1. Your “telescope” is a box with a small pinhole on one side. The pinhole represents the lens on the outside of the telescope. The distance between the front of the box (with the pinhole) and the back of the box will be the focal length (the distance from the lens to where an image is focused) of our telescope.

Our Telescope

Q5. What would happen if you kept moving the lamp away from the box? Is there a distance at which you can no longer find the distance to the star?

Analyze and Conclude

Q2. Was your prediction in Step 12 correct? Why or why not?

Q3. How did your calculation for Star 1 compare with the actual distance?

Q4. Does parallax shift increase or decrease as an object is moved farther away?

Your mission is to find the distance to the light bulb star in the classroom using the parallax shift in a model telescope.

Pap

er

Focal LengthDiam

eter

of O

rbit

(dist

ance

bet

ween

pinho

les)

2. Place a sheet of paper inside the box as shown. Hold the paper in place with two paper clips.

3. Darken the room and turn on the star light.4. Align the box so the left edge of the box lines up with the left edge

of the table. Make sure to line up the pinhole with the mark on the tape.

5. With one color, mark the paper where you see a dot of light. Label this dot A.

6. Move the box so the right edge of the box lines up with the right edge of the table. Make sure to line up the pinhole with the mark on the tape.

7. Mark the paper again using a different color to mark the second dot B. Dot B represents the image of the star as seen 6 months later from the other side of Earth’s orbit. Remove the paper.

January 1st

July 1st

Q6. What are you thinking about now?

StarDiameter of Orbit

(mm)

Focal Length (mm)

Parallax Shift (mm)

Distance to Star (mm)

Distance to Star (m)

1

2

Analyze and Conclude

Q1. What caused the apparent change (parallax shift) in position of the dots of light for each star?

Diameter x Focal length Parallax Shift

Distance =

Star 2

Calculation Area

8. Measure and record the parallax shift (the distance between dots A and B) in millimeters.

9. Measure and record the focal length (distance from the hole (lens) to the back of the box) in millimeters.

10. Measure and record the diameter of orbit (distance between pinholes) in millimeters.

11. You now have the same kind of information that early astronomers had to work with. The properties of similar triangles can be used to determine the distance to the star. Find at least three similar triangles in the diagram below. Draw the lines for the triangles and label the distances you known.

A

B

A

B

12. Move the lamp further away from the model telescope. The bulb now represents Star 2. Predict what you think will happen to the light images on your paper.

_______________________________________________________

13. Repeat the same process as before to find the distance to Star 2.

Sun

Earth

Earth

Pap

er

Focal length

Diamet

er o

f orb

it

(dist

ance

bet

ween

pinho

les)

If you take a very distant object (a galaxy, for example) that doesn't move laterally compared to the Earth, you can compare the angle between a star and that galaxy when the Earth is on one side of the Sun and 6 months later when the Earth is on the other side of the Sun. Since the Earth has moved roughly 186,000,000 million miles, you can draw the triangles with that base length and determine the distance to the star with trigonometry.