One book dominated astronomy for nearly one-thousand four-hundred years. This was the Almagest of Claudius Ptolemy. Why was just one book so influential and so dominant that it took so long to be surpassed?

Ptolemy lived in Egypt in the period when it was part of the Roman Empire. We know very little about him personally and the dates of his life (AD.100 to 178) are only approximate. He worked in the city of Alexandria. Alexandria is located on the northern (Mediterranean) coast of Egypt. It was a city renowned for learning and had a famous library and museum. Possibly, Ptolemy himself worked in one or other of these institutions. Ptolemy wrote in Greek, which was the scientific and philosophical language of his day. He is famous for writing many books on scientific subjects. These are:

Of these books, apart from the Almagest, the two most important are the Geography and the Tetrabiblos,both of which had a great influence on the development of their subjects.

The Almagest was originally called the ‘Mathematical Systematic Treatise’ or ‘Mathematical Syntaxis, it is often just called the Syntaxis. It is Ptolemy’s major work on astronomy. In it he drew on several hundred years of Greek science. He drew on Aristotle for his overall philosophical standpoint. His geometrical methods came from Euclid. His spherical geometry was from Menelaus of Alexandria. He drew on the Astronomer Eudoxus, for his planetary models and on Hipparchus for his catalogue of stars and astronomical methods.

The Almagest covers the whole scope of Greek astronomy in 13 books. All Ptolemy’s writing is very ordered and logical. Therefore, the Almagest has the two advantages of being, totally comprehensive and logical in its structure.

One curiosity about the writing of the Almagest is that the many mathematical and astronomical tables in contains normally are 45 lines in length. This is thought to conform to ancient standard of papyrus size.

It is difficult to summarize the astronomy of the Almagest. You will sometimes see books on astronomy with crude earth-centred diagrams showing sun, moon and five planets circulating around, proporting to be the "Ptolemaic System", these are highly simplified. The astronomy of the Almagest is complicated, the models for the Moon and for Mercury particularly so.

The astronomy of the Almagest, has as its centre the earth, which is spherical, but which remains at rest. The whole heavens, including the stars, planets, sun and moon rotate around the earth every day (24 hours). Additionally, the sun, moon and planets have their own movements, these are explained by the principle of uniform circular motion. Uniform circular motion was part of the philosophical baggage of Greek science which could not be ignored. It meant that the movements of all astronomical bodies had to be explained using circles. Ptolemy uses various arrangements of circles to model the movement of different bodies.

The Planets: Venus, Mars, Jupiter and Saturn, orbit the Earth on Equants, these are circles whose centre of rotation is a point away from the Earth. The Equant, approximates to the elipse used in Kepler’s Laws of Planetary Motion. The Equant was probably the best part of Ptolemy’s astronomy.

In order to explain the retrograde motion of the planets, Ptolemy has the planets carried on additional circles called Epicycles whose centre of rotation is carried on the Equant circle or Deferent.

Various combinations of circles are used to model the movements of astronomical bodies. With varying degrees of success. I do not want to go into this too far, I have difficulty with it myself, it is very technical and deserves a talk just to itself.

The Almagest contains also a catalogue of 1022 stars in 48 constellations. It has descriptions of various sighting instruments and a table of chords to help with calculations.

The Almagest was so successful in encapsulating the astronomy of its day, that many the books of earlier astronomers such as Hipparchus, were no longer copied and are lost to us.

The Almagest is dated to around 150 and it was the last major work in the Greek scientific tradition.

Due to the climate of Egypt a number of papyri from this period have been discovered by archaeologists. Amoung these are a few astrological tables and horoscopes based on Ptolemy’s "Handy Tables". This indicates that the major use of Ptolemy’s work was astrological in nature.

In 203 we have the first of many ‘commentaries’ on Ptolemy, these sought to make Ptolemy accessible or easier to use. Commentaries attempted to explain the models used in the Almagest, they did not represent any advance on Ptolemy and encouraged the reverence in which he was held. Two important commentaries were by, Pappus (320) and Theon of Alexandria (360), these failed to improve on Ptolemy’s astronomy and served mainly to educate students in his methods. About this time, there is evidence of the introduction of the plane astrolabe, although it is debatable if this was an advance on Ptolemy’s own instruments.

In 389 it is possible that the Great Library of Alexandria was destroyed. The fifth century saw further decline and chaos as the Roman Empire colapsed. There were a few more commentaries on the Almagest, by Serapion, Arcadius and the celebrated daughter of Theon of Alexandria, Hypatia, but this was the last dying gasp of Alexandrian science. In 415, Hypatia was murdered by a mob of ‘Christians’ who disagreed with her Pagan beliefs.

