Key concepts
Do you enjoy going stargazing on a warm night? Summer can be a great time to watch the stars as well as other celestial events, such as the impressive Perseids meteor shower that happens annually and peaks this year from August 10 to 13. Did you know that ancient astronomers could actually measure the distance from Earth to faraway stars? How could they do this without modern technology? In this activity you will find out by exploring the relationship between the distance of an object and the viewing perspective (also known as parallax), which can be used to measure how far away distant stars are.
How do astronomers know how far planets, stars and galaxies are from us? They use a visual phenomenon called parallax to measure stellar distances. Parallax is the way an object appears to move, as it appears to change position when it is seen from two different locations, or perspectives.
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 while keeping your thumb in the same position. (You can use your free hand to cover your other eye if necessary.) Notice that the object you were looking at is no longer lined up with your thumb. Don't believe what you're seeing? Repeat again starting with the other eye. This optical illusion is because of parallax. The difference in distance between your two eyes makes your thumb (a relatively nearby object) line up differently with the object that is across the room (a relatively distant object).
When a person looks at stars when Earth is at different positions in its orbit, closer stars will appear to change position much more relative to stars that are farther away. This apparent movement, or parallax, can be used to help figure out distances between Earth and specific stars.

  • A wide open space (It could be your backyard or a park.)
  • Two Hula-Hoops (Alternatively, you could use two flat rocks or bricks that you can sit on.)
  • A yardstick or meterstick (Use one that has clear markings so that it can be read from a distance.)
  • Small table or bar stool (This will need to be able to be taken outside.)
  • A thick rubber band
  • A large rock (It should be at least the size of a baseball, but not so large that the rubber band cannot fit around it.)
  • Measuring tape
  • A scratch piece of paper and a pen or pencil (optional)
  • If you are using a scratch piece of paper, draw three columns on it and label them "Left," "Right" and "Difference."
  • Take your Hula-Hoops (or flat rocks), small table (or bar stool), rubber band, large rock and measuring tape outside to a wide-open space, such as a park or your backyard.
  • Find a distant object—ideally one that is tall and narrow, such as a tree, light pole or post. This will be the "distant object" you use in this activity
  • Giving as much distance between you and the distant object as possible (at least 20 to 30 steps, but farther is even better), face the object and place the two Hula-Hoops on the ground, one on your right and one on your left, so that they are nearly touching. (If you are using flat rocks instead, place them so that they are about three feet apart, to your left and right as you face the distant object.) Each Hula-Hoop (or flat rock) will be an "observation point."
  • Walking from the Hula-Hoops toward the distant object, place the small table about three to five steps away from the edge of the Hula-Hoops (or about five to seven steps away from the flat rocks).
  • On top of the table place a large rock. Loop the rubber band around the rock and yardstick (or meterstick) to hold the yardstick against the rock so that the yardstick is horizontal (running to your left and right) and its markings are facing you (toward the Hula-Hoops). Center the yardstick along the rubber band. The table (with the rock, yardstick and rubber band) will be the "near object" you investigate.
  • Your setup should now be ready for you to do some testing! But before you do, make sure that all of the parts of your setup are lined up well. Specifically, make sure that the space between the two Hula-Hoops, the rubber band and the distant object all roughly line up along an imaginary line. If needed, you can shift the table to the left or right to make the rubber band line up.
  • Sit in the center of the left Hula-Hoop (or on the flat rock on the left) and look toward the distant object. Which number on the yardstick does the distant object appear to line up with? If you are using a scratch piece of paper, write this number down in the "Left" column.
  • Sit in the center of the right Hula-Hoop (or on the flat rock on the right) and look toward the distant object. Which number on the yardstick does the distant object now appear to line up with? If you are using a scratch piece of paper, write this number down in the "Right" column.
  • Now move the table (with the rock and yardstick on it) forward another three to five steps (so that it is closer to the distant object).
  • Sit in each Hula-Hoop again and see which number on the yardstick the distant object appears to line up with now. If you are using a scratch piece of paper, write down your results. Do you see a difference in where the distant object appears to line up on the yardstick compared with when the table was closer to you?
  • Repeat this process at least three more times (each time moving the table a little closer to the distant object and then looking at how the distant object lines up on the yardstick from your Hula-Hoop vantage points). How does where the distant object lines up on the yardstick appear to change as the near object (the table with the yardstick) is moved closer and closer to the distant object?
  • If you used a scratch piece of paper to record your results, you can subtract the number in one column from the other and write your result down in the "Difference" column for each distance you tested. Do you see a relationship between the distance (between the observation points and the near object) and the difference in the measurements you took from the left and right perspectives?
  • What do you think your results tell you about how astronomers use parallax to measure how far away a relatively nearby star is?
  • Extra: In this activity you moved the near object (the table with the rock, yardstick and rubber band) different distances away from the observation points (the Hula-Hoops). Another factor for measuring parallax is the distance between two observation points. Can you think of a similar activity you could do to test this variable? How does the distance between observation points affect parallax?
  • Extra: If you watch your favorite constellation over several nights, 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?
  • Extra: Parallax is similar to the process our brains use when calculating depth perception. Intuitively, you know which objects are near and which are far. You could try testing depth perception by comparing binocular vision and monocular vision. How does each affect depth perception?

Observations and results
As the near object moved farther from the observation points, did the apparent movement of the object decrease (as measured from the left and right observation points)?
When a relatively distant object is viewed from two different points, it appears to move less compared with a relatively nearby object. Similarly, in this activity you should have seen that as the "near object" (the table with the rock, yardstick and rubber band on it) got closer to the "distant object" (the tree, light pole, etcetera), it appeared to move less (when you compared its apparent position between the left and right "observation points" inside the Hula-Hoops). It might have been hard to tell the difference for the first two measurements, but the relationship should have become clearer after that point.
Because the apparent movement of an object (the parallax) depends on how far the object is from the observation points, astronomers can figure out how far away relatively nearby stars are. This is done by looking at a nearby star's apparent movement relative to distant stars when they are all viewed from different observation points (that is, from different points in Earth's orbit around the sun).
More to explore
A Puzzling Parallax, from Science Buddies
Parallax, from
Parallax—Greek Astronomy for Kids, from History for Kids
Parallax, from StarChild

This activity brought to you in partnership with Science Buddies