He was able to attain this precision exclusively with naked-eye observations and the few instruments available at the time – gnomons, astrolabes, and armillary spheres. Hipparchus's catalogue, one of the earliest successful attempts to chart the heavens, lists the positions of 850 stars across the sky with a precision of about one degree (about twice the angular size of the full Moon). To measure angles in the sky, Hipparchus employed the ancient Babylonian practice, still in use today, of dividing a circle into 360 degrees, and each degree into 60 arc minutes. A record of his work was handed down by Ptolemy, an astronomer writing three hundred years later at Alexandria – by then part of the Roman Empire. In the second century BCE, the famed Greek astronomer Hipparchus of Nicaea compiled the first stellar catalogue. The mythological figure of Atlas holds a celestial globe depicting constellations as they may have appeared in the sky in the 2nd century BCE, when Hipparchus compiled his stellar catalogue. Right: The Farnese Atlas, Roman copy of a Greek original, 2nd Century CE. He had the right idea, but the measurement was not very precise current data show that the Sun is about 400 times more distant than the Moon.Ībove: 10th century CE Greek copy of Aristarchus of Samos's 2nd century BCE calculations of the relative sizes of the Sun, Moon and the Earth. With the help of trigonometry, he determined that the Sun is 18 to 20 times more distant from Earth than the Moon. A proficient mathematician, he tried to assess the relative distance of the Sun and the Moon from Earth, by measuring the angle between them when the Moon appears exactly as one quarter. The dominant view of the cosmos among scientists was geocentric, with the Earth being at the centre of the Universe and everything else revolving around it, but there were some who were edging closer to the truth.Īristarchus of Samos was one of the few supporters of the heliocentric system, identifying that the Earth travelled around the Sun rather than the other way around. Among other sciences, astronomy flourished at Alexandria, a Greek colony off the northern coast of Egypt, with a renowned library and museum. It was much later, in the third century BCE, that Greek astronomers first attempted to use astrometry to estimate cosmic scales. ![]() But they had no idea how far away the stars and the planets were. From this cradle of civilisation in Mesopotamia – in the southern part of present-day Iraq – astronomers had built up knowledge of the celestial bodies and recorded their periodic motions. The first documented records of systematic astronomical observations date back to the Assyro-Babylonians around 1000 BCE. Monitoring the motions of stars and planets in the sky was the best tool to track time, which was fundamental for agriculture, religious rituals and navigation. Credit: John GoldsmithĬuriosity alone did not inspire the earliest astronomers: astronomy and astrometry were practical sciences too. The Moon and comet Hale-Bopp over the Great Pyramids of Giza in 1997.
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