Greek astronomy continued at a much lower level in Athens and in Constantinople (the new capital of the Empire). Indeed, it is in Athens, around 450 that we get the first hints of dissatisfaction with Ptolemy. The Neoplatonist philosopher Proclus wrote a commentary (the "Hypotyposis") in which he rejects Ptolemy’s theory of precession. Also in Athens during the period 475-510 two brothers Heliodorus and Ammonius made a series of observations, one of which showed up a gross error in the tables of the Almagest. On 13 June 509 they observed a conjunction between Mars and Jupiter, Ptolemy’s tables predicted the conjunction on the 17th of June by which time the planets were far apart. However, even Athens could not escape the political and religious conflicts convulsing the Empire. In 529 the philosophy schools in Athens were closed, they were opposed by the growing numbers of christians who linked them with supporters of the Pagan religion.

Boethius (c.480-524) is sometimes called ‘The Last Roman’, he was an important official in the Roman Gothic Kingdom in Italy. He translated the Almagest into Latin. This Latin translation was lost shortly after, which probably set back astronomy in Western Europe by 700 years.

In the early seventh century (610-645) Emperor Heraclius in Constantinople took an interest in astronomy and he, or his astronomer Stephanus produced a commentary on Ptolemy’s "Handy Tables".

You can take your pick in theories for the collapse of the Roman Empire, barbarian invasion, dependence on slavery, christianity, ecological disaster or imperial decadence. But whatever, the reason this was not a climate that astronomy or any scientific study could thrive. Instead, the few educated people who could preserve some of the books and knowledge of the past, looked back on Ptolemy’s achievements in awe and reverence.

However, a new empire was now expanding, this was the empire of Islam. The Prophet Mohammed (570-632) and his followers the Caliphs had built up a vast empire, which included Egypt, Syria and Mesoptamia. The common language of the empire was Arabic, the language of the Koran. The Abbasid Caliphs built themselves a new capital in 762, this was Baghdad. In this very early period, Arabic astronomy was influenced by Indian astronomy. I do not have time here to trace the history of Indian astronomy, but it had been influenced by Greek ideas from the time of Alexander the Greats conquests in the east. This increased the arabs receptivity to the ideas of Greek astronomy.

From the 750’s onwards the use of paper was introduced into the Islamic world from China. This made books much cheaper to produce and encouraged scholarship. In Western Europe books were made using parchment which made them enormously expensive.

In the eigth and ninth centruries the Almagest was translated first into Syriac and then into Arabic. The first translation into Arabic (early 800s) was by Al Haggag ben Jusuf ben Matar. An arabic Almagest of 827 still exists in Leiden University Library. With translation into arabic, the Mathematical Syntaxis became the Almagest, meaning ‘The Greatest’, this might at first only refered to the length of Ptolemy’s book, but later came to indicate the reverence in which it was held.

One of the greatest of the Abbasid Caliphs was Al Mamun (786-833), he established the ‘House of Wisdom’ in Baghdad, this was a library and centre of translation where many Greek astronomical works were translated into arabic. An indication of the high esteem in which the Almagest was held is the story that Al Mamun demanded a copy in a peace treaty with the Byzantine Emperor! Al Mamun also founded an observatory in Baghdad (829).

During the ninth century (836-901) a Syraic astronomer Thabit ibn Qurra (or Tabit ben Korra) made a number of criticisms of the Almagest and was at one time credited with the theory of ‘trepidation’. Trepidation was developed to explain an error of Ptolemy. Ptolemy had stated that the rate of precession of the equinoxes is one degree in one hundred years, this rate is too slow. Because of Ptolemy’s authority astronomers believed that the rate of precession was increasing and that it varied over time. It is now understood that trepidation was introduced much later by another Islamic astronomer, with a similar name, working in Andalusia.

In the 850’s Al-Khwarizimi (a Persian astronomer) produced some very influential astronomical tables (zij), based on Ptolemy’s ‘Handy Tables’.

Muhammed Al Battani (Latin: Albategnius) who lived c.850 to 929 worked at Al-Raqqa on the Euphrates. He made a commentary on Ptolemy’s astrological work the "Tetrabiblos". He also improved on Ptolemy’s model for the orbit of the sun.

Around 860, Ahmed ben Muhammed Al Fargani (Latin: Alfraganus) wrote a book called the "Elements of Astronomy" he accepted Ptolemy’s ideas. This book was later translated into Latin and was very influential in Europe.

Abd Al-Rahman b Umar Al-Sufi (903-986) wrote his "Book of the Fixed Stars", this was based on the star catalogue of the Almagest, but Al-Sufi did update the star-magnitudes by making his own observations. A number of attactively illustrated manuscripts of Al-Sufi’s book have survived, including one at the Bodelian Library, Oxford.

Mohammed Abu ‘Wefa Al Buzjani (940-998) working at the Sharaf Al Daula observatory in Baghdad, improved on the accuracy of Ptolemy’s trigonemetric tables. He also produced a commentary on the Almagest for students, called the Katib al-Kamil.

