A Brief History of the Universe Page 3
During more than two centuries of Persian rule, Babylonian astronomy continued to improve in the accuracy of both observation and mathematical predictions. The city of Babylon again shifted its political allegiance in 330 BC when the Persians were conquered by the armies of Alexander the Great. This began one of the most significant periods of cultural diffusion in all of history: the Hellenistic period.
Alexander planned to restore some of the glory of Babylon by making the city his eastern capital. He died there in the palace of Nabuchadrezzar II in 323 BC, and in the wars between his successors, Mesopotamia suffered much from the passage and the pillaging of armies. When Alexander’s empire was divided in 321 BC, one of his generals, Seleucus, received the province of Babylonia to rule.
With the aid of Ptolemy, Seleucus was able to enter Babylon in 312 BC. He held Babylon against the forces of Antigonus for a short time before marching east, where he consolidated his power. It is uncertain when he returned to Babylonia and re-established his rule there; it may have been in 308, but certainly by 305 BC he had assumed the title of king.
With the defeat and death of Antigonus at the Battle of Ipsus in 301 BC, Seleucus became the ruler of a large empire stretching from modern Afghanistan to the Mediterranean Sea. He founded a number of cities, the most important of which were Seleucia, on the Tigris, and Antioch, on the Orontes River in Syria.
As a result, the population of Babylon was forced to move to the newly founded metropolis of Seleucid 62 miles (100 kilo-metres) north. The old city of Babylon never recovered from the removal of its intellectual and political core.
At Seleucid, a highly advanced form of Greek astronomy developed. The community made a particularly important advance by reviving Aristarchus of Samos’ hypothesis that the sky could be explained by assuming the Earth turns on its axis once every 24 hours and along with the other planets revolves around the Sun. Most Greek philosophers who could not believe that the big, heavy Earth could revolve around light, celestial bodies, later rejected this explanation.
The Babylonian scribes had eventually learned to produce ephemerides, tables illustrating future positions of the Sun, Moon and planets. So in addition to the very careful record of observations that were available, the scribes made use of their numerical system and their knowledge of mathematics to take full advantage of the cycles revealed by their observational records. Once the ephemerides were completed, astrologers could make predictions without the observations and in all kinds of weather. This new mode of providing written, predictive information had hugely important implications to the future of astronomy, meteorology and navigation.
The marriage between astrology and astronomy has had a strange history. There is no question that during the Babylonian era the motivation for collecting celestial data was to satisfy the inclination for astrological knowledge. For many hundreds of years, particularly during the late first century BC, scribes and priests increasingly obtained this data.
The Babylonians were never attracted to the form of astrology popular today, that which is based on Greek geometric cosmology. Greek cosmology uses the celestial sphere model and the zodiac to interpret personality traits for horoscopes, with birth charts drawn from ancient myths and legends. With Babylonian astrology, celestial configurations were not the final word and emphasis was placed on the avoidance of unfavourable prognostications. This philosophy was therefore quite different than the version of astrology handed down to today’s practitioners from the Greeks.
The difference between these two approaches was mainly due to the influence of two major writings from Ancient Greece, Plato’s Timaeus (360 BC) and Ptolemy’s Tetrabiblos (late second-century BC). Thereafter, classical astrology gave a naturalistic rationale for natal horoscopes, marking the split between astrology as ‘divination’ (Babylonian) and astrology as ‘science’ (Greek). The latter eventually split off and grew into what today is recognized as the science of astronomy. It is unfortunate that historians of science do not today interpret this split between astrology and astronomy in a positive way. This is undoubtedly due to the proliferation of untenable claims to interpret birth charts and horoscopes, which have since popularized the field of astrology, though in an unscientific way.
There can be little doubt that without the influence of astrology, astronomical observations from the ancient world would never have existed. Though many astronomers and other scientists loathe the concept, astrology has made an important contribution to the history of science. The thousands of dusty clay tablets in the British Museum, a product of early man’s attempt to placate the gods, may hold yet more secrets of ancient celestial configurations. A major exhibition of Babylonian artefacts was mounted in 2008 at the British Museum in London and notably featured the importance of these artefacts in early pre-Greek Astronomy.
2
GREEK ASTRONOMY AND THE BIRTH OF NATURAL PHILOSOPHY
The next phase of the development of astronomy shifted dramatically from the priests of Mesopotamia to the philosophers of the Aegean. These inhabitants of the Mediterranean set the foundation for what was to become the science of astronomy as it was passed into Western Europe and then down to the present day.
There is a direct connection between the Babylonians and Hipparchus, the earliest of the great Greek astronomers, who proposed a geometric model for the non-uniform motion of the Sun based on Babylonian observations. Hipparchus lived from 190 BC to 120 BC, mostly on the island of Rhodes, and is only known by one surviving work and fragments from his lost star catalogue. Both of these documents later influenced the Alexandrian Claudius Ptolemy, who lived in the second century AD and helped develop the geometric model of the heavens that lasted until the time of Copernicus, fourteen centuries later.
