The Principle of Parallax

We’ve been looking at the indirect methods employed by scientists through the ages as they struggled to determine the distance of our Earth from its sun.   Today we’ll take a look at one of those indirect methods, known in scientific circles as the Principle of Parallax.

 Parallax is an optical effect which results in a three dimensional view of the world as seen through our eyes.   Because our eyes are spaced a distance apart from each other on the relatively flat surface of our face, each of them will perceive an object along a different line of sight and thus offer differing perspectives.   Our brain makes use of the parallax phenomenon to automatically process these differing perspectives into a single image, along with providing us with a three dimensional view of the object, something that comes in handy when negotiating our physical world.

As an example, let’s say you’re sitting at the kitchen table staring at a cup of coffee.   Your left eye sees the cup from one perspective, your right eye another.  To prove it, cover each of your eyes in turn as you look at it.   You’ll see that the perspective changes between your two eyes.   These two perspectives are processed by your brain into an internal calculation which enables you to adjust your muscle response accordingly.   The result is that you are able to deftly pick up the cup of coffee and bring it to your lips rather than clumsily toss it to the floor.

Parallax is at work when viewing objects both near and far.   Consider Figure 1.

 Suppose we want to measure the distance from Point A to a tree several blocks away.   That’s too far away to use a tape measure, but we can employ the parallax phenomenon and trigonometry, the study of angles, to easily determine the distance.