Mobile Stars: The Apparent Movement of Stars

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Author: Janice VanCleave

Stars do move through space. But, as observed from the Earth, which is also moving, the position of stars in relation to each other is generally the same. The position of all the stars in the sky appears to change from one hour to the next. This apparent change of position is due to the motion of the Earth.

In this project, you will study apparent star movement due to parallax. You will learn how parallax distance indicates the distance of stars from the Earth and how astronomers determine this distance. Stellar parallax is measured in seconds of arc, and you will get an idea of how small this measurement is. You will make a star clock to determine the direction of the apparent movement of stars during the night and from one month to the next. A method for identifying circumpolar stars at given latitudes will also be determined.

Getting Started

Purpose: To determine how the Earth's rotation makes stars appear to change position.


  • Pencil
  • Scissors
  • 1-inch (2.5-cm) square of stiff paper (index card works well)
  • 1-quart (1-liter) jar
  • Metric ruler
  • Sheet of typing paper
  • Two marble-size pieces of modeling clay


  1. Draw a five-pointed 1 × 1-inch (2.5 × 2.5-cm) star on the stiff paper square. Cut out the star.
  2. Use the ruler to draw a straight line from top to bottom down the center of the sheet of typing paper.
  3. Lay the paper on the edge of a table with the line perpendicular to the table's edge.
  4. Mobile Stars: The Apparent Movement of Stars

  5. Use one of the clay pieces to stand the ruler on edge parallel to the edge of the paper with the metric measurements at the top and the 15-cm mark of the ruler at the end of the line on the paper.
  6. Stand the star upright in the second piece of clay with the legs of the star in the clay and its point up.
  7. Set the star in the center of the line on the paper.
  8. Kneel beside the table with your nose at the end of the line on the paper.
  9. Close your right eye and using your left eye observe the position of the star against the ruler behind it (see Figure 6.1). Read the measurement on the ruler behind the tip of the star's top point.
  10. Without moving your head, open your right eye and close your left eye. With your right eye, again read the measurement on the ruler behind the tip of the star's top point.
  11. Calculate the absolute difference between the readings in steps 8 and 9 by subtracting the smaller number from the larger one.
  12. Example:

      16.7 cm – 13.1 cm = 3.6 cm


The star appears to move first to the right, then to the left. In the example, the distance between the star's two apparent positions is 3.6cm.


A straight line drawn between the two points from which an object is observed is called a baseline. In this experiment, the position of each eye represents the different positions of the Earth during its rotation around the Sun. This is much like observing the stars from the Earth at different points along its orbit. The baseline is an imaginary line between your eyes, as your eyes sight the star from two different points (see Figure 6.2). Each eye sees the star from a different angle; thus each eye sees a different background behind the star. The star seems to move from its actual location. The apparent change in the position of an object when viewed from two different points is called parallax. In the example, the star's parallax distance (linear distance between the apparent positions of an object due to parallax) is 3.6 cm.

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