Parallax

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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 half-angle of inclination between those two lines. Due to foreshortening, nearby objects show a larger parallax than farther objects, so parallax can be used to determine distances. To measure large distances, such as the distance of a planet or a star from Earth, astronomers use the principle of parallax. Here, the term parallax is the semi-angle of inclination between two sight-lines to the star, as observed when 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"; they are the first in a succession of methods by which astronomers determine the distances to celestial objects and serve as a basis for other distance measurements in astronomy, which form the higher rungs of the ladder. Because parallax is weak if the triangle formed with an object under observation...

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