When early astronomers observed the sky, they imagined the vast array
of stars embedded in a giant "crystalline sphere" that encompassed the
Earth and the Sun. They believed that the planets resided on a smaller
transparent sphere concentric to the one on which the stars moved.
Only over time did astronomers realize that as the
Earth moved around the Sun, some stars shifted in position relative to
the background of others. They realized that this shifting occurred because of a
parallax effect when the Earth was on opposite sides of the Sun.
In this activity, students will use a piece of black paper to create a
sample star field. They will transform this two-dimensional star
field into a three-dimensional star field by shining light from
an overhead projector across the room.
Attach each Styrofoam ball to the end of a length of string by using bits
of tape.
Make several holes in the black construction paper. Place holes
in random locations about the paper. Make them a variety of sizes by
using the single-hole punch, the pen or pencil, and the pin.
Tack the black sheet to the front of the classroom to form a screen for the
overhead projector.
Place the overhead projector in the back of the classroom to make its image as
large as possible on the black screen.
Place a construction-paper star field on the projector so that the holes in
the paper create points of light on the sheet.
Explain to students that we on Earth see the stars in the sky as if
they were on a flat plane. However it only appears as though they are all
the same distance away, since our eyes can't detect
their relative distances because they are points of light.
Give each student a ball and string. Ask them to find a star beam by
moving their hands in the area between the overhead projector and the sheet. When
they find a star beam, have them tack their string to the ceiling in a
manner that allows the star beam to strike directly on the sphere.
Encourage students to place their spheres at varying distances from the
overhead projector to increase the 3-D effect of the demonstration.
Gather students in the back of the classroom and explain that this arrangement is
more representative of the arrangement of stars. They are at different
distances from the Earth, but because they are ALL so far away from the
Earth, we have a difficult time seeing their varying distances.
Optional: To emphasize the need for parallax to determine distances,
have kids pair up and stand at arms' length facing their partner. One
student should hold up a finger. The second should reach out and grab
hold of it. Explain that they used both their eyes and the principle of
parallax to determine the distance with which they needed to reach out
and grab the other's finger. Now have students repeat the process with
one eye closed. The difficulty results from the lack of parallax with
only one eye.
Have advanced students use astronomy
reference books to pick a constellation and identify by name the stars
in the constellation. Students should look up the distance in light-years that those stars are from Earth. Have these students reproduce their
constellation on their black construction paper and, using an
appropriate scale, place their spheres at the appropriate distances for
the stars they represent.
