About the theory of Heliocentrism
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Heliocentrism[a] is the astronomical model in which the Earth and planets revolve around the Sun at the center of the Solar System. Historically, heliocentrism was opposed to geocentrism, which placed the Earth at the center.
The heliocentric theory argues that the sun is the central body of the solar system and perhaps of the universe. Everything else (planets and their satellites, asteroids, comets, etc.) revolves around it.
The first evidence of the theory is found in the writings of ancient Greek philosopher-scientists. By the sixth century BC they had deduced that Earth is round (nearly spherical) from observations that during lunar eclipses Earth’s shadow on the moon is always a circle of about the same radius wherever the moon is on the sky.
Only a round body can always cast such a shadow.
Despite this discovery, the prevailing theory at that time was that of a geocentric (Earth-centered) universe, in which all celestial bodies were believed torevolve around Earth. This was seen as more plausible than the heliocentric theory because to a casual observer, all celestial bodies seem to move around a motionless Earth at the center of the universe.
Over 200 years later Aristarchus of Samos (310-230 BC) attempted to measure the sun’s distance from Earth in Earth-moon distance units by measuring lunar intervals. Observing the new moon to the first quarter and the first quarter to full moon, then using geometry and several assumptions, Aristarchus used the time-interval differences to calculate the sun’s distance from Earth. The smaller the difference between the intervals, the more distant the sun. From this value he determined the sun’s distance and the relative sizes of Earth, the moon (about 1/4 that of Earth), and the sun.
Aristarchus concluded that the sun was several times larger than Earth, and thought it reasonable that the smaller Earth revolved around the larger sun.
Because the stars are all located on an enormous celestial sphere (the entire sky) centered on the sun, Earth’s yearly motion around the sun is reflected in the stars. Those most likely to show the effect of this yearly motion are those in Gemini, especially its brightest stars: Castor and Pollux, which are about 4.56° apart and close to the ecliptic, the sun’s yearly path among the stars. In heliocentric theory, the ecliptic is the projection of Earth’s orbit onto the sky. If one views the heliocentric model from the north ecliptic pole in Figure 1 we see the sun, Earth (E) in several positions in its orbit, Castor (C), and Pollux (P) on the celestial sphere. If Castor and Pollux are fixed on the celestial sphere, then the distance CP between them is a fixed length.
Because they are fixed objects, the distance CP in this case appears largest when closest, and smallest when most distant.
This effect was not detected with even the best astronomical instruments during the time of the ancient Greeks.
Copernican revival of the heliocentric theory Nicholas Copernicus (1472-1543) revived the heliocentric theory in the sixteenth century, after hundreds of years of building on Claudius Ptolemy’s (c. AD 90-168) geocentric cosmological model (“proving” Earth is at the center of the universe). In his book, De revolutionibus orbium coelestium (On the revolutions of the celestial spheres), he placed the sun at the center of the universe with the planets revolving around it in epicycles (a circle around which a planet moves) and deferents (the imaginary circle around Earth in whose periphery the epicycle moves). He argued that the planets in order from the sun are Mercury, Venus, Earth (with the Moon orbiting it), Mars, Jupiter, and Saturn. The celestial sphere with the stars is far beyond Saturn’s orbit. The apparent daily westward rotation of the celestial sphere, the sun, moon, and of the planets is the result of Earth’s daily eastward rotation around its axis.
If one assumes that the orbital velocities decrease with increasing distance from the sun, then the apparent retrograde (backward) motion of the planets on the zodiac could be explained by Earth overtaking Mars, Jupiter, and Saturn near opposition (when they are 180° from the sun on the zodiac), and by it being overtaken by the faster-moving Mercury and Venus when they pass between the sun and Earth (inferior conjunction).
Copernicus’s heliocentric model achieved a simpler cosmology than did the modified (although not more accurate) Ptolemaic geocentric model that existed in the sixteenth century.
The major advantage of Copernicus’s system was the aesthetic appearance of a system of concentric orbits with ever-widening separations and, ironically, the return to some of the ancient Greek fundamentals, including purely circular motions. Copernicus’s heliocentric model, however, did not accurately represent the observed planetary motions over many centuries. His model had many critics and was generally not accepted. An interesting variant of a geocentric model was developed at the end of the sixteenth century by the Danish astronomer Tycho Brahe (1546-1601), who placed Earth at the center of the solar system, except for Mercury and Venus, which revolved around the sun, which in turn revolved with them around Earth.