Study how Ptolemy tried to use deferents and epicycles to explain retrograde motion Encyclopædia Britannica, Inc. NARRATOR: Aristotle's model of the universe had trouble explaining some planetary phenomena. The most striking of these was retrograde motion. In retrograde motion each planet seems to slow down at times, then move in reverse, or retrograde, before resuming its course. Planets also grow brighter or dimmer as they move through the sky. Aristotle's model could account for neither phenomena very well. The most important solution to this problem was proposed by Claudius Ptolemy in the 3rd century AD. He argued that planets move on two sets of circles, a deferent and an epicycle. This explained retrograde motion while keeping the planets in their circular orbits around the Earth. Where this did not fit, Ptolemy proposed an eccentric. An eccentric orbit had a center different from the Earth and accounted well for changes in a planet's brightness. Ptolemy's last device was the equant. In an equant, a planet speeded up and slowed down but when seen from an off-center point actually appeared to be moving with uniform speed. From Earth, however, the planet's motion was quite irregular. The Ptolemaic system held ground for centuries until too many discrepancies cried for new solutions.
Early Models of the Universe Motions of the Planets The Modern Lineage of Astronomy The history of astronomy is based on written accounts, but of course we have only a fraction of what had been written, much of which is now lost. We know that "Western" civilization became well established by 3000 B.C. in the Middle East, Egypt, and Mesopotamia. The great pyramids were constructed in Egypt between 2700-2100 B.C., and were aligned using astronomical observations of the "north star" of the time, Thuban. The Egyptians believed that the Sun was a god of fire, Ra, which was born anew each morning, traveled across the sky, and was destroyed each night.The Ptolemaic (Earth-Centered) Model What do we mean by a scientific model? It is a framework, or conceptual model that can be used to explain currently known facts and make predictions that can be tested. The starting point for the Ptolemaic model is the common sense idea that the Earth is flat and immobile, and that all of the celestial bodies (the Sun, Moon, planets, stars) move around the Earth. The Ptolemaic model was also based on the idea that the heavens should be built on geometrical ideals.The Islamic Role The book by Ptolemy that described this model would have been kept at the library in Alexandria. However, in 415 A.D., around the time of the fall of the Roman Empire, the library was destroyed (burned) by anti-intellectual mobs, and many many irreplaceable manuscripts were lost. Much more would have been lost, but a new center of scholarship arose about the same time in Baghdad (present-day Iraq). There, the book by Ptolemy received its name, the Almagest. Many of the star names we use today date from this time, and have Arabic names (e.g. Betelgeuse--armpit of the giant; Algol--the Ghoul--actually representing the eye of Medusa).The Heliocentric Model of the Solar System
Last Days Before the Telescope
Galileo's Historic Observations
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