Ancient Astronomy (-1500s)
On a clear night, about two thousand stars can be seen. These orbit the Earth in a fixed pattern, as if they are attached to a giant sphere. The skies also contain seven moving objects, the Sun, the Moon and five other 'wandering stars', Mercury, Venus, Mars, Jupiter and Saturn. All of these objects can be used for both navigation and predicting the seasons.
There is evidence that the stars played an important role in human life from at least 32,000 BC when a picture of the constellation Orion was carved onto a small mammoth tusk in Germany. Other evidence of prehistoric star maps comes from cave paintings found in the Lascaux caves in France, which depict the constellation now known as the Summer Triangle and the Pleiades star cluster, part of the constellation of Taurus. These date back to 14,000 BC (Rappengluck, pp.51).
Orion
The Star Garden
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Taurus Chinese star chart
By 4000 BC, the Egyptians had built a calendar by arranging stones in a pattern that could be used to map astronomical alignments (Brophy and Rosen, pp.1259) and the Chinese had begun to depict constellations (Lee, pp.247). The first evidence of written language was created in Sumer in around 3500 BC. Sumer is the earliest known civilisation and the Sumerians are also thought to have invented the wheel, the plough, agriculture and irrigation, they were also the first to record the names of the constellations (Ridpath, pp.14).
By 3000 BC, the Egyptians had divided the year into twelve months, one for each full Moon, they also divided the day into twenty four hours. Within a thousand years, Sumer had become part of Babylon. Much of the Sumarian's knowledge was conserved including the names of their constellations and their calendar system. The constellations that were most important to the Sumerians were those closest to the path of the Sun, the Moon and the wandering stars. There is evidence that the Babylonians were the first to divide the ecliptic, the path taken by these objects, into twelve equal segments, one for each month.
Many of the zodiacal constellations originate from Babylon including Taurus, Cancer, Virgo, Scorpio, Capricorn and Pisces. There is evidence that the Babylonians used a twelve month calendar, they also had a seven day week and celebrated a 'holy-day' at the end of each one. The Egyptians were not as interested in constellations as the Babylonians, a star map created by Amenhope in 1100 BC names only five. In contrast to this, the Babylonian's had recorded at least sixty six stars and constellations by 1000 BC.
The first evidence of Greek astronomy comes from Homer's epic poems, the Iliad and the Odyssey, which mention six constellations and the star Sirius, and Hesiod's poem Works and Days which makes references to a calendar system. These are thought to have been composed in about 800 BC. Aristotle considered Thales of Miletus to be the first Greek philosopher (Aristotle, pp.13). He predicted the Solar eclipse of 585 BC and believed the Earth to be a flat disc (Krupp, pp.327). Pythagoras was the first to suggest that the Earth is spherical in about 500 BC and this was accepted by most Greek philosophers.
The Ancient Greeks traded with Egypt and Babylon and in around 370 BC, Eudoxus of Cnidus learnt the names of forty seven Babylonian constellations including those of the zodiac (Ridpath, pp.2). His work is lost but a complete guide to his constellations can be found in the poem Phaenomena by Aratus, first published in 275 BC.
The Greeks also devised their own names for the constellations, naming them after animals, objects and Gods. The constellations that we have now are almost exclusively Greek and Babylonian in origin. The word zodiac derives from the Greek word for animal, since eleven of the signs are animals.
Greek constellations depicted in the late 1700s - The Northern and Southern hemispheres
Eudoxus was also the first to present a mathematical theory of the universe, he placed the spherical Earth in the centre and the Sun in a giant transparent sphere that orbits the Earth every twenty four hours. The stars were attached to a larger sphere beyond this, also orbiting the Earth.
Eudoxus's view was accepted by Aristotle in the 4th century BC. Aristotle's universe was finite, eternal, and geocentric, with the Sun, Moon, planets and stars all orbiting the Earth inside of giant transparent spheres (Cohen, pp.649). Aristotle considered the sphere to be the perfect shape, he argued that the heavens are perfect and unchanging, unlike the imperfect Earth which is covered in valleys and mountains. Comets were considered to exist inside of the Earth's sphere as they do not move in spherical orbits. Aristotle's contemporary, Heraclides Ponticus, suggested that it would be simpler for the Earth to rotate than for the whole of the heavens to orbit around it (Stahl, pp.321), and Aristarchus, born in 310 BC, also suggested that the Earth revolves around the Sun. These claims were generally dismissed, however, and Aristotle's cosmology remained dominant until German astronomer Nicolaus Copernicus popularised the heliocentric theory in 1543.
Aristarchus was the first to provide an estimate of the relative distance between the Earth and the Sun, concluding that the Sun is about twenty times further away than the Moon and is about twenty times larger (Gutzwiller, pp.589-639). We now know that the Sun is almost four hundred times further away than the Moon and four hundred times larger.
The first estimate of the circumference of the Earth was provided by Eratosthenes who was born in 276 BC. Eratosthenes knew that the Sun would appear directly overhead in the Egyptian city of Swenet at noon on the day of the summer solstice. The summer solstice is the longest day of the year, when the Sun is at its highest point in the sky.
Eratosthenes measured the angle of Sun in Alexandria at that same time and found it to be 1/50 of a full circle. He realised that this meant the distance between Swenet and Alexandria must be 1/50 of the circumference of the Earth. He measured the distance between cities by timing how long it took to ride there by camel. Eratosthenes concluded that the Earth has a circumference of 252,000 stadia which became the accepted value (Diller, pp.6-9). The Greek stadium was about 185 metres, this leads to a circumference of 46,620 kilometres, an overestimation. If we assume that he used the Egyptian stadium of about 157.5 metres, however, then the circumference would have converted to 39,690 kilometres, which is very close to the 40,075 kilometres value accepted today.
