How did Ptolemy explain retrograde motion?
Ptolemy's theory of the solar system.
Encyclopædia Britannica, Inc.
-
NASAThe Boeing-built Lunar Roving Vehicle transported astronauts and equipment across the Moon during the Apollo 15, 16, and 17 missions.
-
Encyclopædia Britannica, Inc.Cloud seeding can produce an extra 5 to 15% of precipitation.
-
Encyclopædia Britannica, Inc.Every year, monarch butterflies fly south for winter—sometimes traveling up to 1,800 miles.
-
© Open University (A Britannica Publishing Partner)A discussion of the Nice model (named for Nice, France) of the formation of the orbital structure of the outer planets and the Kuiper belt. The Nice model also accounts for the age of some Moon rocks.
-
NASA/JPLComputer animation showing a brown dwarf surrounded by a swirling disk of planet-building dust.
-
© Open University (A Britannica Publishing Partner)Learn about comet debris, including meteors (shooting stars).
-
Encyclopædia Britannica, Inc.How big is our world?
Transcript
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.
The Ptolemaic system held ground for centuries until too many discrepancies cried for new solutions.