Passage VI

When viewed from Earth, the other planets in the solar system usually appear to move prograde (eastward relative to the stars). Occasionally, however, each planet appears to briefly move retrograde (westward relative to the stars). For example, Figure 1 shows Mars's position relative to the stars on 9 dates between July 24, 2005, and February 26, 2006.

Two hypotheses were proposed to explain why the planets occasionally appear to move retrograde.

Hypothesis 1

Earth is the solar system's central body, and the other bodies move around Earth in looped orbits. Each body (except Earth) has 2 circles associated with it: a deferent and an epicycle. Both circles rotate counterclockwise, and their combined motions result in a body following a looped orbit. In Figure 2, the left panel shows Mars's deferent and epicycle, and the right panel shows Mars's orbit.

As a body passes through a loop. the body's motion changes from prograde and back. The larger a body's deferent. the more loops in the body's orbit. and the more often that body passes through a loop.

Hypothesis 2

The Sun is the solar system's central body, and the planets move counterclockwise around the Sun in elliptical orbits. The larger a planet's orbit, the more time the planet takes to complete a revolution around the Sun. As a result, the line of sight from Earth to a given planet drifts over time. Figure 3 shows the orbits Earth and Mars, and the positions of Earth and Mars, on each of the 4 dates labeled in Figure 1. For each date, the line of sight from Earth to Mars is projected onto a view of the sky.

There are 2 rules for apparent retrograde motion:

• A planet with an orbit larger than Earth's appears to move retrograde whenever Earth passes between the Sun and that planet. The larger that planet's orbit, the more often a pass occurs.

• A planet with an orbit smaller than Earth's appears to move retrograde whenever that planet passes between the Sun and Earth. The smaller that planet's orbit, the more often a pass occurs.

34. Which of the following statements best describes a primary difference between the two hypotheses? Hypothesis 1 claims that all planets follow:

Answer and Explanation

Your Answer is

Correct Answer is F

Explanation

This article explains the causes of retrograde Mars. Hypothesis 1 is based on the geocentric theory. Since the geocentric theory is a wrong theory, it cannot explain planetary retrograde very well. The epicycle mentioned in it is based on Ptolemy's cosmic model. Ptolemy believes that the planets roughly revolve around the earth along a deferent cycle. However, while revolving around the earth, they will also draw a circle along a small circle (that is, an epicycle). So the dotted line in the picture on the right side of Figure 2 shows the orbit of the planet (that is, the looped orbit mentioned in question 1).

Hypothesis 2 is based on the heliocentric theory. The earth and other planets revolve around the sun from west to east, and their orbits are elliptical orbits. The orbital lengths and angular velocities of the planets are different, so taking the earth as the observation point, the position of the earth sometimes surpasses other planets and sometimes lags behind other planets, so the phenomenon of prograde and retrograde planets will be observed on the earth (as shown in Figure 3). At this stage when the earth surpasses Mars, Mars retrograde will be observed. PS The Mercury retrograde (Mercury retrograde) that everyone usually mentions is also the same principle. It is not that the planets will really rotate in reverse, but in some special time periods, Mercury is overtaking the earth. , so we see Mercury retrograde.

Therefore, the difference between the two hypotheses in the article is obvious.