Hold out a pencil at arm's length so that it covers your view of a more distant object. Now close each eye in turn. The pencil seems to move relative to the distant object when a different eye is closed. Each eye looks at the pencil from a slightly different direction. With both eyes open you get more visual clues as to how far away any object is. The following diagram shows the situation:
Your brain instinctively determines the object's distance from the slight change in direction (measured by the angle - P). This method of measuring distances is called PARALLAX.
* Click here to find out how astronomers use parallax to measure the distances to the stars.
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