(figure available in print form)
TALKING HEADS ¥ PROFILES
(Lesson Three Handout)
(340 B.C.) realized that the Earth was round in two ways, which he wrote about in his book
On the Heavens.
First he saw that the shadow the Earth cast on the surface of the moon during eclipses (when the Earth is between the sun and the moon) was always round. If the Earth had been a flat disc, then it would have cast a long oval shaped shadow instead. He also realized from Greek travelers that the North Star shown lower in the sky the further south one traveled, and higher the further north one traveled. (From the North pole, the star would appear overhead; from the equator, it appears to lie at the horizon.) Yet another clue to the round earth idea was the fact that when one observed a ship sailing into shore, he saw the sails of a ship coming over the horizon first and as the ship came closer to shore, he saw its hull. Aristotle also believed that the Earth was the center of the universe and that the universe had always existed and would continue to exist forever.
(6th c. A.D.), in his book
The City of God,
expressed that the universe probably began about 5,000 B.C., which also fit with the story of “Genesis” from the Bible. Jewish and Muslim beliefs also asserted that the universe had a beginning since everything that existed had been caused or created by something that existed before it, therefore there had to be a “First Cause” to explain how the universe came to be. St. Augustine also believed that time was part of the universe that God had created and so, it too, had a beginning.
(2nd c. A.D.) believed that the Earth was the center of the universe and that it was surrounded by eight concentric spheres that carried the heavenly bodies around it. The first sphere carried the moon; the next, Mercury; then Venus, the sun, Jupiter, and Saturn; the final sphere held the fixed stars, which did not move individually, but orbited around the Earth in a fixed group.
(1514) did not believe that the Earth was the center of the known universe, but that the planets orbited the sun. He proposed a simpler model than Ptolemy’s, but circulated it anonymously. As a Polish priest, he felt he had to be careful since the Catholic Church at that time felt it necessary to punish people whose ideas might challenge their own religious beliefs.
(1609) believed in Copernicus’ theory. With the new invention of the telescope, he observed that the moons of Jupiter orbited around the planet, which also meant that everything did not directly around the Earth.
(1609)—in the same year that Galileo discovered Jupiter’s moons—suggested that the planets did not move in circular orbits around the sun, but elliptical (elongated circle) orbits. He was not particularly happy with this discovery because it was believed (since the time of Aristotle) that the circle was a pure and perfect form. Therefore, elliptical orbits seemed less perfect, but they did seem to explain the pattern of planets that could actually be observed in the night sky far better than earlier ideas.
(1687)—the author of one of the most important works ever published in the physical sciences,
Philosophiae Naturalis Principia Mathematica—
postulated (claimed as a principle of truth) that each body in the universe was attracted to every other body. The larger and closer the bodies, the more attracted to one another they became. This attraction is called universal gravitation and Newton was able to explain mathematically how planets and their satellites (moons) followed elliptical paths around the sun.
(1781)—in his work,
Critique of Pure Reason
—called the questions about whether or not the universe had a beginning “antinomies” (contradictions) because the “thesis” (proposed idea believed to be true) that time had a beginning and the “antithesis” (proposed idea believed to be untrue) that it did not, could be argued equally. So far as Kant was concerned, both his thesis and antithesis were backed by his assumption that time continued backwards forever. If the universe didn’t have a beginning, then there would be no starting point, which meant that time was infinite backwards and forwards. If the universe did have a beginning, he proposed that there would be an infinite period of time before it, so why would it start at any particular point?
(1929) made the landmark observation that distant galaxies were moving away from us, which meant that the universe was expanding. With the knowledge that the universe was expanding, it could be reasoned that it had been doing so over a period of time, which also meant that at some point in the very distant past, it had been contracted into a very small, infinitely dense mass (since all this expansion had to come from somewhere). Since time is the product of matter being energized to move through space from one place to another—if nothing was moving in this tiny, supercondensed point, then time would not exist. Then something moved. Bang!—a great explosion that dispersed all the matter of the universe into space and in this process, time itself, was created.
A Brief History of Time
by Steven Hawking;
Exploring The Universe,
Prentice Hall, 1994.)