Parallax

Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines. The term is derived from the Greek word παράλλαξις (parallaxis), meaning "alternation". Due to foreshortening, nearby objects show a larger parallax than farther objects when observed from different positions, so parallax can be used to determine distances.

Astronomers use the principle of parallax to measure distances to the closer stars. Here, the term "parallax" is the semi-angle of inclination between two sight-lines to the star, as observed when the Earth is on opposite sides of the Sun in its orbit. These distances form the lowest rung of what is called "the cosmic distance ladder", the first in a succession of methods by which astronomers determine the distances to celestial objects, serving as a basis for other distance measurements in astronomy forming the higher rungs of the ladder.

Parallax also affects optical instruments such as rifle scopes, binoculars, microscopes, and twin-lens reflex cameras that view objects from slightly different angles. Many animals, including humans, have two eyes with overlapping visual fields that use parallax to gain depth perception; this process is known as stereopsis. In computer vision the effect is used for computer stereo vision, and there is a device called a parallax rangefinder that uses it to find range, and in some variations also altitude to a target.

Astronomical parallax

Astronomical parallax is used to measure the distances of nearby stars. The Earth's orbit around the Sun causes nearby stars to appear to move in small ellipses in the sky. The distance to a nearby star can be calculated by measuring the parallax angle and knowing the radius of the Earth's orbit.

Stellar parallax

Stellar parallax is so difficult to detect that its existence was the subject of much debate in astronomy for thousands of years. It was first observed in 1838 by the astronomer Friedrich Bessel.

Distance measurement

Parallax is used to measure distances to nearby stars in a method officially known as trigonometric parallax. The method uses the motion of the Earth around the Sun and the change in position of a star relative to more distant stars.

See also

• Aberration of light
• Depth perception
• Disparity
• Parallax scrolling
• Stereopsis

References

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