During the tenth century there was a number of arabic works which cast doubt on Ptolemy. These are sometimes called ‘Shukuk’ meaning ‘doubts’.

Ibn al-Haytham (Latin: Alhazen) (965-1040) lived in Cairo near the Al-Azhar Mosque, he copyed books for a living, but also wrote 92 books on a wide variety of subjects. One book was ‘Al-Shukuk ala Batlamyus’ or ‘Doubts concerning Ptolemy’. This contained a series of 28 essays highly critical of Ptolemy’s books the Almagest and "On the Planetary Hypotheses". He wrote another book ‘Hay’at Al-Alam’ or ‘The Configuration of the World’ in which he tried to put the models of the Almagest into a physical system.

In the later Islamic period the study of the Almagest spread to the farthest parts of the Islamic world. In Spain (which was then called Al-Andalus) an Islamic scholar, Maslama Al-Majriti was studying the Almagest in the 10th century. By the 12^th century a number of Islamic astronomers were working in Spain. Gabir ben Aflah (Latin = Geber, died 1145) and Nur ed-Din al Betrugi (Latin = Alpetragius) both studied the Almagest and made criticisms of Ptolemy, but they could not develop any alternative to Ptolemy’s system.

In Persia, a keen astronomer called Nasir al-Din al-Tusi (1201-1274) tried to travel to Baghdad to pursue his studies, but was captured by the Assassins. The Assassins were a kind of religious-military brotherhood who used political murders to obtain power. In 1256 Al-Tusi gave away secrets of the castle he was held in by the Assassins to an invading Mongol army. The Mongols captured the castle and in return offered to build Al-Tusi an observatory!

Al-Tusi built his observatory at Meragha in Persia. It was built on an impressive scale with the assistance of Chinese astronomers. Al-Tusi modified some of Ptolemy’s models and criticised the Almagest in his book 'Memorandum on Astronomy'. His pupil Nizam al-A’Raj, also made a commentary on the Almagest.

Further East, in Samarkand, in the 15^th century, the ruler of Turkestan (and a grandson of the famous Timur) took an interest in Astronomy. This was Ulugh Beg (1393-1449), who built an observatory with gigantic instruments (quadrants, armilliarys etc.) He was able, through accurate observation, to point out errors in Ptolemy. He was also the first since Ptolemy to issue a star catalogue (of 992 stars). Unfortunately, he was not such a good politician as an astronomer and was executed by his own son who took power. The ruins of his observatory can still be seen in Samarkand.

The Islamic astronomers kept the stronomy of the Almagest alive for over a thousand years. The arabic translations of the Almagest were widespread and insured the survival of the book. Islamic astronomers criticised Ptolemy and improved on his planetary models, but were unable to free themselves from the concepts of geocentricism and uniform circular movement. However, only a tiny proportion of Islamic astronomical books have been studied in the west and it is not impossible that we may have to reassess our appraisal in the light of new manuscript discoveries. Certainly, the Europeans of the middle ages and renaissance eagerly seized on the few Islamic astronomical books they could access with great interest.

Byzantium was the Greek-speaking survival of the Eastern half of the Roman Empire. It was in continual decline until 1453 when it consisted of little more than its capital of Constantinople, that fell to Turkish invaders that year.

During the 9^th Century old classical manuscripts were sought out and re-copied onto parchment, this included the Almagest in the original Greek.

During the 11^th-13^th centuries, under the influence of Islamic astronomy, it became known that Ptolemy’s latitude for Constantinople was wrong.

Astrology, rather than astronomy, was the dominant activity of Byzantine scholars interested in the stars.

In 1158 an Almagest was given by the Byzantine Emperor (Manuel I Comnenus) as a gift to King William I of Sicily. This was an indication of the high regard in which the book was still held.

Theodorus Metochites (1270-1332) produced a treatise ‘Elements of Astronomy’ demonstrating his mastery of the mathematics of the Almagest. His pupil Nicophorus Gregoras was able to predict a solar eclipse (16 July 1330) from Ptolemy’s tables.

However, the Turkish capture of Constantinople in 1453 led to a flight of scholars (such as Cardinal Bessarion) and books including Greek Almagests, to the Latin west.

The Almagest first started to creep into Western Europe in the 12^th century. In the 1160’s a poor Latin translation was made in Sicily from Greek. A better translation was made from Arabic to Latin by Gerard of Cremona (1114-87) in Toledo, Spain. In Toledo there was a school of translators who, with the help of Arab and Jewish scholars, translated a host of scientific books into Latin. Gerard of Cremona himself is said to have translated over 70 books.

The translation of the Almagest into Latin was of great importance as Latin was the scientific and learned language used throughout Western Europe at that time, particularly within the Church.

