Parallax

Parallax is the optical illusion that two stationary points change their position relative to each other, due to the motion of an observer. By observing parallax, measuring angles, and using geometry; one can determine the distance to various objects. When this is in reference to stars, the effect is known as stellar parallax. The first measurements of star parallax were made by Bessel, in 1838.

The two points in question will be different distances from the observer and the illusion of parallax is caused by the fact that light follows straight lines. When the observer views the nearer point, the line of his vision toward that point is at a given angle within the full arc of his vision. For example, let us say that the view straight ahead is zero degrees, and one point, nearer the observer, is at minus five degrees while a point which is farther away is at minus two degrees. The apparent angular distance between the points is a subjective three degrees to the viewer. If the viewer moves ten meters to his right, the angular direction to the nearer object, as it is on a shorter radius, will change more than the angular direction to the farther object. So, for instance, when the angular direction to the nearer object is at minus ten degrees, the father object may only have moved to minus three degrees. Now the subjective angular difference in position is seven degrees. The objects appear to have moved relative to each other.

On a macro scale, this effect is responsible for the fact that, in a moving car, one can look at distant mountains and see them seem to move (retard) in position beneath a seemingly motionless moon. The moon is at such a distance that the subjective angular change in position relative to an earth-bound observer is extremely slight, even as many miles are covered. The mountains, much closer, exhibit a much greater apparent change in angular position.

Put differently and somewhat more generally, distant objects seem to move with the car. This can be explained as follows: all objects move backward relative to the car, and for nearby objects the speed of change in direction is what the observer considers the normal consequence of his own movement; however, for distant objects the backward change in direction is slow and much less obvious than the forward change in direction relative to nearby objects. It seems as if distant objects move parallel to the car with the same speed or a little slower.