Starry Science: Measuring Astronomical Distances using Parallax
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, like the Perseids, which is an impressive meteor shower that happens each year from mid-July to late August. Did you know that ancient astronomers could actually measure the distance from Earth to faraway stars? How could they do this without modern technologies? 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.
This activity is not appropriate for use as a science fair project. Good science fair projects have a stronger focus on controlling variables, taking accurate measurements, and analyzing data. To find a science fair project that is just right for you, browse our library of over 1,200 Science Fair Project Ideas or use the Topic Selection Wizard to get a personalized project recommendation.
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, looking like it changes 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) and you will notice that the object you were looking at is no longer lined up with your thumb – the two objects appear to have moved away from each other. 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 the Earth is at different positions in its orbit, closer stars will appear to move position relative to more distant stars. This apparent movement, or parallax, can be used to figure out distances between Earth and specific stars.
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 measuring 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 binocular vision and monocular vision 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 to 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, etc.), 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 a 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 (i.e., from different points in Earth’s orbit around the sun).
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Teisha Rowland, PhD, Science Buddies
Science Buddies |
Stars, physics, optical illusions
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