| Early astronomical measurements |
The first scientific measurement of a cosmic distance was accomplished, to the year 240 BC JC, by Eratosthenes of Cyrene - director of the Library of Alexandria, the then most advanced scientific institution in the world - who noticed that the June 21, when the Sun, at noon, was exactly at its peak in the city of Siena (Egypt), he was not also, at the same time, in Alexandria, some 750 km north of Siena. Eratosthenes concluded that the explanation must be that the Earth's surface, being round, was always furthest from the Sun at some points than others.
 Building on the length of the shadow in Alexandria at noon on the solstice, the already advanced geometry could answer the question concerning the extent to which the Earth's surface curved in the 750 km route between Siena and Alexandria . From this value could be calculated the circumference to the diameter of the Earth, assuming it had a spherical shape, a fact that Greek astronomers then accepted without hesitation. Eratosthenes made the corresponding calculations (in Greek units) and, as we can judge, their numbers were approximately 12,000 km for the diameter and 40,000 for the circumference of the Earth. Thus, although perhaps by chance, the calculation was quite correct. Unfortunately, this value did not prevail for the size of Earth. Approximately 100 years BC J. C, another Greek astronomer, Posidonius of Apamea, repeated the experience of Eratosthenes, reaching the very different conclusion that the Earth had a circumference of about 29,000 km. This value was smaller than Ptolemy accepted and therefore it was considered valid during medieval times. Columbus also accepted this figure and thus believed that a journey of 3,000 miles to the West would lead to Asia. If he had known the true size of the earth, perhaps would not have ventured. Finally, in 1521-1523, the fleet of Magellan - or, rather, the only ship left of it - first circumnavigated the Earth, allowing restore the correct value, calculated by Eratosthenes. Based on the diameter of Earth, Hipparchus, about 150 years a. JC, calculated the Earth-Moon distance. He used the method that had been suggested a century earlier by Aristarchus of Samoa, the most daring of the Greek astronomers, who had assumed as lunar eclipses were caused by the earth is interposed between the Sun and Moon. Aristarchus discovered that the curve of the Earth's shadow across the front of the Moon indicates the relative sizes of Earth and Moon. From this, the geometric methods offered a way to calculate the distance to the Moon was, according to the diameter of Earth. Hipparchus, repeating this work, calculated the distance of the Moon to Earth was 30 times the diameter of Earth, this meant that the moon must be at about 348. 000 km from Earth. As we see, this calculation is also quite correct. But finding the distance that separates us from the moon was all he could get Greek Astronomy to solve the problem of the size of the universe, at least correctly. Aristarchus also made a heroic attempt to determine the Earth-Sun. The geometric method he used was absolutely correct in theory, but by the measure of such small differences in the angles, without the use of modern instruments, was ineffective in providing an acceptable level. According to this measurement, the Sun was about 20 times farther from us than the moon (when in fact it is about 400 times more). Regarding the size of the sun, Aristarchus concluded - although their numbers were also erroneous - that the size must be at least about 7 times larger than Earth, then noting that it was unreasonable to assume that the Sun, of its size, would revolve around our little Earth, so it decided, finally, that our planet revolved around the sun Unfortunately no one took up his ideas. Later astronomers, beginning and ending Hipparchus Ptolemy, issued all sorts of assumptions about celestial motions, always based on the notion of a stationary Earth at the center of the universe, with the Moon at 384. 000 km of distance and other bodies located beyond it, to an undetermined distance. This pattern was maintained until 1543, when Nicolaus Copernicus published his book, which again give effect to the view of Aristarchus and forever dethroned the Earth from its position as the center of the universe.
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