Did you ever ask yourself why we have two eyes? Try the following experiment. Hold some object with your hand (a pencil or a stick), extend your arm in front of you and look at it, now with the left, then the right eye. See how the object you hold projects against different parts of the distant background as you blink. The size of that apparent displacement, technically known as "parallax", depends on how far you keep the object from your eyes: the displacement is larger if the object is close, smaller if you keep your arm fully extended. Parallax could not be measured if we had only one eye.
Our brain estimates distances to nearby objects by automatically carrying out this experiment for objects in our vicinity. It is so used to doing it, that we don't need to blink and we are not even aware that the associated computations are being made.
The same principle helps astronomers.
As the Earth revolves around the Sun, our perspective of the heavens changes just so slightly. If you look at a given nearby star twice, six months apart, it's as if you blinked with a pair of eyes separated from each other by the diameter of the Earth's orbit around the Sun: the apparent displacement, or parallax, of the star against the background of more distant objects (which is very tiny and imperceptible by the naked eye) tells astronomers how far away the star is.
Ps. Its also used in 3D movies like Avatar to make us go wow!
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