Hipparchus’ discussion of the extremely slow motion of the solstice and equinox points along the ecliptic from east to west against the background of the fixed stars is perhaps his most famous achievement. This is now referred to as the ‘precession of the equinoxes’. This basic idea was also adopted – virtually unchanged – three hundred years later by Ptolemy.
It is clear that Babylonian astronomical records influenced Hipparchus, who made use of their methods as well as their observations to develop quantitative geometrical models, which describe the motion of the Sun. Though many questions remain unresolved regarding the relationship between Babylonian and Greek astronomy, Hipparchus’ work provides a clear link between the two. Historians argue that he was responsible for the direct transmission of both Babylonian observations and methods and the successful synthesis of Babylonian and Greek astronomy.
Hipparchus was born in Nicaea, in what is now known as Iznik, Turkey and probably died on the island of Rhodes where he is known to have been a working astronomer from 147 BC to 127 BC. He is considered by many historians to be the greatest astronomer of antiquity and is the earliest Greek whose quantitative, accurate models for the motion of the Sun and Moon survive to the present. In order to achieve this, he had to make use of the observations and perhaps even the mathematical techniques accumulated over centuries by the Chaldeans from Babylonia. He thus demonstrated from the very beginning of his creative work in astronomy, that there is a fundamental importance in the interplay between observation and theory.
During his lifetime he formulated the world’s first accurate star map, a catalogue of over 850 stars with estimates of their relative brightness. He also developed the system of epicycles that preserved the Earth-centred universe of Aristotle and supported the motion of all heavenly bodies in perfect circles. This system was in approximate agreement with the observations of the time, almost three hundred years before Ptolemy, who is introduced later in this book. Perhaps what is more surprising to any student of the history of astronomy is that Hipparchus also produced the first eccentric orbital construction to predict the motion of the Sun. This simple construction was used almost unaltered (but acknowledged) by Ptolemy three centuries later as part of the elaborate model published in The Almagest
, which was then handed down to Copernicus in the sixteenth century.
In Hipparchus’ simple geometry the Sun orbited the Earth on a circle, moving about the centre of the circle at a uniform speed (and therefore satisfying Aristotle’s conditions). But Hipparchus knew unquestionably, probably from his access to Babylonian records, that seasons are different lengths and that this must be accounted for in any orbital model. Clearly, the Earth could not be located in the centre of the Sun’s orbit, but must be eccentric (i.e. off centre) in order to reproduce its non-uniform motion. He was able to calculate just how far the Earth had to be displaced from the centre of the Sun’s orbit in order to give agreement with the speeding up and slowing down of the Sun during its journey around the ecliptic – the path of the Sun as seen from the Earth on the background of the stars. This single off-centre eccentric was sufficient to reproduce the motion of the Sun throughout the year.
Incorporated directly into Ptolemy’s model, the eccentric later became quite controversial as Copernicus and Kepler began undermining geocentrism, as we will see in subsequent chapters. The Moon has a more complicated motion than the Sun. It is in Hipparchus’ model for the Moon’s motion, that we begin to more clearly recognize the shift to geometric astronomy that the Greeks began through their assimilation of the Babylonian commitment to observations. Hipparchus used astronomical records and parameters derived from Babylonian sources in his development of quantitative geometric models for the motion, of both the Sun and the Moon. In so doing he ultimately demonstrated the fundamental importance of the interplay of observation and theory for the future development of astronomy
Hipparchus’ Star Catalogue
Study of the Babylonian records produced other rewards for Hipparchus. As a systematic observer himself and one of the few Greek astronomers who valued the early records of the Babylonians, he was able to compile a remarkable catalogue of stars. By combining his own observations with the older, more extensive measurements from Mesopotamia, he was able to observe the movement of any one star over the centuries. He continued to pursue the historical plotting of stars and was eventually able to distinguish the slow movement of the entire sky, including the drifting of the equinoctial points, the astronomer’s main reference frame, from east to west among stars. Equinoctial points are the places where the celestial equator is crossed by the path of the Sun, i.e. the ecliptic. The Vernal, or spring equinox, is the point at which the Sun’s path crosses the celestial equator in the ascending direction. This precise crossing had, for centuries, been understood by astronomers to mark the beginning of the celestial year. On the autumn equinox, the Sun crosses in the descending direction.
Hipparchus’ discovery, that this particular point is steadily moving, indicated that the measured position of a star varies with the date of the measurement. This is called the precession of the equinoxes and is now known to be approximately 1 degree every 70 years. To detect such a small change without a telescope was quite an achievement for an observer in the second century BC.
This was Hipparchus’ single most important discovery. Knowing as he did that very long-term observations would be necessary to confirm this phenomenon, he recorded the positions of hundreds of stars. This enabled successive astronomers to measure and compare the future movements of stars. In the process Hipparchus compiled the first comprehensive catalogue of stars in the western world. His star catalogue has been since lost, although a few partial star positions are recorded in his only surviving work, the Commentary (c.276 BC). In addition, Professor Bradley E. Schaefer of Louisiana State University in Baton Rouge, Louisiana has found evidence of an image of Hipparchus’ star catalogue on a marble celestial sphere, part of an extant ancient Roman sculpture now in the Farnese collection of the National Archaeological Museum in Naples.
