[MUSIC] Hello again. In the previous unit, we saw how the theory of the Great Year, as mentioned by Plato in the Timaeus, was received in antiquity. In this video, we'll explore the same topic further. But now, we'll focus on the reception of the doctrine in medieval Islam, and then in the Latin Middle Ages in Europe. In Islam, during the Middle Ages, astronomy and astrology were usually mingled with one another under a single designation, 'ilm at nujum, literally, the signs of the stars. It is commonly acknowledged that by the end of the 8th century CE and therefore, coinciding with the glorious reign of the Abbasid caliphs in Baghdad, the signs of the stars had already fully absorbed components stemming from three great cultures of the past, the Greek, the Iranian, and the Indian. This assimilation was made possible through an extraordinary movement of translation. A movement which has possibly no equivalent in history. For about two centuries, in Baghdad and in many other places of the Arab Muslim world, an astonishing number of scientific and philosophical works originally written in Greek but also in Persian and Sanskrit, were translated into Arabic. And made available to the intellectuals of the time. Among these scientists, special note should be made of Abu Mashar al Balkhi, called Albumasar in the Latin tradition. His influence upon astrology in the Middle Ages, whether in the East or in the Latin West, remained unparalleled. In particular, Abu Ma'shar was the author of various works of historical or mundane astrology. The aim of these works was to account for the influence of great planetary conjunctions and millennial periods on large areas and communities of people in this world. Curiously enough, one of these books the Kitab al Uluf, the book of thousands, mentioned several Indian astronomical systems that incorporate the doctrine of the Great Year in very much the same sense as Plato in the Timaeus. These systems differ from one another in the measurement of the great cycle, which ranges from 180,000 years to 4.32 billion years, according to the system used. But they all postulate that the beginning of the present cycle took place in the year 3,101 BCE. And that it was marked by a universal flood, as well as by the conjunction of all the planets in the first degree of the zodiac, or the zodiacal signs of Aries. In his masterly reconstruction of Abu Ma'shar's lost treatise, David Pingree situates the direct provenance of these astronomical speculations in the Brahmasphutasiddhanta. A treatise written by the Indian astronomer Brahmagupta in the year 628 of the Common Era. Brahmagupta, in turn, may well have taken out these materials from Aryabhata, who lived one century before and is often regarded as the first major astronomer and mathematician in the Classical period in India. In their astrological epistle on cycles and revolutions, the 10th century group of thinkers known as Ikhwan al Safa, Brethren of Purity, also explicitly attribute to the Indians, the origin of the conjunctional Great Year with an alignment of all the planets in the first degree of Aries. In this case, however, the periodicity of the cycle is mentioned as 360,000 years. Interestingly enough, in another of their epistles, the Ikhwan also report an Indian fable or allegory designed to illustrate, rather as we did with the clock example in the previous unit, the problem of the conjunctional Great Year. Or in other words, the way to find the least common multiple of the planetary revolutions. They do this by taking the example of seven men who are asked to run around the walls of a circular city, each at his own speed. The reader is asked to calculate the number of revolutions each one will perform, before they all come back together to the starting line. It was, no doubt, a very nice attempt to explain the mechanism of the Great Year. But unfortunately for our authors, who were clearly not experienced in mathematics, the values provided by the calculation led them to a wrong result. In the history of the Great Year doctrine, medieval Islam also had its share of confusions and distortions of the original concept. The most serious of these is undoubtedly the amalgamation of our conjunctional Great Year with another great cycle of the world, technically called the cycle of equinoctial precession. It is impossible to determine exactly when this amalgamation of theories took place. But what is certain is that it already appears in the work of the astronomer Farghani, the Alfraganus of the Latin tradition, who was also active during the 9th century. But what is the equinoctial procession? Well, in the 2nd century BCE, it was discovered, presumably by the Greek Astronomer Hipparchus, that the axis of the Earth is not absolutely fixed in space. Rather, it slowly rotates like a spinning top around the poles of the ecliptic. The ecliptic being the circle representing the sun's apparent path during the year. Likewise, the vernal point, that is, the point of intersection between the ecliptic and the celestial equator, moves at the same speed along the ecliptic with a retrograde motion. This movement has immense implications, since it affects the coordinates of all stars. And it was called equinoctial precession because it brings, every year, an adriance vernal equinox from east to west, through the stars of the zodiac signs. It was also in order to account for this movement of precession that many astronomers began to introduce into their system, a ninth sphere, a sphere without stars, in addition to the starry sphere, and to the seven planetary spheres. We do not know the value assigned by Hipparchus to the processional cycle, since his work has been lost. But we do possess the estimate provided in the 2nd century C.E. by Ptolemy, in his famous Almagest. Ptolemy's estimate was 36 seconds of arc per year. Or 1 degree in 100 years, which comes to a complete revolution in 36,000 years. Owing to the incomparable authority of the Almagest as a model of theoretical astronomy, this value of 36,000 years became the canonical figure for the movement of precession during most of the Middle Ages. It goes without saying, that from a strictly astronomical point of view, the movement of precession and the doctrine of the Great Year, have absolutely nothing to do with one another. The latter involves all the heavenly spheres. Whereas the equinoxal precession is involved with only one of them, namely the stratosphere. But since these two periods could both be regarded in some way as the greatest cycle of the universe, we may perhaps understand the reason why these two theories were confused with one another. The confusion may well have been facilitated by the close resemblance of the values attached to each cycle. 36,000 years for the precession, and as we have seen with the Ikhwan al-Safa, 360,000 years for the conjunctional Great Year. I shall develop this a little further in another video. [MUSIC]
