Astronomy in Alexandria

Astronomy in Alexandria

In the second century AD, the Greeks combined their celestial theories with observations translated into planes.
By replacing the Eudoxus spheres with a more flexible system of circles, they proposed a series of eccentric circles, with the Earth near a common center, to represent the general movements towards the East around the zodiac at different speeds of the Sun, the Moon, and the planets.




To explain the periodic variations in the speed of the Sun and the Moon and the recoils of the planets, they said that each of these bodies rotated uniformly around a second circle called an epicycle, whose center was located in the first. By the appropriate choice of diameters and velocities of the two circular motions attributed to each body, their observed movement could be represented. In some cases, a third body was needed to square the calculations.

Ptolemy compiled the astronomical knowledge of his time in the thirteen volumes of the "Almagest." He exhibited a system in which the Earth, in the center, was surrounded by crystal spheres of the other 6 known stars. This scheme is known as the Ptolemaic Geocentric System.The earth did not exactly occupy the center of the spheres and the planets had an epicycle (system created by Apollonius of Pergamum and perfected by Hipparchus) whose axis was the line of the orbit that revolved around the so-called deferential earth.


As the planet revolves around its epicycle, it approaches and moves away from the earth, sometimes showing a retrograde movement. This system allowed to make predictions of the planetary movements, although it had a very poor precision. In spite of this, it was popularized and accepted more than as a true model as a useful mathematical fiction. It is estimated that the Ptolemaic universe only average 80 million kilometers.

Another thinker who, like Ptolemy, kept alive the tradition of Greek astronomy in Alexandria in the first centuries of the Christian era, was Hypatia, a disciple of Plato. She wrote commentaries on mathematical and astronomical subjects and is considered as the first scientist and philosopher of the West. The movie "Agora" recreates its history.

Other achievements of the Astronomy in Alexandria were the calculation of the circumference of the earth by Eratosthenes and the first measurements of the distances to the Sun and the Moon. Stellar catalogs were designed as those of Hipparchus of Nicea and the discovery of the precession of the equinoxes.


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