Geocentric and Heliocentric theory
In astronomy, the geocentric model (alsoknown as geocentrism, or the Ptolemaic system) is a description of the cosmoswhere Earth is at the orbital center of all celestial bodies. This model servedas the predominant cosmological system in many ancient civilizations such asancient Greece including the noteworthy systems of Aristotle (see Aristotelianphysics) and Ptolemy. As such, they assumed that the Sun, Moon, stars, andnaked eye planets circled Earth.
Heliocentrism, or heliocentricism, is theastronomical model in which the Earth and planets revolve around a relativelystationary Sun at the center of the Solar System. The word comes from the Greek(ἥλιος helios "sun" and κέντρον kentron "center").Historically, heliocentrism was opposed to geocentrism, which placed the Earthat the center. The notion that the Earth revolves around the Sun had beenproposed as early as the 3rd century BC by Aristarchus of Samos, but at leastin the post-ancient world Aristarchus's heliocentrism attracted littleattention—possibly because of the loss of scientific works of the HellenisticEra —until Copernicus revived and elaborated it.
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For example, Greek astronomers madeexcellent, very accurate observations of the movements of the planets, but theobservations revealed a bit of a problem. The geocentric theory said, that theplanets would move around the Earth in one direction. However, astronomersnoticed that at times, several planets seem to stop moving in one direction andstart moving backward in their orbits around the Earth, and they came up with atheory that these planets themselves moved in smaller circles called epicyclesas they travelled around the Earth. Here’s a picture of what they imagined. Yousee how this epicycle theory could account for the seemingly backward motion ofthe planet. Of course, today we know that this appearance of backward motion iscaused by the fact that Earth, as well as other planets, all move in their ownorbits around the Sun, and the relative movements of the planets with respectto each other can get quite complex.
However, there were a few astronomers inGreece and other places who didn’t agree with the geocentric view, for example,a Greek astronomer who lived in the third century B.C.E. He proposed the theorythat our planetary system might be heliocentric, his name was Aristarchus.Heliocentric means Sun-centered, that the Earth revolves around the Sun.Aristarchus recognized from his calculations that the Sun was much larger thanthe Earth and other planets. It was probably this discovery that led him toconclude that the universe is heliocentric. I mean, isn’t it more sensible tothink that a smaller heavenly body would orbit a larger one, rather than theopposite?