# 61. PARALLAX TWO

SUBJECT: Science
GROUP SIZE: Large or Small
TIME: 45 minutes
TYPE OF ACTIVITY: Student Investigation TEACHING STRATEGY Expository Guided Discovery
CONCEPTS: Parallax Relative Distance Relative Motion
SKILLS: Measuring Graphing Plotting Data
Objective: To learn how parallax can be used to determine the relative distance to the stars; to calculate, using parallax, the distance to an object in the classroom.

relative distance measurement diagram

star chart

Materials: Meter stick; 2 styrofoam balls; chalkboard; block with mounted protractor; ruler; graph paper.
Teacher Background Information: Before class begins, draw and number about 10 vertical lines on the chalkboard with 30cm between them.
Procedure:

Have the students prepare a piece of graph paper by drawing their classroom to scale. You can discuss with your students the appropriate scale to use given the size of the classroom. Ask the students to use a whole sheet of paper and to indicate the positions of the numbered chalkboard lines on the drawing
Now,

1. Have students move to the back left hand corner of the room (indicate this as point A on their drawing) and sight along the edge of a ruler at “star” S. Have them note the number of the line that appears to be in line with “star” S and indicate this position on their paper as “a”. Have them draw a dotted line from A to “a”.
2. Without moving, have the students sight at star S1. The students should note the number of the line that appears to be in line with star S1 as a1 and draw a dotted line from A to a1.
3. Have student move to the right hand back corner of the room and mark this point on his/her paper and sight along the ruler at star S and then star S1 indicating the apparent positions of the chalk lines on the board as b and b1. Draw lines from B to b and from B to b1.
4. Have students draw a small circle where line Aa crosses Bb and indicate this circle as S, Have them draw another circle when line Aa1 crosses Bb1 and indicate this S1.
5. Have students measure the distance from the back of the room, to the positions of S and S1 on their paper and calculate these distances using the same scale you came up with earlier. Now, have them measure the actual distance from the back of the room to S and to S1 with a meter stick.
6. Discuss the following with your students as a way to apply this activity to using parallax to measure distances to stars:

On July 1, consider the earth’s position as A. We sight on two nearby stars S and S1 against their background of stars. On January 1 we consider the earth’s position as B and we sight on the same two stars against their backgrounds. What is the distance between points A and B? We notice that the apparent shift behind star S is very slight, but the apparent shift behind star S1 is twice as great. Which star is closest to the earth?