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A Puzzling Parallax

Time Required Very Short (≤ 1 day)
Prerequisites None
Material Availability Readily available
Cost Very Low (under $20)
Safety No hazards


Did you know that ancient astronomers could measure the distance to other stars? They could also distinguish between stars and planets. How could they do that without modern technology of telescopes? See if you can discover the link between distance and perspective used to measure stars.


To discover the relationship between the distance of an object and the viewing perspective, also known as parallax.


Sara Agee, Ph.D., Science Buddies

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MLA Style

Science Buddies Staff. "A Puzzling Parallax" Science Buddies. Science Buddies, 20 June 2014. Web. 30 July 2014 <http://www.sciencebuddies.org/science-fair-projects/project_ideas/Astro_p006.shtml?from=Blog>

APA Style

Science Buddies Staff. (2014, June 20). A Puzzling Parallax. Retrieved July 30, 2014 from http://www.sciencebuddies.org/science-fair-projects/project_ideas/Astro_p006.shtml?from=Blog

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Last edit date: 2014-06-20


How do astronomers know the distances of planets, stars and galaxies? They use a visual phenomenon called parallax to measure stellar distances. Parallax describes the way a near object moves its position relative to distant objects when seen from two different places, or perspectives. When you look at stars during different positions of the earth's orbit around the sun, the near stars will move position relative to more distant stars.

Stellar Paralax

(Diagram adapted from Carroll, 2003.)

To see parallax for yourself hold out your arm and stick up your thumb. Closing one eye, line up your thumb with an object across the room. Now quickly switch your eyes and you will notice that the object you were looking at has moved away from your thumb. This optical illusion is because of parallax. The difference in distance between your two eyes makes your thumb (which is nearest) line up differently with the distant object. If this were the model of parallax, then the star who's distance you're measuring would be like your thumb, the two different positions of the earth would be like your eyes and the distant stars would be like the distant object in the background.

In this experiment you can investigate how the distance of the near object is related to how far it moves when you view it from two perspectives. To set up your two perspectives, you will use two hula hoops placed next to each other. You will use a ruler to measure the visual shift of the near object (the coffee can) with respect to a distant object (a tree for example).

Terms and Concepts

To do this project, you should do research that enables you to understand the following terms and concepts:
  • astronomy
  • orbit
  • near star
  • distant star
  • galaxy
  • perspective
  • parallax
  • How does distance change the movement of a near object during parallax?
  • Will objects move more as they become closer or more distant?


Here are some helpful websites:

Materials and Equipment

  • a yardstick or meterstick (one that is clearly marked so that you can read it from a distance)
  • a thick rubber band
  • a long stick, about 2–3 feet
  • a coffee can
  • beans or sand
  • 2 hula hoops
  • measuring tape
  • a wide open space, like a back yard or park

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Experimental Procedure

  1. Attach one end of the ruler to one end of the stick using the rubber band. It should look like an "L" shape.
  2. Fill the coffee can with beans or sand to weigh it down, and use it to anchor your stick. Make sure that the can is heavy enough to support the ruler in the air without tipping over. If you have more time, you can use plaster to permanently mount the stick in the can.
  3. Take your apparatus, hula hoops and measuring tape outside to a wide open space, like the park or your back yard.
  4. Find a distant object to use as a visual guide to represent a distant star. The object should be tall and narrow, if possible, to make a clear measurement. Good examples are: trees, light poles, posts, or a parent who is willing to stand up for a while.
  5. Giving as much distance between you and the object as possible, face the distant object and place the 2 hula hoops on the ground, one on each side of you. The center of each hula hoop will be an observation point.
  6. Now walk ahead towards the distant object about 3–5 steps and put down your apparatus with the ruler markings facing the hula hoops.
  7. Use your measuring tape to measure the distance from the can to the hula hoops, and record this data in your data table.
    # of Steps Distance (ft) Left Hoop Right Hoop Difference
  8. Sit in the center of the left hula hoop and look towards the distant object. Which number on the measuring stick does it line up with? Write this down in your data table.
  9. Sit in the center of the right hula hoop and look towards the distant object. Which number on the measuring stick does it line up with? Write this down in your data table.
  10. Now get up and walk the coffee can forward another 3–5 steps towards the distant object and put down the apparatus with the ruler markings facing the hula hoops.
  11. Repeat steps 7–10 several more times, collecting data and writing it in your data table each time.
  12. Now you can go back inside and analyze your data. First you will need to figure out how much the distant object appeared to move along the ruler when your perspective changed from one hula hoop to the other.
  13. To calculate this, you will subtract the measurement taken in one hula hoop from the other for each distance.
  14. Now you can make a graph of your data. You should put the distance of the object on one axis, and the shift in measurement on the other.
  15. Are there any patterns that you see? Is there a relationship between the distance between you and the coffee can and the amount of optical shift that was measured between perspectives?

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  • Parallax is similar to the process our brains use when measuring depth perception. Intuitively, you know which objects are near and which objects are far. Try an experiment comparing binocular vision and monocular vision and the effect on depth perception.
  • Astronomers use parallax to determine the distances of stellar objects in space. But parallax can also be used to calculate and measure distant objects on the earth. Builders often use parallax to measure the height of buildings or the location of a building site. City planners use parallax to measure city parks and roads. Can you think of an inventive way to use parallax around your city or town?
  • Parallax shows us that near stars will move more quickly than distant stars. We can see this if we watch the change in the location of stars in the night sky. Pick your favorite constellation and watch it over several days, you will notice that the stars move together as a group. Compare the movement of the constellation to nearer objects, like the moon or a planet. Which objects move more quickly? Can you pick out the difference between planets and stars using this method?
  • In this experiment we moved the near object (our coffee can and ruler) different distances away from us. Another factor for measuring parallax is the distance between the two viewing perspectives. Can you think of a similar experiment to test this variable? Try moving the two hula hoops different distances apart instead of moving your coffee can. How does the distance between viewing perspectives affect parallax?

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