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Author: Janice VanCleave

Apparent Shift of an Object

When an observer changes viewing positions, a nearer object appears to move somewhat in relation to objects in the distant background. Scientists call this apparent movement parallax. Astronomers use parallax to measure the distance of stars.

In this project, you will discover how two factors, the distance from an object and the baseline (distance between observing points), affect parallax. You will learn how to measure the distance to a nearby object using parallax shift. You will also find out how to measure the distance to faraway celestial bodies.

Getting Started

Purpose: To determine how the distance to an object affects parallax.


  • marking pen
  • yardstick (meterstick)
  • 22-by-28-inch (55-by-70-cm) sheet of white poster board
  • masking tape
  • 7-foot (2.1-m) string
  • flat toothpick


  1. Draw 27 lines 1 inch (2.5 cm) apart across the poster board parallel to the short ends. Number the lines from left to right.
  2. Tape the poster board to the edge of a table so that the lines run vertically.
  3. Measure 14 inches (35 cm) from one end of the string. Place a piece of tape on the string at this point. Label the piece of tape "1."
  4. From the tape on the string, measure 1 foot (30 cm) and place a second piece of tape. Label the piece of tape "2." Repeat this step four times, numbering the pieces of tape in order. There should be six pieces of tape on the string.
  5. Place the yardstick (meterstick) so that about 6 inches (15 cm) of it extends over the center of the poster board. Tape the measuring stick to the table.
  6. Tape the toothpick to the extended end of the measuring stick, pointing down.
  7. Lay 2 inches (5 cm) of the end of the string that is near tape 1 on the extended end of the stick. Tape the string to the stick.
  8. Sit on the floor in front of the toothpick. Stretch the string and adjust your sitting position so that label 1 touches your nose. In this position, your eyes are in line with and about 1 foot (0.3 m) from the toothpick.
  9. Close your left eye and look at the toothpick with your right eye (see Figure 9.1). Move your head until the toothpick aligns with one of the vertical lines on the poster board. Note the number of that line.
  10. Apparent Shift of an Object

  11. Without moving your head, open your left eye and close your right eye. Note the number of the line the toothpick appears to move to. If the toothpick falls between lines, estimate the line number to the nearest tenth.
  12. Record the parallax measurement in a table like Table 9.1. Determine the parallax by finding the difference between the positions of the toothpick as seen with your right and left eyes. For example, if you see the toothpick in front of line 4 with your right eye and in front of line 9 with your left eye, the parallax equals 9 minus 4, or 5 spaces.
  13. Repeat steps 8 to 11 at each of the marked distances on the string.


Parallax decreases as distance from the object increases.

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