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The first scientific measurement of a cosmic distance was made around the year 240 a. J. C. By Eratosthenes of Cyrene – director of the Library of Alexandria, by then the best scientific institution in the world – who appreciated the June 21, when the Sun at noon was exactly at its zenith 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 surface of the Earth being round, was ever farther 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 surface of the Earth is curved in the journey of 750 km between Syene and Alexandria . From this value could be calculated the circumference and diameter of the Earth, assuming it had a spherical shape, a fact that Greek astronomers back 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 a. J. C, another Greek astronomer, Posidonius of Apamea, repeated experience of Eratosthenes, reaching the very different conclusion that the Earth had a circumference of approximately 29,000 km.

This smaller value was agreed that Ptolemy 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 I had known the actual size of the land may not have ventured. Finally, in 1521-1523, Magellan’s fleet – or rather, the only ship left of it – first circumnavigated the Earth, allowing to restore the correct value, calculated by Eratosthenes.

Based on the diameter of Earth, Hipparchus, 150 years a. J. C. , Calculated the Earth-Moon distance. He used the method that had been suggested a century earlier by Aristarchus of Samos, the boldest of the Greek astronomers, who had assumed since the lunar eclipses were caused by the Earth stood between the Sun and the Moon. Aristarchus discovered that the curve of the Earth’s shadow across the moon ahead of indicating 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, estimated the distance to the Moon to Earth was 30 times the diameter of the Earth, this meant that the moon must be about 348. 000 km from Earth. As we see, this calculation is also quite correct.

But finding the distance to the moon was all he could to get the Greek Astronomy to solve the problem of the dimensions of the universe, at least correctly. Aristarchus also made a heroic attempt to determine the Earth-Sun distance. The method used geometric was absolutely correct in theory, but involved the measure such small differences in the angles, without the use of modern instruments, was ineffective to provide an acceptable value. According to this measurement, the Sun was about 20 times farther from us than the moon (when in reality it is about 400 times more).

Regarding the size of the Sun, Aristarchus deduced – although their numbers were also erroneous – that size must be at least 7 times greater than Earth’s, indicating that it was illogical to assume then that the Sun, of such large dimensions, revolve around our little earth, so he decided at last that our world 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 384. 000 km away and other bodies located beyond this, to an indeterminate distance. This pattern remained until 1543, the year that Copernicus published his book, which returned to 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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