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Gallery 3

Parallax

Photo of Parallax exhibit

Objects closer to you appear to move more than objects further way when you look from different angles.

How it works

Stand at different points on either side of the exhibit and look at the objects hanging in front of the wall.

Things to try or ask around the exhibit

  • Look from the left side of the exhibit. Where is the star in relation to the background?
  • Look from the right side of the exhibit. Is it still in the same place?
  • Try doing the same thing with your thumb. Close one eye and look past it, then swap over and close your other eye. Does your thumb appear in the same place relative to objects behind it?

Background

Parallax involves measuring angles to distant objects in the sky at different times of the year. The Earth is on different sides of the Sun at this point, which means you see the object from slightly different angles. Its apparent position will change in relation to things further away (which look like they don’t move at all). Using the angle difference and the distance from the Earth to the Sun (1 Astronomical Unit or AU) we can determine how far away these objects are. Hence, parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines.

Stellar parallax measures in arcseconds, or even in thousandths of arcseconds (milliarcseconds). The distance unit parsec is defined as the length of the leg of a right triangle adjacent to the angle of one arcsecond at one vertex, where the other leg is one AU long. Since stellar parallaxes and distances all involve such skinny right triangles, a convenient trigonometric approximation can be used to convert parallaxes (in arcseconds) to distance (in parsecs).

Finding the science in your world

Parallax occurs at small scales as well. When you look out the window of a car as you drive along you can see how objects close to you like bushes appear to move past you faster than objects further away (like trees or hills). Parallax can also produce errors.
Measurements made by viewing the position of some marker relative to something to be measured are subject to parallax error if the marker is some distance away from the object of measurement and not viewed from the correct position.
Cameras with a viewfinder separate from the photo lens exhibit parallax error (which is why you sometimes accidentally cut peoples’ heads off in photos). Those cameras that have a viewfinder using the same lens as the camera don’t suffer from this issue.
Parallax error also impacts 'image stitching' where multiple pictures taken from different angles are joined together. The slightly different angles of closer objects are viewed relative to distant ones which means that closer objects appear to move more, making it harder to line up the images to stitch together.