Of Sun and Earth: The Greek Who Hazard A Theory

     If asked, a great many people might inform you that Galileo Galilei was the first man to bring about the determined theory that the solar system was heliocentric (Sun-centered solar system), and not, as the church deeply suggested, geocentric (Earth-centered solar system).     
    Galileo did play a significant role in the Scientific Revolution, and his contributions included improvements to the telescope, the confirmation of the phases of Venus, the observation and analysis of sunspots, and the discovery of the four largest satellites of Jupiter. However, Galileo's proposal of heliocentrism was not without its opponents, such as fellow astronomers who doubted heliocentrism due to the absence of a stellar parallax, and, more seriously, the rebuffing of the Vatican. 
    But what is the stellar parallax--that vital missing component to solving the heliocentric conundrum? Well, imagine having six tennis balls placed at random positions on a table. Now move your head left and right across the plane of the table surface. Notice how the tennis balls appear to be moving? Of course it is not the tennis balls, but you, who is moving. 
    Stellar parallax--parallax being a displacement, or difference, in the apparent position of an object viewed along different lines of sight--is the effect of parallax on distant stars in astronomy. Seeing as astronomers, during Galileo's time, did not know of this, it was accepted that the stars and planets were moving around the Earth.
    But was Galileo the first astronomer to introduce this idea of heliocentrism? And what's more, were the arguments against such a theory always met with hesitation and fierce criticism? 

Aristarchus of Samos
(Ἀρίσταρχος, Aristarkhos, 310 BCE – ca. 230 BCE)

    In 310 B.C.E. a man named Aristarchus was born, and lived until sometime around 230 B.C.E. He was the first astronomer to place the Sun at the center of our solar system, over 1,700 years before Galileo was born, and Aristarchus maintained that not only did the Earth revolve around the Sun, but that it also rotated on its axis. 
    Aristarchus based his heliocentric model off of Philolaus's (470 – c. 385 BCE) idea that the Earth was not the center of the universe. Philolaus proposed that all things in the universe revolved around a hypothetical astronomical object he called the 'Central Fire'. 
    Aristarchus adopted this Central Fire idea, and identified it with the Sun. He then positioned the planets in their appropriate order of distance around the Sun. But like Galileo after him, Aristarchus was met with an age old rival, the geocentric theory of Aristotle and Ptolemy. 
    To counter this, Aristarchus proposed that because the stars were such a great distance from Earth, there was no observable parallax, and without a telescope capable of seeing just how far the stars were, the possibility of the introduction of the stellar parallax was highly improbable. Thus without the stellar parallax, Aristarchus's calculations were not exact; such as his claiming that the Sun was 18 to 20 times the distance away, instead of its actual 390 times the distance. However, in retrospect, his model is still sound, even if the spacial dimensions are incorrect.
    Unfortunately, Aristarchus's heliocentric solar system would be rivaled and pushed aside by the widely accepted geocentric theory. Although heliocentrism found a Hellenistic supporter in Seleucus of Seleucia, an astronomer who lived over a century after Aristarchus, it would take over 1,800 years later for astronomers like Copernicus, Johannes Kepler, and Isaac Newton to breath new life into the ancient heliocentric theory.
    Like Galileo and Copernicus, Aristarchus suffered harsh criticisms, and venomous accusations of his Sun-centered solar system. Greek philosophers like Cleanthes wrote in his 'Against Aristarchus' that Aristarchus should be indicted for impiety for "putting into motion the hearth of the universe." 
    Fortunately this sour look on heliocentrism was not shared by all. Archimedes mentions Aristarchus's theory in his work, 'Sand-Reckoner', and stated that if Aristarchus was indeed correct, it would make the universe massively larger than had been believed. Centuries later, Aristarchus would be an influence for 1600s Polish astronomer, Nicolaus Copernicus, who cited in his work "Six Books Concerning the Revolutions of the Heavenly Orbs" that Aristarchus was the ancient authority who espoused the motion of Earth. (However, this reference was excluded from the eventual published version.)

    The only surviving work of Aristarchus is "On the Sizes and Distances of the Sun and Moon." From this geometric work we get the following premises from Aristarchus, though the following degrees and distances are incorrect:

• (1) As observed during a lunar eclipse, the diameter of Earth’s shadow is twice the diameter of the Moon.
• (2) The Moon and Sun are each 2 degrees in angular diameter.
• (3) At the time of quarter Moon, the angular distance between the Moon and the Sun is 87 degrees.

*Aristarchus showed that the Sun is between 18 and 20 times farther away from Earth than the Moon is. (The actual ratio is about 390.)

*Aristarchus found the Moon’s diameter to be between 0.32 and 0.40 times the diameter of Earth and the Sun’s diameter to be between 6.3 and 7.2 times the diameter of Earth. (The diameters of the Moon and the Sun compared with that of Earth are actually 0.27 and 109, respectively.) 

*Aristarchus stated that the angle subtended by the Sun's diameter is 1/2 degree; a near accurate measurement. (The actual average value is 0.53 degrees.) 

*Aristarchus's 87° degrees is slightly off. The true angle is 89°.