What is Parallax? - Definition of Parallax
Welcome to Quatr.us Study Guides!

What is Parallax?

Parallax and your finger
Parallax and your finger

Parallax comes from the Greek word "para", meaning "alternate" and "alla", meaning "change". Greek people used the word parallax to describe how an object seems to move when you look at it from two different places. The easiest way to see parallax in action is to hold out your left arm straight in front of you and close one eye. Look at your hand. What is in back of it?

Now close the other eye instead, but don't move your hand. Look at the same hand with the other eye. Whatever's in back of it will seem to have moved a little bit. That's because you are looking from slightly different places. Your brain uses parallax to figure out how far away something is, like a cup, so you can pour water into it accurately.

You can also use parallax to figure out how far away something is. The less the background changes when you look at the object from two different places, the further away it is from you. Test this out easily: hold your finger close to your face and close first one eye, then the other. Now hold your finger at arm's length and try it again. Does close or far make the background seem to change more?

About 130 BC, the Greek astronomer Hipparchus used parallax to calculate how far away the moon was from the earth, by seeing how much the moon appeared to move relative to the stars behind it if you stood at different places on the earth. He calculated that the minimum average distance to the moon was 59 times the radius of the earth - and the real average distance is 60.3 times the radius of the earth (238,857 miles). So Hipparchus was pretty close!

But when Hipparchus tried the same method for the sun, he couldn't see any parallax there. That's not because the sun is too bright; it's because the sun is so far away that you can't see the parallax with just your eyes - you'd need a telescope to see enough stars to notice the difference in the background.

More about Hipparchus

Bibliography and further reading about Greek astronomy:

The Librarian Who Measured the Earth, by Kathryn Lasky (1994). An account of the life and work of Eratosthenes, who figured out the circumference of the earth. Explains how he did it. Easy reading.

The Shining Stars: Greek Legends of the Zodiac, by Ghislaine Vautier, Kenneth McLeish and Jacqueline Bezencon (reprinted 1989). A Greek myth for each star sign, with drawings of the constellations so you can find them in the sky. Easy reading.

Greek Astronomy, by Thomas Heath (1932). A collection of what ancient Greek writers had to say about astronomy, in their own words, with a long introduction. For adults.

The History & Practice of Ancient Astronomy, by James Evans (1998). Includes both the history, and directions to actually re-do the experiments that ancient Greek astronomers used to figure out their conclusions. For adults.

Greek Science After Aristotle, by G. E. R. Lloyd (1975).

More about Hipparchus
Ancient Greece
Quatr.us home

LIMITED TIME OFFER FOR TEACHERS: Using this article with your class? Show us your class page where you're using this article, and we'll send you a free subscription so all your students can use Quatr.us Study Guides with no distractions! (Not a teacher? Paid subscriptions are also available for just $16/year!)
Please help other teachers and students find us: link to this page from your class page.
Karen Carr is Associate Professor Emerita, Department of History, Portland State University. She holds a doctorate in Classical Art and Archaeology from the University of Michigan. Follow her on Instagram or Twitter, or buy her book, Vandals to Visigoths.
Cite this page
  • Author: K.E. Carr
  • Title:
  • Site Name: Quatr.us Study Guides
  • Publisher: Quatr.us
  • Date Published:
Did you find what you needed? Ask your teacher to link to this page so other people can use it too! Send it in and win a Quatr.us "Great Page!" award!
Sign up for more free articles and special offers in Quatr.us' weekly newsletter:
We will never share your e-mail address unless you allow us to do so. View our privacy policy. Easy unsubscribe links are provided in every email.
Comment on This Article

Does your class page honor diversity, celebrate feminism, and support people of color, LBGTQ people, and people with disabilities? Let us know, and we'll send you a Diversity Banner you can proudly display!
Looking for more?
Quatr.us is loading comments...
(Comments will appear after moderation, if they are kind and helpful. Feel free to ask questions, and we'll try to answer them.)
Cite this page
  • Carr, K.E. . Quatr.us Study Guides, . Web. 24 April, 2017