Jacob ben Machir ibn Tibbon (1236-1312), known in Latin as Prophatius, he was a Jewish doctor working at the University of Montpellier, translated the Almagest from Arabic to Hebrew. He also designed a new kind of quadrant, explained in his book as ‘Jacobs Quadrant’. His books were used later by Copernicus. The role of Jewish astronomers during this period deserves greater study.

Also during this period, the myth that Ptolemy was a king of Egypt emerged. This was due to confusion of Ptolemy with the dynasty of Greek kings of Egypt, the Ptolemies (ruled 305BC to 30AD). There is no evidence that Ptolemy was even related to this royal family, but many pictures of him from the middle ages and renaissance depict him wearing a crown.

In 1460 the Greek Cardinal Bessarion (1403-72) arrived in Vienna, he was on a diplomatic mission. His object was to obtain the military assistance of the German Emperor against the Turks who had recently captured Constantinople. He was a keen scholar and brought with him a great collection of Greek manuscripts. He had even started on a translation of the Almagest into Latin himself, but he was too busy to complete it. In Vienna he met up with two German astronomers, Georg Peurbach (1423-1461) and Johann Müller, also known as Regiomontanus (1436-1476). Regiomontanus was the pupil of Georg Peurbach and they worked at first together on a book, which Regiomontanus completed in 1462-3. This was the ‘Epitome of the Almagest’. It was not simply a translation of the Almagest, instead it took the astronomy of the Almagest and put it into a clear modern light. It was a triumph of renaissance Greek study.

1515 saw the first printed edition of the Almagest in Venice. It was the Latin translation of Gerard of Cremona. Then in 1538 the Greek text of the Almagest was printed in Basel (by Hervagius), the text came from a Greek codex once in the possession of Regiomontanus.

At this time, the man who was to render the Almagest obsolete was making observations and working on the theories which would transform astronomy.

Nicolaus Copernicus (1473-1543) was born in Torun, Poland, his uncle was Bishop of Varmia and he enjoyed the patronage of the church throughout his life. He had studied in Italy at Bologna and learned Greek in order to better understand the Almagest. In 1513 he had circulated a brief anonymous phamphlet called the ‘Commentariolus’ in which he put forward a heliocentric (sun centred) universe. But he felt he lacked evidence for this view and spent the next 17 years gathering observations as evidence. He was influenced by the Almagest and by Regiomontanus’s ‘Epitome of the Almagest’. Finally in the year of his death, his great book ‘De Revolutionibus Orbium Coelestium’ (On the Revolutions of the Heavenly Spheres) was published.

In some respects Copernicus had retained parts of Ptolemy’s astronomy. Uniform circular motion and the use of epicycles was still to be found in the Copernican system. But the long reign of the Almagest was now over.

The final blow came with Johannes Kepler (1571-1630) whose laws of planetary motion introduced the elipse and did away with uniform circular motion for good.

I would not argue that study of the Almagest stopped after Copernicus, it certainly did not. But it no longer reigned supreme over astronomy and interest in it became increasingly historical.

Ptolemy was regarded in a kind of benevolent way by the early 20^th century, he had preserved Greek astronomy, ancient observations and had made his own contributions.

But in 1977 the American astronomer Richard R. Newton published a book called "The Crime of Claudius Ptolemy". In it he uses modern statistical and analytical techniques on the tables and observations of the Almagest, he concludes that Ptolemy:

• Fabricates his observations and missreports those of earlier astronomers in order that they match his theories.
• Passed work of the work of Hipparchus as his own (in the case of the star catalogue).
• Makes failures of comprehension.
• Promotes models for the Moon and Mercury that are clearly wrong.
• Lost for us the genuine astronomy of the ancient world.

The only good thing he has to say, is that the equant model of Ptolemy has some value for Venus and the outer planets. He refuses to use the term ‘Almagest’ for Ptolemys book, using its original title ‘Syntaxis’. Finally he says (page 378):

So, from being King Ptolemy, the greatest astronomer of antiquity, Ptolemy had sunk to being an academic criminal! More recently, attempts have been made to restore Ptolemy’s reputation: amoung them G.J. Toomer the author of the most recent (1984) English translation of the Almagest. From this I leave you with a quotation from the Almagest (Book 4, Chapter 9):

So, in conclusion, why did the Almagest reign for so long? Once answer could be its own strengths as a book, it is logical, well written and comprehensive. The models of planetary motion worked for most planets to an accuracy of a degree or less. Also, there was little alternative to the astronomy of the Almagest, the works of other Greek writers on astronomy were superseded by the Almagest. There were other external factors, the decline of the Roman Empire meant successors to Ptolemy did not have a stable environment in which to work. There was also the philosophical conservatism of the Islamic astronomers, who had the Almagest for so long. Then in Europe, there was an inferiority complex regarding the ancients which took hundreds of years to throw off.