Monday, October 8, 2018

What is universe? - Part 2 - The earliest cosmology

The earliest cosmology cartoon

The Greek word kosmos means ‘orderly arrangement’, and from it we derive today’s words cosmos and cosmology. Cosmology is the branch of astronomy that goes about answering that most fundamental of questions, ‘What is the Universe?’, by studying the way the Universe behaves, how it began, and how it will all eventually end.
Cosmology as a true science only really started in 1916, when Albert Einstein published his General Theory of Relativity. Before this, astronomers lacked the mathematical framework in which to describe the behaviour of the whole Universe, and so pre-20th-century cosmology tended to be an uneasy mixture of speculation and religious sensibility. Ancient cosmology especially was usually inspired by religion and the assumption that Heaven was located somewhere above our heads, in space.
The Egyptians based their cosmology on the human reproductive cycle. They believed that the sky goddess Nut gave birth to the Sun god Ra every year and that the changing altitude of the Sun with the seasons was its gestation in Nut’s star-studded body. The Sun was said to be reborn every winter solstice, and returned inside Nut through her mouth at the spring equinox. In this way, Ra continually recreated himself, making the Universe an eternal, self-sustaining entity.
Early civilizations told stories inspired by the patterns of stars in the night sky. They imagined the lines joining stars to form pictures of familiar or mythical characters. In Mesopotamia (modern-day Iraq) archaeologists have unearthed stone tablets and clay ledgers dating back to 1300BC, which detail many such ‘constellations’ including the 12 signs of the zodiac. These zodiacal constellations were given special significance because they inhabited regions of the sky through which the Sun passed, and they were subsequently adopted by the Greeks – the Assyrian Hired Man and the Swallow became Aries and Pisces, for example, and the Goatfish and the Great Twins became Capricorn and Gemini. In Ancient Greece wandering minstrels would drift from village to village, recounting the star myths in exchange for food and lodgings. At the same time, philosophers would come up with their own fanciful tales to explain the nature of the Universe. One of the earliest was the philosopher Thales, in the sixth century BC. He put forward the idea that space was filled with water, in which the Earth floats, that earthquakes were caused by waves in this water, and the stars moved because they were caught in gentler currents.

greek philosopher Plato
Greek Philosopher - Plato
‘Astronomy compels the soul to look upwards and leads us from this world to another.’ Plato 4th century B.C. Greek astronomer

The Greek astronomer Claudius Ptolemy, who lived in the first century AD, compiled a list of 48 constellations, but since not all the sky could be seen from Greece, the regions around the South Pole remained uncharted until intrepid astronomers ventured far from Europe during the 16th and 17th centuries in order to chart the southern stars. Other new constellations were also proposed to fill gaps in Ptolemy’s classical sky map. Inevitably this led to arguments as astronomers disagreed. In England, Edmond Halley proposed a constellation called Robur Carolinum (Charles’ Oak), after the tree in which Charles II had hidden from the Round heads following the battle of Worcester. Whilst the King was delighted with the honour, some of Halley’s fellow astronomers were not keen, and quietly discarded the constellation from their maps.

Much later, in 1922, things were finally put on a firm footing when the International Astronomical Union ratified 88 constellations with defined boundaries, mostly based on the Greek model. These were not the only aspects of Greek astronomy to pass into modern usage. There was one Ancient Greek in particular who was not prepared to tell stories about the stars, or speculate about them; he realized that the first step on the way to true understanding was to measure them. That man was Hipparchus and he defined a system of classification for the stars still in use today. 

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