The Greek Philosophers
Though Hipparchus may with some accuracy be called Greece’s first astronomer, he was not the first philosopher from the Aegean to contemplate the heavens. Nearly all historians give the ancient Greeks credit for introducing the idea that numerical relationships can manifest themselves in the physical world. As we have seen, they did so by using arithmetic and geometry combined with logic to provide an explanation for astronomical events. On another level, Greek metaphysics implored humans to be curious, to seek truth, to look for patterns and to use reason to solve problems. These ideals remain the central tenets of science and scientific exploration today, in the twenty-first century.
Perhaps the first man to call himself a philosopher was Pythagoras, who was born on the island of Samos, close to the mainland of what is now known as Turkey. Pythagoras made influential contributions to philosophy and religious teaching in the early sixth century BC. He also exercised a marked influence on another important Greek philosopher, Plato. Pythagoras founded a religious movement based on the belief that everything was related to mathematics and that numbers represent the ultimate reality.
Pythagoras taught that all observable phenomena could be measured and predicted based on rhythmic patterns or cycles. Unfortunately, very little is known about the man himself because none of his writings have survived. Some historians believe that his colleagues and successors may actually have made many of the accomplishments credited to Pythagoras. Nevertheless, his name has come to symbolize the order, harmony and simplicity of numbers that have applied by subsequent generations to the structure of the universe.
We can be more certain about the existence of the next giant of classical philosophy. Socrates lived and taught in Athens during the Golden Age of Greece (circa 546–404 BC). The age began with the unlikely victory of a badly outnumbered Greek army over a vast Persian army. Following this victory, significant advances were made in a number of fields, including the formulation of the structure of democracy, art, philosophy, drama and literature. Some of the Greek names most familiar to us, such as the renowned military and political leader Pericles, lived in this exciting and productive time. It was an era marked by such high and diverse levels of achievement that classical scholars coined the term ‘the Greek miracle’.
Although Socrates did not leave any written work, he is included in this listing because his ideas were documented by his student Plato and later transmitted to Aristotle, the man who first documented a theory of science. Plato witnessed the notorious death of Socrates, a result of being charged with ‘corrupting the youth of Athens’. In spite of this, Socrates’ views on morality and his disdain for the physical world set the stage for Plato and his prize student Aristotle to build wide reaching and coherent world views. These later provided the foundation for much of western thought.
Early in the fourth century BC, Plato adopted an intellectual approach to the study of celestial motion. He defined the problem to his students in the Agora in Athens. It is with Plato that we find the birth of the notion of the ‘perfection of the heavens’ that was to hinder astronomical thinking for two millennia.
In terms of quality and importance, Plato contrasted bodies in the heavens with objects on the Earth. The stars, he said, represent eternal and divine unchanging things that move with uniform speed around the Earth in the most regular and perfect of paths, the circle. This idea took root even though it was well known that while the motion of the Sun and Moon seemed regular, the planets – which are also celestial objects – wandered across the sky in complex paths, occasionally veering back before moving forward again in their orbits.
Plato contended that if the motions of heavenly bodies do not move in a perfect circle then the movement must be described by some combination of perfect circles. This planted the idea for the Greeks that the motion of all heavenly objects could only be described by perfect circles and uniform speed. And thus Plato’s devastating legacy took root. This principle, which was clearly inspired by the strong influence of Pythagoras, became a major restriction on the thinking of generations of philosophers and scientists in Greece and beyond.
Aries (which corresponds to the time period of mid-March to
mid-April), each covers 30 degrees of the sky. The zodiac constellations used on the celestial sphere model of the heavens to locate the Sun, Moon and planets is a scheme that has essentially lasted to the present day.
Aristotle
Not long after Plato’s pronouncements, the principle of uniform circular motion in the heavens was further ingrained into Greek philosophy. This was achieved as a result of an elaborate scheme for defining the natural world pioneered by Plato’s most famous student, Aristotle. The legendary Athenian – soon to become the master of Greek thinkers and famous as the teacher of Alexander the Great – set forth a detailed list of the ‘natural conditions of things’ which included Plato’s heavenly principle as well as many other ideas on what we would today call ‘science’. Much of the subsequent thinking of Greek philosophers on nature would follow from his ideas.
Here is a summary of his postulates:
There is order in the universe: the baser elements lie at the bottom and the nobler elements at the top.
The Earth is the basest of all objects in the universe, therefore it is at the bottom.
The Earth is composed of four elements: air, water, fire and earth. These elements always seek each other. Thus, air and fire rise and water and earth fall.
Once the materials have regained their rightful place, their natural tendency is to remain motionless.
Sideways motion is caused by violence (or force). Eventually this violent motion runs out and the object either rises or falls according to its nature.
Violent and natural motion affects only the four elements.
Celestial bodies are made of a fifth element called quintessence and their natural behaviour is to move uniformly in circles indefinitely around the Earth (which does not move) either by spinning or by moving through the heavens.