Aristarchus of Samos, an early Greek astronomer (about 310 to 230 BC), was the first to suggest that the Earth revolved around the Sun, rather than the other way around. He gave the first estimate of the distance of the Moon (section (8c)), and it was his careful observation of a lunar eclipsepinpointing the Sun's position on the opposite side of the skythat enabled Hipparchus, 169 years later, to deduce the precession of the equinoxes). Except for one calculationan estimate of the distance and size of the Sunno work of Aristarchus has survived. However, one could guess why he believed that the Sun, not the Earth, was the central body around which the other one revolved. His calculation suggested that the Sun was much bigger than the Eartha watermelon, compared to a peachand it seemed unlikely that the larger body would orbit one so much smaller. Here we will develop a line of reasoning somewhat like the one Aristarchus used (for his actual calculation, see reference at the end). Aristarchus started from an observation of a lunar eclipse (section (8c)). At such a time the Moon moves through the Earth's shadow, and what Aristarchus saw convinced him that the shadow was about twice as wide as the Moon. Suppose the width of the shadow was also the width of the Earth (actually it is lesssee below). Then the diameter of the Moon would be half the Earth's. 
Aristarchus next tried to observe exactly when half the moon was sunlit. For this to happen, the angle EarthMoonSun (angle EMS in the drawing here) must be exactly 90 degrees. Knowing the Sun's motion across the sky, Aristarchus could also locate the point P in the sky, on the Moon's orbit (near the ecliptic), which was exactly 90 degrees from the direction of the Sun as seen from Earth. If the Sun were very, very far away, the halfmoon would also be on this line, at a position like M' (drawn with a different distance scale, for clarity). Aristarchus estimated, however, that the direction to the halfMoon made a small angle a with the direction to P, about 1/30 of a right angle or 3 degrees. As the drawing shows, the angle ESM (EarthSunMoon) then also equals 3 degrees. If R_{s} is the Sun's distance and R_{m} the Moon's, a full 360° circle around the Sun at the Earth's distance has length of 2pR_{s} (p = 3.14159...). The distance Rm = EM is then about as long as an arc of that circle, covering only 3° or 1/120 of the full circle. It follows that R_{m} = 2pR_{s}/120 ~ R_{s}/19
making the Sun (according to Aristarchus) 19 times more distant than the Moon. Since the two have very nearly the same size in the sky, even though one of them is 19 times more distant, the Sun must also be 19 times larger in diameter than the Moon. If now the Moon's diameter is half the size of the Earth's, the Sun must be 19/2 or nearly 10 times wider than the Earth. The effect described in the figures of section (8c) modifies this argument somewhat (details here), making the Earth 3 times wider than the Moon, not twice. If Aristarchus had observed correctly, that would make the Sun's diameter 19/3 timesa bit more than 6 timesthat the Earth. Actually, he had not! His method does not really work, because in actuality the position of the halfMoonis very close to the line OP. The angle p, far from being 3 degrees, is actually so small that Aristarchus could never have measured it, especially without a telescope. The actual distance to the Sun is about 400 times that of the Moon, not 19 times, and the Sun's diameter is similarly about 400 times the Moon's and more than 100 times the Earth's But it makes no difference. The main conclusion, that the Sun is vastly bigger than Earth, still holds. Aristarchus could just as well have said that the angle p was at most 3 degrees, in which case the Sun was at least 19 times more distant than the Moon, and its size at least 19/3 times that of Earth. In fact he did say sobut he also claimed it was less than 43/6 times larger than the Earth (Greeks used simple fractionsthey knew nothing about decimals), which was widely off the mark. But it makes no difference: as long as the Sun is much bigger than the Earth, it makes more sense that it, rather than the Earth, is at the center. Good logic, but few accepted it, not even Hipparchus and Ptolemy. It took nearly 18 centuries before the idea surfaced again, in the mind of Copernicus.

Exploring Further:Note: The actual calculation by Aristarchus was more complex and less transparent. See A. Pannekoek: A History of Astronomy, Interscience, 1961, p. 118120 and Appendix A. While the method used by Aristarchus to estimate the EarthSun distance does not give the correct valuethe Sun is too fara variation of it was used by a Danish student, successfully, to estimate the distance to Saturn, in terms of the SunEarth distance. Saturn has a wellknowen system of rings around its equator. Using a good telescope, one can observe the planet's shadow on the rings, and note its position. Viewing the rings as a round dial, one notes the position of the edge of the shadow relative to the point where the rings are lined up with the center of the planet, as seen from Earth.
That gives the (small) angle between the SunSaturn line and the
EarthSaturn line. Knowing the positions in the sky of Saturn and
of the Sun gives another angle of the EarthSunSaturn triangle.
Regarding the EarthSun distance as one "astronomical unit" (AU),
one can now calculate the EarthSaturn distance (and the SunSaturn
distance) in astronomical units.

Related Calculation: #9b The Earth's Shadow
Next Stop: #9c Discovery of the Solar System