Ancient Astronomy, Page 18: [Previous Page] [Next Page] [Class Home Page] Textbook pages 33 - 50

An Earth-Centered versus a Sun-Centered 'Universe'

Parallax
Ancient mathematicians were quite clever. They knew that if the Earth moved relative to the Stars, they would observe the nearby stars to show a parallax: Stellar parallax occurs as the Earth orbits the Sun and our line of sight to a nearby star varies. The effect is to make the star appear to shift position over the course of the year. In reality, stellar distances are so great that parallax shifts are less than an arc second, completely unobservable to the unaided eye.

Other (less scientific) evidences supporting the Geocentric Universe:


1. The Earth is not part of the heavens.
2. The celestial objects are bright points of light while the Earth is an immense non-luminous sphere of mud and rock.
3. There is little change in the heavens: Stars are the same night after night. Earth is home of birth, change, and destruction. Celestial bodies have immutable regularity that is never to be achieved on the corruptible Earth.
4. Our senses show Earth is stationary:
Air, clouds, birds, and other things unattached to Earth are not left behind, as they would be if the Earth were moving. There is no strong wind.
If the Earth were moving, then a man jumping from a high point would hit the Earth far behind from the point where the leap began.

Aristarchus (another Greek philosopher born around 310 B.C.) realized that although he couldn't feel any motion of the earth, other observations (motions in the sky, etc) were simpler to explain if the Sun were at the center. However, Aristarchus' model was rejected because of his inability to explain the lack of sensation of motion of the Earth. Aristarchus measured the ratio of the distances to the moon and to the sun and, although his methods could never yield accurate results, they did show that the sun was much further from the earth than was the moon. His results also showed that the sun was much larger than the earth, although again his measurements were very inaccurate. Some historians believe that this knowledge that the sun was the largest of the three bodies, earth, moon and sun, led him to propose his Heliocentric theory. His sun-centred universe found little favour with the Greeks, however, who continued to develop more and more sophisticated models based on an earth centered universe.