Motion Parallax. Image Credit: Wikipedia
Motion parallax is a depth cue that our minds associates with out movement. We see objects that are closer to us as moving faster than objects that are in the distance. We are able to see the full speed motion of the close items, but distant objects seem to be moving at a slower rate.
Motion parallax is different from the parallax that is repeatedly spoken of in astronomy. That term refers to an apparent 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. While it is difficult to put into words, it is easy to visualize. Sit with your hand extended towards the middle of this article. Close your left eye, open it. Close your right eye, open it. Notice the shift of the screen. That is a narrow example of parallax. Now, blow that image up onto a cosmic scale and you can see how entire planets appear to disappear depending on your vantage point.
To help you understand how motion parallax works, there is a great animation at http://psych.hanover.edu/krantz/motionparallax/motionparallax.html. The site is very simple and takes the point of view of a passenger in a speeding car. What it lacks in sophistication it makes up for in transferring its information.
We have written many articles about motion parallax for Universe Today. Here’s an article about stellar parallax, and here’s an article about the nearest stars.
If you’d like more information on motion parallax, check out NASA’s StarChild article about Parallax, and here’s an article about a new method in determining the parallax of the Sun.
We’ve done many episodes of Astronomy Cast about Astrometry. Listen here, Episode 182: Astrometry
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