Parallax

An example of the parallax of an object against a distant background due to a change in location. When viewed from "Viewpoint A", the object appears to be in front of the blue square. When the viewpoint is changed to "Viewpoint B", the object appears to have moved in front of the red square.

Parallax is the perceived change in position of an object seen from two different places.

In astronomy, annual parallax is the only direct way to measure distance to stars outside the solar system. In essence, parallax is the perceived shifting phenomenon which occurs when an object is viewed from different positions.

It is measured by the angle between two lines of observation. Nearby objects have a larger parallax than more distant objects when observed from identical positions, so parallax can be used to determine distances.

Astronomers use the principle of parallax to measure distances to celestial objects including to the Moon, the Sun, and to stars beyond the Solar System. Here, the term "parallax" is the angle between two sight-lines to the star.

Astronomical measurement of position are taken at different times of the year. Since the Earth's orbit is known exactly, the distance from position 1 to position 2 can be worked out. The angle from the horizon to the object can be measured precisely. This gives a triangle whose baseline and angles are known accurately. From the triangle, the distance is calculated by trigonometry and expressed in parsecs.

The method only fails with objects which are so distant that the earth's orbit is too small to get a large enough parallax angle to measure accurately. This distance is about 100 light-years. Astronomers have invented various ways to solve this problem, though none are so accurate as the parallax method is for relatively nearby objects. This provides a basis for the cosmic distance ladder of techniques to calculate greater distances.

From 1989 to 1993 the Hipparcos satellite took measurements for over 100,000 nearby stars. Gaia (spacecraft) is intended to make similar measurements of about a Billion stars.

Many animals, including humans, have two eyes which provide depth perception; this is called stereopsis. Because the two eyes are at different places on the head, this gives the basis for an automatic sense of distance. We perceive this as a normal 3-D scene.

Parallax Media

• 

  A simplified illustration of the parallax of an object against a distant background due to a perspective shift. When viewed from "Viewpoint A", the object appears to be in front of the blue square. When the viewpoint is changed to "Viewpoint B", the object appears to have moved in front of the red square.

• 

  This animation is an example of parallax. As the viewpoint moves side to side, the objects in the distance appear to move more slowly than the objects close to the camera. In this case, the white cube in front appears to move faster than the green cube in the middle of the far background.

• 

  In this photograph, the Sun is visible above the top of the streetlight. In the reflection on the water, the Sun appears in line with the streetlight because the virtual image is formed from a different viewing position.

• 

  Parallax theory for finding naval distances

• 

  The correct line of sight needs to be used to avoid parallax error.

• 

  Simple animation demonstrating the effects of parallax compensation in telescopic sights, as the eye moves relative to the sight.

• 

  Contax III rangefinder camera with macro photography setting. Because the viewfinder is on top of the lens and near the subject, goggles are fitted in front of the rangefinder and a dedicated viewfinder is installed to compensate for parallax.

• 

  Failed panoramic image due to the parallax, since the axis of rotation of the tripod is not the same as the focal point.

Other websites

• Parallax Citizendium