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Chapter 4: Problem 6

How did the models of Aristarchus and Copernicus explain the retrograde motion of the planets?

Short Answer

Expert verified
The models proposed by Aristarchus and Copernicus were heliocentric, explaining the retrograde motion of planets as an apparent change in the direction of a planet's orbit around the Sun as viewed from Earth. This apparent change is caused due to the different orbital speeds of Earth and the other planets. Both Aristarchus and Copernicus used this concept to describe the phenomenon of retrograde motion.

Step by step solution

01

Understanding Aristarchus's Model

Aristarchus of Samos proposed a heliocentric model, suggesting that Earth revolves around a stationary Sun. This explained the apparent retrograde motion as Earth overtakes the other planets in its orbit. As Earth catches up with and passes a slower planet, that planet seems to pause and move backward (retrograde motion) before continuing its usual eastward movement.
02

Understanding Copernicus's Model

Nicolaus Copernicus also presented a heliocentric model, which was more detailed than that of Aristarchus. Copernicus described the orbits of the planets around the sun as being circular and also explained that the apparent retrograde motion is a result of the different orbital speeds of Earth and other planets.
03

Comparing Both Models

While Aristarchus and Copernicus both proposed heliocentric models, Copernicus's model was more detailed and had more predictive power. Both models, however, effectively explained retrograde motion of the planets by using the idea that Earth and other planets orbit the Sun at different speeds, which creates the illusion of a planet moving backwards when Earth overtakes it in its orbit.

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Most popular questions from this chapter

Explain why the semimajor axis of a planet's orbit is equal to the average of the distance from the Sun to the planet at perihelion (the perihelion distance) and the distance from the Sun to the planet at aphelion (the aphelion distance).

(a) Search the World Wide Web for information about Kepler. Before he realized that the planets move on elliptical paths, what other models of planetary motion did he consider? What was Kepler's idea of "the music of the spheres"? (b) Search the World Wide Web for information about Galileo. What were his contributions to physics? Which of Galileo's new ideas were later used by Newton to construct his laws of motion? (c) Search the World Wide Web for information about Newton. What were some of the contributions that he made to physics other than developing his laws of motion? What contributions did he make to mathematics?

Figure 4-21 shows the lunar module Eagle in orbit around the Moon after completing the first successful lunar landing in July 1969. (The photograph was taken from the command module Columbia, in which the astronauts returned to Earth.) The spacecraft orbited \(111 \mathrm{~km}\) above the surface of the Moon. Calculate the period of the spacecraft's orbit. See Appendix 3 for relevant data about the Moon.

A general rule for superior planets is that the greater the average distance from the planet to the Sun, the more frequently that planet will be at opposition. Explain how this rule comes about.

What are Kepler's three laws? Why are they important?
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