Notice that the “wandering” of the Sun and Moon in our sky is very simple: Day by day, both of them gradually move on a fairly steady path around the celestial sphere, with the Sun taking a year to make one trip around the ecliptic (look back at Figure 2.6) and the Moon taking about a month to make one circuit around a path that stays close to the ecliptic.
The wandering of the planets was much more mysterious. Instead of moving steadily eastward relative to the stars, like the Sun and Moon, the planets vary substantially in both speed and brightness. Moreover, while the planets usually move eastward through the constellations, they occasionally reverse course, moving westward in the way that you’ve already seen for Mars in Figure 2.33. These periods of what we call apparent retrograde motion (retrograde means “backward”) last from a few weeks to a few months, depending on the planet. For ancient people who believed in an Earth-centered universe, apparent retrograde motion was very difficult to explain. After all, what could make planets sometimes turn around and go backward?
Today, we know that planets never really go backward. Instead, the backward motion is an illusion created by the fact that Earth and the other planets are all orbiting the Sun. The following activity will help you understand how this illusion occurs.
Activity
Apparent Retrograde Motion Demonstration
Find a spot outside where you can put down a ball to represent the Sun. You will represent Earth and a friend will represent Mars. As shown in Figure 2.34a, you and your friend should both walk around the ball in the same direction (counterclockwise) so that you are like planets orbiting the real Sun. Your friend should walk more slowly and in a bigger circle than you, because Mars orbits the Sun more slowly and at a greater distance than Earth. Remember that each step represents a few weeks of real time, so you are demonstrating changes that occur over weeks and months, not over a single night.
Background trees or buildings can represent stars or constellations, as long as you do one more thing: Pretend you don’t realize that your friend (Mars) is closer to you than the stars. Then your demonstration will be just like the night sky, in which we can’t tell that planets are closer than stars just by looking.
Watch carefully as you walk several times around the Sun. Most of the time your friend will seem to move right to left through the background stars and constellations. But during the times when you are “lapping” your friend, she or he will seem to move backward (left to right), even though both of you are always walking in the same direction around the Sun. Figure 2.34b shows how the real Mars does the same thing as Earth laps it in its orbit.
After you do this, switch roles, so that your friend can now see how apparent retrograde motion occurs.
Teacher Notes: This activity should be done in pairs as indicated. The key to making it work successfully is to find a place with a background (such as trees or buildings) against which it will be easy to see the apparent retrograde motion as shown in Figure 2.34a. Be sure to scout out a good spot before you have students actually do the demonstration.
To make sure you understand what you’ve learned in the activity, try the following questions, discussing with classmates if you wish. Then click to see the answers.
1. Your demonstration should have looked just like Figure 2.34a. Trace the lines extending from the numbered points in the inner person’s (Earth’s) orbit. During which time did the person representing Earth see the person representing Mars appear to move backward?
Tracing the paths, you should see that the outer person appears to move backward between points 3 and 5, which is when the inner person is catching up to and passing the outer person.
2. Now trace the lines for the real orbits represented in Figure 2.34b. When does Mars appear to move backward (compared to the stars) in our sky?
Notice that the situation is virtually identical to your demonstration, so again, Mars appears to go backward as Earth passes Mars in its orbit between points 3 and 5.
3. The outer person represented Mars in the demonstration. What would change if the outer person represented a different planet, such as Jupiter or Saturn?
The same basic thing would happen, except that a more distant planet would be farther away and would move slower. Therefore the apparent retrograde motion would last longer.
4. The inner planets (Mercury and Venus) also exhibit apparent retrograde motion in our sky. How could you modify the demonstration to understand their motions?
Just switch who is looking to the background: the inner person now represents Mercury or Venus, and the outer person represents Earth and watches the inner person’s motion against the background.
5. Why do we say that apparent retrograde motion is an illusion rather than real?
Because you’ve seen that no person in this demonstration, or planet in the real solar system, ever actually goes backward around the Sun. They only appear to go backward because of the relative motions of the planets in their orbits.
6. We also see planets vary noticeably in size and brightness at different times; you can see this clearly for Mars in Figure 2.33. When would you expect Mars to look biggest and brightest in our sky?
Mars looks biggest and brightest when it is closest to us in its orbit, which is at Point 4 in Figure 2.34b. Notice that this point also is when Mars is in the middle of its “backward” loop, and you can see this fact by noticing how big and bright Mars looks at this time in Figure 2.33. Moreover, Mars is also directly opposite the Sun in our sky (we say that Mars is at “opposition”), which means that Mars rises around sunset, reaches the meridian around midnight, and sets around sunrise during this period.
Given how easy it is to explain apparent retrograde motion in a Sun-centered system, you might wonder why most ancient people continued to believe that Earth was the center of everything. We’ll discuss the answer in the next chapter, and in the process you’ll see how our ancestors’ observations of the sky led directly to modern science and technology.