How do epicycles explain retrograde motion

These smaller circles were called epicycles, and they allowed the planets to move backward relative to the background stars. Ptolemy's model took epicycles even further, using them to explain the brightening and dimming of the planets as well, by having epicycles attached to epicycles.

While these epicycles did not perfectly explain the motion of the planets, it was the most accurate model until Kepler's laws simplified things. How did Ptolemy's model account for retrograde motion? Astronomy Introduction to Astronomy History of Astronomy. Zack M. Draw a graph of planetary motion given its celestial coordinates at various times. Demonstrate ability to read a graph and its coordinates. Demonstrate undersanding of retrograde motion of a planet against the fixed star background Demonstrate undersanding of the geocentric and heliocentric explanations for retrograde motion Demonstrate ability to make conclusions about planetary motion from analysis of pictorial and graphical information.

Discuss the difficulties in determining the correctness of either the geocentric or the heliocentric explanations using observational data alone. Also see the relevant sections of program 4 [the celestial sphere] [the planets] ,program 7 [Plato's Question] , program 8 [Ptolemaic System] , program 9.

Retrograde motion refers to the change of direction of the planets as they wander through the fixed background of the stars. Most of the time the nightly course is east to west in fairly predictable paths near the ecliptic. Occasionally each planet turns around and moves to the east 'backwards' for awhile, then turns around to head westward again, making a loop or zig-zag against the star background. There are two good explanations or models to explain retrograde motion. One involves a geocentric view, where the sun and planets go around the Earth which is stationary.

The other is a heliocentric view, where the earth and planets go around a stationary sun. For twenty five centuries or more people thought that the earth was the center of the universe.

They believed that the geocentric model explained the movements of the stars and planets, thinking that they revolved around the earth the way the celestial sphere appears to from our vantage point on earth. Today we have much evidence that the earth is just one of the many planets, and that it revolves around the sun once per year as it rotates on its own axis once per day.

It is easy to see why our ancestors thought the earth was the center of the universe when we watch the movement of the heavens. It is much easier to imagine that we are motionless at the center of a universe which revolves around us than to imagine the result of the combined daily rotation and annual revolution of earth.

This is especially true when it comes to the motion of the other planets in relation to the earth's own moions.. It was the erratic motions of the planets that gave early astronomers problems.

Placement of Mercury and Venus closer to the Earth than the Sun was problematic. The theory was based upon the idea that those that appeared to move the slowest must be farthest away from Earth.

The problem is that the epicycle containing Mercury, the epicycle containing Venus, and the Sun all orbited around the Earth in one year!

So their order was reluctantly agreed upon because Mercury moved fastest on it epicycle, Venus next fastest, and of course the Sun had no epicycle because it never retrograded. The epicycle sizes are based on arbitrarily assumed distances from Earth. The fact that Venus is farther away than Mercury from the Earth in this model requires it to be considerably larger than one might expect, but these are to scale to create the properly sized retrograde patterns.

As time is progressed a trace can be turned on which shows the retrograding patterns of the planets. Figure 2 shows a close up of Mercury and Figure 3 that of Venus.

Does Earth go retrograde? Do all planets have retrograde motion? How does heliocentric explain retrograde motion? How does heliocentric theory answer retrograde motion? How do epicycles explain retrograde motion?

How did Ptolemy explain retrograde motion? Why did people not believe in heliocentric? How often does retrograde motion occur? Does sun have retrograde motion? What is a retrograde in astrology?

What is the annual motion? Does Jupiter have retrograde motion? What happens during retrograde? Is Earth prograde or retrograde? What never goes in retrograde motion? What two motions do all planets have? The two planetary movements are revolution and rotation.