The precision of the equinoxes was first measured by Hipparchus who was born in 190 BC. This determines the difference between the sidereal year and the tropical year. The sidereal year refers to the time taken for the Earth to orbit the Sun once around the ecliptic, measured with reference to fixed stars, and the tropical year refers to the seasonal year, such as the time between the two longest days of the year.
Hipparchus measured the angle of the Earth with respect to the ecliptic to be changing at a rate of less than two degrees every one hundred and fifty years. He did this by measuring the longitude of a number of stars and comparing them to the measurements of 3rd century BC Greek astronomers Timocharis and Aristillus. Hipparchus measured the tropical year to be 365 days, 5 hours and 55 minutes long and took the length of the sidereal year from Babylonian texts to be 365 days, 6 hours and 33 minutes long (Plackett, pp.130-135).
Hipparchus compiled a star catalogue of over eight hundred and fifty stars and devised the magnitude system. This categorises stars according to their brightness, the brightest are given a magnitude of one and the faintest six. In 1902, Dutch astronomer Jacobus Kapteyn introduced the term 'absolute magnitude', this allows stars to be compared by stating the magnitude they would have if they were viewed from an equal distance, ten parsecs away.
Roman astronomer, Ptolemy reproduced many of Hipparchus' constellations in the Almagest written in about 150 AD. All of Ptolemy's forty eight constellations are used today with the exception of Argo Navis (the Ship Argo), which has been broken up into smaller parts, Carina (the Keel), Puppis (the Stern), and Vela (the Sail) (Ridpath, pp.7). Ptolemy is known for developing Aristotle's geocentric theory of the universe. The Ptolemaic system popularised the use of epicycles to explain why the planets do not appear to orbit in perfect circles.
After the fall of Rome, the demise of scientific progress in the west was countered by the Islamic Golden Age. In 964, Persian astronomer Abd al-Rahman Al-Sufi published a revised edition of Ptolemy's Almagest, the Description of the Fixed Stars. Al-Sufi determined the magnitudes for most stars and added two images of each of Ptolemy's forty eight constellations. Single stars, which did not fit into a constellation, were named after different animals or people. Al-Sufi also made the first recorded observation of Andromeda, the nearest spiral galaxy to our own, describing it as a 'small cloud'.
Al-Sufi's Description of the Fixed Stars
The Almagest was updated once again in 1437, when Persian astronomer Ulugh Beg determined the new positions of almost one thousand stars. He had previously commissioned one of the largest observatories of his time to be built in Samarkand, Uzbekistan.
Europeans began to map the southern skies again in 1595, when Dutch navigators Pieter Dirkszoon Keyser and Frederick de Houtman were asked to fill in the gaps around the South Pole whilst they explored the East Indian oceans. They added twelve new constellations, the southern triangle, the Indian, a term which could refer to a native Asian or American at the time, and ten others which were named after animals, in the Greek tradition. They represented; a mythical phoenix, a bird of paradise, a peacock, a toucan, a crane, a fly, a chameleon, a flying fish, a coryphaenidae or dolphinfish and a water snake. These were first published in Johann Bayer's star catalogue of 1603.
References
Aristotle, 'Metaphysics', 1998, Lawson-Tancred, H. (trans.), Penguin Classics, London
Brophy, T.G. and Rosen, P.A., 2005, 'Satellite Imagery Measures of the Astronomically Aligned Megaliths at Nabta Playa', Bulletin of the American Astronomical Society, Vol.35
Cohen, M.S., 2005, 'Readings in ancient Greek philosophy: from Thales to Aristotle', Cohen, M.S., Curd, P., Reeve, C.D.C. (eds.), Hackett Publishing, Indianapolis
Diller, A., 1949, 'The Ancient Measurements of the Earth', Isis, Vol.40
Gutzwiller, M.C., 1998, 'Moon-Earth-Sun: The oldest three-body problem', Reviews of Modern Physics, Vol.70
Krupp, E.C., 2003, 'Echoes of the Ancient Skies: The Astronomy of Lost Civilizations', Dover Publications, Mineola
Lee, A., 2008, 'Warp and Weft, Chinese Language and Culture', AEG Publishing Group, New York - See also 'Early Depictions of Constellations'.
Plackett, R. L., 1958, 'Studies in the History of Probability and Statistics: VII. The Principle of the Arithmetic Mean', Biometrika, Vol.45
Rappengluck, M., 2003, 'The anthropoid in the sky: Does a 32,000-year old ivory plate show the constellation Orion combined with a pregnancy calendar?', Calendars, Symbols, and Orientations: Legacies of Astronomy in Culture; Proceedings of the 9th annual meeting of the European Society for Astronomy in Culture (SEAC), Blomberg, M., Blomberg, P.E. and Henriksson, G. (eds.), Uppsala Astronomical Observatory Report No. 59, Uppsala - See also 'Oldest star chart found' and 'Ice Age star map discovered'.
Ridpath, I., 1989, 'Star Tales', James Clarke & Co, Cambridge, pp.14
Stahl, W.H., 1945, 'The Greek Heliocentric Theory and Its Abandonment', American Philological Association, Vol.76