and the eccentric (since these were deviations from true circular motion). These two concepts could not be reconciled with the physical reality of Aristotle model of heavenly spheres, so they tried to remove them

(Nasir) Al –Tusi in the 13^th century and Al Shatir in the 14^th century both developed systems that needed deferents and epicycles only. Although we do not know for sure, it seems as though Copernicus may have been familiar with the work of Al- Tusi in particular as he uses techniques that are very similar (something called the Tusi – couple). Al – Tusi also was the driving force behind the constructin of Maragheh - the major Arabic observatory of this period. Built in 1262, it became a centre of Arabic science. Copernicus did know about Al-Battani’s work, as he quotes him directly in Revolutions, so he may have also used the techniques of other Arab astronomers.

The Arabs did not develop any new cosmology of their own, but seemed content to accept the Greek geocentric models of Aristotle and Ptolemy. Their major contribution is to improve on these works and preserve for the later renaissance scientist, as well as developing better mathematics for the European astronomers to use.

Europe in the Middle Ages and Copernicus

Chapter 4 Cambridge

During the middle ages the ability to read Greek was lost in Europe and only a few scattered texts about science and astronomy were kept in monasteries in Europe. Knowledge wasn’t entirely lost, as the Church still had to be able to calculate the date for Easter. Some knowledge of constellations was stilled maintained, and some rudimentary astrological knowledge.

However in the first couple of centuries of the second millennium, stable powers began to emerge in Europe and with stable countries the quest for knowledge and the existence of universities and libraries emerge.

When the Arabs were pushed out of the Iberian peninsula, they left behind the written texts at their centres of learning, particularly in Toledo, Spain. Gerard of Cremona, in particular did a great deal of work in translating texts from the Arabic into Latin or European languages. This gives the Europeans access to works of Ptolemy, Aristotle, Plato etc.

In this time frame astronomy and astrology are grouped in as a branch of mathematics. Because Aristotle gave us the ‘macrocosm’ (universe) is reflected in the ‘microcosm’ (human body), physicians were also trained in astrology as part of the diagnosis of what was wrong with a patient, depending on what happened in the skies. (page 73)

Thomas Aquinas (a catholic saint), who was and Italian Dominican friar is responsible for the transposing of the world view of Aristotle and other Greek philosophers into the Christian viewpoint, and in this way Aristotlian physics and the geocentric model of the universe become part of the Christian view of the world. The spheres of the Aristotle’s model fit nicely fit the Christian interpretations of the Bible and with the Book of Genesis. The outermost sphere becomes the firmament, the crystalline heavens book the ‘waters above the firmament’ and the outer most part the container of the Universe is the ‘Empyreum’, the domain of God and the Angels (page 77)

At this time the Alphonsine Tables, are translated from an Arabic text based on the Almagest, and become the major source on planetary motion. In the more mathematical sense, Ptolemy’s model dominates the way in which planetary motions are calculated. Earlier Toledo Tables of motion are replaced by the Alphonsine tables (named after Alphonse X who was the patron) . These tables are based on the calculations from the Almagest.

In the early 1400’s there is an influx of Greek scholars into Italy and that part of Europe as they are fleeing the Muslim/Arabic conquest of the middle east, and the fall of Constantinople in 1453, bringing with them Greek versions of the works of Aristotle, Ptolemy and others.