To measure large distances, we employ an important method called the parallax method.
If we look at an object with one eye closed, its position seems to be moved a little from the original position.

eg.
If you look at a ball held in front of your eyes against a fixed reference point on a plain wall, with your left eye, closing your right eye and then with your right eye, closing your eye you observe that the position of the ball seems to change with respect to the fixed reference point on the wall. This is called parallax.

we may call the two eyes are two different viewpoints, since when used separately, they create different lines of sights.
 
Distance between these two viewpoints or points of observation is called basis.