SUBJECT: Science
GRADE: 7.8.9
GROUP SIZE: Small or Large
TIME: 45 minutes
TYPE OF ACTIVITY: Teacher Demo or Student Investigation
TEACHING STRATEGY: Expository Guided Discovery
CONCEPTS: Parallax Measuring Long Distances
SKILLS: Observation Measuring Plotting Data
Objective: To provide students with an understanding that near objects appear to change their positions more than distant objects when the position of the observer changes.

parallax illustration

Materials: Two paper cups; meter stick; table or desk; chalkboard or wall; chalk.
Teacher Background Information:

As we drive down the highway and look at a tree in the field, the tree seems to change position against the woodland in the distant background. If the tree is close to the road, this apparent shift is great; if the tree is close to the woods, its apparent shift is small.

Such a shift in apparent position is called parallax and is useful in determining the relative distance to far off objects such as stars. Astronomers observe a star’s location with respect to more distant stars. They will sight the more distant star with a closer star and then wait a few months while the earth moves in its orbit around the Sun. Then, they observe the star’s position again. Nearby stars appear to shift to a slightly different position with respect to the more distant one.

This shift is called parallax and it can be measured. The nearer star will appear to shift farther than more distant stars and the very distant stars may have shifts too small to measure. This activity gives students the opportunity to see how parallax works. The activity which follows, Parallax Two, provides students with the opportunity to do some calculations regarding parallax.

Adapted from NASA educational materials

Have the students put two paper cups about 30cm apart on the desk or table, as shown in the illustration.

Make 6 chalk marks 50 cm apart on the wall or chalk board on the other side of the classroom and number them 1 through 6 as shown in the illustration.

  1. Have a student place himself or herself lm behind the table like the student in the illustration. Make sure the eyes are level with the paper cups.
  2. Ask the student to draw a picture of the paper cups as they appear from that vantage point. Include the positions of the chalk marks. Then,
  3. Have the student move lm to the left and draw the paper cups as they appear now. Show their position in relation to the chalk marks on the wall. Have the student move to two other positions and draw what is seen.
  4. Ask the students to think about these questions:
  • What do the two cups represent?
  • When the student shifts position what does that shift represent?
  • How are the drawings different from each other?
  • Which paper cup appeared to move the most?
  • What effect does the observer’s position have on the apparent position of the things seen?

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