Aristotle & Aristarchus
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Aristotle (4th century B.C.)
Aristotle was the main source of scientific thought during his time and for many centuries after. He came up with many ideas for how things move and why. A good physics course with historical overtones could detail more than we will here. We are mostly interested in his ideas on astronomy.
Aristotle believed the earth was the center of the universe (a geocentric universe). He argued (and correctly so) that if the earth moved through the universe that nearby stars would move with respect to farther stars. This is the concept of parallax. You experience parallax when you walk through the parking lot. As you pass nearby cars, they seem to move in position with respect to farther cars. Try it and see. Another method to see parallax is to close one eye and hold your hands in front of you at different distances. Hold up your index finger for each hand and align them in the view of the open eye. Now switch eyes. The fingers will have apparently moved when you switched eyes, even though you did not move your fingers. This is the idea of parallax that Aristotle used to argue against the earth moving through the universe.
Interestingly enough, we can detect parallax in the earth's movement around the sun. However, this requires photographic comparison or fine instrumentation to achieve. Aristotle had no chance to see it with the unaided eye. This parallax will be very important in our discussion of distances later in the class.
Aristotle also believed in a round Earth. He came to this conclusion because of observations of the Earth's shadow upon the moon during lunar eclipses. The shadow of the Earth is always round. If the earth were flat, even if it was circular and flat, the shadow of the earth would sometimes be elongated, like an ellipse. This has to do with the orientation of the Earth. In the figures below, you see a circle. If that same circle is turned about the axis shown, it becomes elongated. Its shadow would have a similar shape. Since the Earth's shadow is always round, the Earth must be a sphere. Its interesting that Aristotle, the word of science, made this statement almost 2000 years before Columbus sailed the ocean blue.
Can you think of other observations you can make which cannot be explained by a flat world, but only by a spherical one?
Aristarchus (3rd century BC)
Aristarchus conceived of the method for establishing the relative size of the Earth and the sun and the Earth and the Moon. We shall not go into the geometry of this method (but the hyperlink above will), but suffice it to say that Aristarchus established that the sun was much larger than the Earth. Even though he was wrong about the ratio of the sun size to the Earth's size the fact that the Earth was smaller than the sun was of great importance to Aristarchus. It made no sense to Aristarchus that the large object like the sun would orbit of smaller object like the Earth. Therefore, Aristarchus believed that the Earth went around the sun. Because Aristotle had already said that this was not the case, no one accepted the arguments of Aristarchus.
By similar measurements Aristarchus did correctly measure the relative size of the Moon to the Earth to within acceptable error. Aristarchus even proposed that the stars were objects like our sun at huge distances from the Earth.
This page was last updated on 06/06/01.