Babylonian Astronomy and Celestial Forecasting in Ancient Mesopotamia, Lecture notes of Astronomy

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History of Cosmology in Western Civilization Bob Joseph Institute for Astronomy University of Hawaii at Manoa Outline • Babylonian Celestial Forecasting • The Greek Invention of Scientific Theory • The Medieval Islamic Empire Preserves the Greek Tradition • Synthesis of Astronomy & Cosmology in Medieval Europe • Copernicus, Galileo, & Kepler: A Heliospheric Universe • Newton and the 17th Century Scientific Revolution • Intellectual Implications of the Scientific Revolution in the Enlightment • 20th Century Relativistic Cosmology • Summary: Cosmology and Civilization Lecture 1 The Babylonians 1.1 Pre-history Paleolithic era • During the Paleolithic era, from about the time of Homo erectus 2 million years ago, to the end of the Ice Age, about 12,000 years ago, our ancestors were hunter- gatherers. • Even during this early time people were observing the heavens and finding patterns in the motions of the Sun, Moon, planets, and stars. • There are apparently thousands of fragments of bone with records of observations of the Moon, some as early as 40,000 years ago. Engraved mammoth tusk, ca. 15,000 BCE Neolithic era • Dates from ca. 10,000 BCE. • Rapid appearance of: ! houses ! growing crops ! domesticated animals ! technologies of textiles, pottery, copper tools ! villages with populations of 100-1000 (e.g. Jericho) • By about 5000 BCE civilization established both in Egypt and in Mesopotamia, with many small villages. • Astronomy was important: witness neolithic stone monuments such as Stonehenge. 1.2 A potted history of Mesopotamia The Ancient Near East .Cairo Euphrates R. Tigris R. Nile R. AKKAD SUMER CHALDEA Sumerians • There was continual fighting for supremacy among various tribes in the region over the centuries. • However there was remarkable continuity of civilization, with conquerors largely adopting and preserving the existing culture. • The Sumerians (from the south) were the first to settle the area, ca. 4000 BCE. • They established irrigation, grew crops, domesticated animals, made pottery and bronze tools & weapons. • By about 3300 BCE they developed a system of writing, cuneiform, by making wedge-shaped marks in soft clay which was then baked in the Sun. • First use of wheeled carts about this time. Cuneiform tablets Akkadians & Amorites • About 2300 BCE a northern tribe established a unified kingdom in Mesopotamia. • Sargon I conquered all the Sumerian city-states and built a new capital city called Akkad. • The Akkadian empire spread Sumerian culture throughout the Middle East. • About 1900 BCE the Amorites, a tribe from the west, conquered Akkad. • Their capital was Babylon, built straddling the Euphrates River. • The sixth Amorite king, Hammurabi, began his reign in 1792 BCE. • Under Hammurabi Babylon became the most splendid and important city of the ancient Near East. 1.4.1 The problem • The Babylonians had a large, land-based empire to administer from a central location, Babylon. • A common calendar was essential for activities like planting, harvesting, paying taxes, etc. • (N.B. the valley was so fertile they could plant three cycles of crops in a year, if irrigated properly.) • We now know the various regularities and irregularities of celestial phenomena; the Babylonian problem was to predict all these, and in particular: ! when the new moon was first visible on the western horizon, ! when solar and lunar eclipses would occur, ! reappearance of planets and constellations. 1.4.2 Arithmetical methods • Methods of the Babylonian astronomers were based on: ! Long, continuous records. ! A social structure to produce and store these records. ! Searching for cycles or patterns in the records. ! Finding patterns or cycles in deviations from the regularities, and fit these with linear zig-zag and step functions. ! They then used the results to extrapolate to the future. Example 1. Period relations • N events of one sort correspond to M events of a different sort. 235 synodic months = 19 years 1 year = 235/19 months = 12.3684 synodic months Subtract off the even 12 months: remainder 0.3684 months. 19x0.3684 months = 7 months, so 7 intercalated months are required every 19 years. Example 2. Zig-zag functions • Here we have an average period relation, but one quantity increases by a constant increment to some maximum and then decreases by that decrement to some minimum, then repeats the cycle. • Consider the amount of time the Moon is visible over the course of a month: zero at new moon, 12 hr at full moon about 15 days later. • The Babylonian astronomer would use a constant increment of 12/15 hr per night = 48 min per night for the waxing half of the cycle, and then a decrement of 48 min per night for the waning half the lunar cycle. • A similar technique was used to calculate the times of each new moon, and ephemerides of the planets. Zig-zag function for