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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.
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.
- Shorter Oxford English Dictionary. (1968). “Mutual inclination of two lines meeting in an angle”
- "Parallax". Oxford English Dictionary (Second Edition). (1989). “Astron. Apparent displacement, or difference in the apparent position, of an object, caused by actual change (or difference) of position of the point of observation; spec. the angular amount of such displacement or difference of position, being the angle contained between the two straight lines drawn to the object from the two different points of view, and constituting a measure of the distance of the object.”
- Parallax Citizendium