Heliocentric Theory

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 Greece. Greek philosopher-scientists deduced by the sixth century B.C. that Earth is round (nearly spherical) from observations that during eclipses of the Moon, 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.

The prevailing theory of the universe at that time was a geocentric (Earth-centered) one, in which all celestial bodies were believed to revolve around Earth. This was seen as a more plausible theory than the heliocentric one 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 B.C.) attempted to measure the distance of the Sun from Earth in units of Earth-Moon distance by measuring intervals of the Moon. Observing the new Moon to the first quarter and the first quarter to full Moon, then using geometry and several assumptions, Aristarchus used the difference of the intervals of time 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 of the Sun. Aristarchus concluded that the Sun was several times larger than Earth. Aristarchus 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, not Earth, Earth's yearly motion around the Sun shows up in observations of the stars. The stars most likely to show the effect of this yearly motion are those in Gemini. The two brightest stars in Gemini, Castor, and Pollux, are about
4.56° apart and are close to the ecliptic, the Sun's yearly path among the stars. In the 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 Fig. 1 we see the Sun, the 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.