Moon’s illumination 0 25 50 75 100 0 7 14 21 28 35 42 49 56 63 70 Moon’s Illumination P e rc e n ta g e i llu m in a ti o n Day 1.4.3 Results • Excellent celestial forecasting. ! They could predict the seasons and the time of new moon very reliably. ! Estimates of the synodic month and length of a year were accurate to a couple of minutes. ! Very accurate calendar based on the knowledge that 235 lunar months very nearly equals 19 solar years. ! They could predict lunar eclipses reliably. ! They knew about the 223-month Saros cycle after which eclipses repeat themselves. ! (They could not predict solar eclipses reliably, of course.) • We can date events from 7th C BCE to 1st C to a precision of better than a day using Babylonian calendar records. Solar & lunar eclipses Ideas of space & time • The Babylonians had no concept of abstract space. • They only considered concrete places. • Similarly, they had no concept of abstract time which flows independently of anything else. • Time for them was found in the periodicities and rhythms of Nature and life. • Their concept of time, like many other ancient civilizations, was a recurring cycle. No cosmological theorizing • What the Babylonians apparently did not do: they did not theorize about what made the heavenly bodies behave the way they did. • They were apparently content to be able to forecast celestial events, and did not worry about an underlying picture or model of what was happening. • In the following section we attempt to understand why this was so. 1.5 Astronomy in the context of Babylonian culture Mesopotamian culture • Society in Mesopotamia seems to have rather suddenly taken characteristic shape in the 4th millenium BCE. • Villages developed into cities. • Governing authority became centralized and a kind of democracy emerged: ! a general assembly (of men) ! a council of elders ! in times of crisis a king was appointed temporarily Mesopotamian cosmology • This sort of democracy seems to have been applied to the Mesopotamia idea of the Universe. • Among the influences affecting the view of the world was that Nature was unpredictable: rivers could suddenly rise and destroy crops, there could be torrential rains that washed everything away, or strong winds, or scorching heat. • All these were seen as gods with wills and personalities of their own. • To bring order to the cosmos required the command of some more powerful god. • Bringing order to the cosmos also required the gods all to be participants in a sort of cosmic state. • The leader was the god of heaven, Anu. • The “seven gods who determine destinies” were a sort of council of elders. • Anu’s son, Enlil, the god of the storm, was a sort of executive assistant who carried out the decisions of the elders. • Human society on earth was not a parallel, but an extension of the cosmic state. • Kings on earth were appointed by Anu, and their commands were understood to be commands from Anu: kings ruled by divine right. • Anu therefore not only controlled Nature, but also required obedience to the laws and customs of society. Cosmos as a State The Enuma Elish • This is the great cosmological epic poem from Mesopotamia. • It dates from the time of Hammurabi, ca. 1800 BCE. • It has two sections, one dealing with the origin of the Universe, and the other showing how the order on earth was established. • In the earliest versions the hero was Enlil, but he was replaced by Marduk, the god of Babylon, in the earliest known version extant. • (Later the Assyrians replaced Marduk with their god, Assur.) • Each year in Babylon the victory of Marduk over the forces of chaos was re-enacted at the New Year festival. 1.6 The relation between cosmology and culture: the Enuma Elish Subject of the myth • The Enuma Elish has two major sections. • The first deals with how the structure of the Universe came to be. • The second shows how the present world order was established. • However, note the ways in which these two subjects are woven throughout the story. Cast of characters • Apsu--represents the sweet waters & male fertility. • Tiamat--represents the sea or salt water & chaos. • Mummu--probably represents cloud banks & mist. • Lahmu & Lahamu--silt, born of the union of Apsu & Tiamat. • Anshar & Kishar--the horizon, born of the union of Lahmu and Lahamu. • Anu--god of the sky & born of the union of Anshar & Kishar. • Nudimmud, also Ea or Enki--here represents the Earth. • Marduk, the god of Babylon. Summary of the Enuma Elish • Salt and sweet water comingle to produce silt, which in turn forms the horizon. • The sons of the gods make so much commotion that Apsu decides to kill them. • Ea learns of the plot and kills Apsu. • Tiamat becomes enraged that her husband has been killed and vowed revenge. • Marduk says he is willing to fight Tiamat. • The gods give Marduk authority and test him, and he goes out and kills Tiamat. • He divides her corpse in half to create the earth and sky. • Marduk then establishes dwelling places for the gods.