The Science of Discworld IV

present-day Luxor had a ritualfunction as a representation of this primal mound, but there may be more to it than that. Recently archaeologist Angus Graham has been making geophysical surveys; using electrical resistivity tomography to detect the former course of the Nile through the silt it deposited, he has shown that in ancient times Karnak was located on an island in the middle of the Nile. As the annual floods receded, it would have re-enacted the emergence of the primal mound literally, not just symbolically.
    Despite their religious interest in the night sky, the Egyptians seem not to have made a systematic study of astronomy for its own sake. For that, we must turn to another ancient culture: Babylon.
    Babylon was one of a variety of civilisations in the Mesopotamian region, the fertile lands between the Tigris and Euphrates rivers. Today this area comprises Iraq, plus parts of Iran, Syria and Turkey. The city of Babylon was in central Mesopotamia, about 80 km south of present-day Baghdad.
    During the Bronze Age, Mesopotamia included the empires of the Old Babylonians, Sumerians, Assyrians and Old Akkadians. The New Babylonian and New Akkadian empires followed in the Iron Age. The Sumerians invented cuneiform writing, triangular marks in clay made by a stick, around 3500 BC . They studied the heavens, were aware of the ‘wandering stars’ that we now call planets, and worshipped them as gods. An ancient Sumerian tablet refers to seven heavens and seven Earths.
    Babylonian history is usually divided into two periods. The city-state of Babylon became a regional power when the sixth king, Hammurabi, took charge in 1792 BC , and the Old Babylonian period dates from then until about 625 BC . The New Babylonian period followed when Nabopolassar took control following a civil war triggered by the death of the Assyrian king Ashurbanipal. Many more cuneiform astronomical texts survive from the New period than from the Old, but there are enough Old Babylonian texts to demonstratethat their study of the heavens was systematic and organised. The Old Babylonian astronomers produced the first known star catalogue around 1200 BC , but many of the star names are Sumerian, so Sumerian astronomers must have made systematic studies of the heavens even earlier.
    The Babylonians paved the way for modern astronomy, and possibly science as well. They observed the motions of celestial bodies, especially the planets, carefully and accurately. Then they looked for patterns, using mathematics to analyse the data. They discovered that many astronomical phenomena are periodic: they repeat at fairly regular intervals. There is a tablet that records how the amount of daylight changes over the year, and a series of tablets called the
Enûma Anu Enlil
contains the Venus Table of Ammisaduqa, a 21-year record of the motions of Venus and the earliest known discovery of periodic events in planetary motion. This tablet, made around 700 BC , is a copy of an older one, possibly from the early part of the Old Babylonian era.
    The Babylonians were assiduous observers, but they had no great interest in theoretical explanations, and we know little about their cosmology. Tablets contain phrases such as ‘the circumference of heaven and earth’, suggesting that they imagined the Earth and the cosmos united in a single round object. The two components were of equal importance, and both revolved in circles. The Babylonians did not link their scientific studies of the planets to their religious views of the cosmos, and they seem not to have thought that the planets themselves move in circles.
    After 400 BC , the centre of natural philosophy in the ancient world shifted to Greece. Philolaus, a member of the cult founded by Pythagoras, viewed the cosmos as a central fire, around which the Sun, Moon, Earth and planets all revolve in circles. We don’t notice the fire because it is hidden by the bulk of the Earth. Around 300 BC , Aristarchus of Samos came up with perhaps the first heliocentric cosmology, by the simple expedient of replacing the central fire by the Sun.
    The novel idea that the Earth revolves around the Sun received a general thumbs-down from almost everyone else, including virtually all early Greek philosophers. Thales thought that a flat Earth floated on water. Anaximander thought it was a thick disc with a flat top. Anaximenes held that a flat Earth rode on air like the other celestial bodies. Xenophanes maintained that we lived on the