![]() ![]() ![]() Discuss: Use what you’ve learned to explain why the fact that Earth orbits the Sun means we should see parallax as we look to the stars.Discuss: Note that in step (1), you observed no parallax when you stood still in the center of the “orbit.” Is there any circumstance in which you could be walking around your “orbit” but still see no parallax for an object beyond the flagpole? How?.How does the amount of parallax you see depend on the distance of the object you are looking at? Why? Continue walking your “orbit” and look at a variety of objects at different distances beyond the flagpole (such as other poles or trees).Do you see parallax of the flagpole? Why or why not? Move about 2 steps away from the ball and start walking in a small “orbit” around it as you watch the flagpole (see figure).Do you see any parallax of the flagpole while you stand still? Why or why not? Stand over the ball (“Sun”) without moving as you look toward the flagpole.Let’s continue the activity to see how this works, and you’ll then be able to understand how it accidentally led the ancient Greeks to an incorrect conclusion.Ĭontinuing near the flagpole outside, bring a ball to represent the Sun and place it at least 10 steps away from the flagpole. This brings us to the key point: If Earth is orbiting the Sun, then our viewing position is always changing as we look to the stars. And it is depth perception that makes it possible to reach for objects, catch balls, and so on.Īs you’ve seen, we should notice parallax for relatively nearby objects whenever our viewing position changes. (6): Parallax is in essence the method by which we judge depth perception: we use our two eyes to judge distances because more distant objects have smaller parallax.It should be clear that if they were far enough away, they would no longer be able to notice the parallax. For (4) and (5): They should notice that the pole’s apparent shift, or parallax, gets smaller as they move farther away from it.Again, help them understand that the parallax is observed because of the changing position from which they are viewing the pole as they move back and forth. They can keep moving back and forth between their original position and the one step left to see the parallax clearly. For (3): We’ve suggested taking only 3 steps away from the flagpole so that the parallax shift should be quite obvious as they step one step to the left.For (2): The answer is that parallax gets smaller with distance (to their finger), and they should be able to see this.For the explanation, try to get students to recognize that they are looking at their finger from a different position (angle) from their left and right eyes. For (1): This should be easy for students to see.This first part of the parallax activity is to make sure students understand the basic idea of parallax as an apparent shift in an object’s position, and to realize that the amount of parallax gets smaller as the object gets farther away. Discuss how parallax helps us judge distances of objects around us, and how it can help us reach to pick up a glass of water or to catch a ball. Can you move far enough away so that you can no longer notice the parallax?Ħ. Describe how its parallax changes as you move farther away from it. Repeat the experiment several more times, each time moving farther from the pole. Is the parallax of the pole bigger or smaller than it was when you were closer? Explain why.ĥ. Move another 3 steps away from the pole and repeat your experiment. How is this similar to what you did with your extended finger? Explain why you see this parallax of the pole.Ĥ. If you keep stepping back and forth, you’ll see the pole seem to keep shifting back and forth, even though the pole is not moving. Notice how the pole appears to shift against the background as you step to the left. Then take one step to the left and look at it again (see figure below). Take 3 steps back from the pole and look at it against the background in the distance. Go outside and find a flagpole (or other pole). (Fill in the blank with either “bigger” or “smaller.”)ģ. The farther away my finger, the _ its parallax. ![]() Then decide how you would complete this sentence: You can try it with your finger at different distances. Repeat the experiment while bending your arm so that your finger is closer to your face. ![]()
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