Description: Parallax is the apparent change in position of a celestial object on the sky due to a change in the observer’s point of view. The position change in the sky, expressed as an angle, is determined relative to the most distant objects we know – historically, distant stars; in modern astronomy, extremely distant objects called quasars, which determine what is called the International Celestial Reference Frame. The angle of apparent position change is inversely proportional to the object's distance from us, making parallax measurements a powerful tool for determining distances in our cosmic neighborhood ("parallax method"). For Solar System objects, simultaneous observations from different locations on Earth can yield useful parallax values. For stars, parallax angles are given for a standard shift of observer position by one astronomical unit (the average Earth–Sun distance) at right angles to the line of sight. Observer position shifts of that magnitude can be achieved by making observations several months apart, with Earth moving along its orbit around the Sun in between. By definition, an object whose parallax angle under those conditions is 1 arcsecond is at a distance of 1 parsec (3.26 light years) from Earth. Over one year, the apparent position of a star in the sky traces out an ellipse, whose semi-major axis is the parallax angle. The most accurate stellar parallaxes to date are supplied by ESA's Gaia mission, a space telescope specifically designed for that task.

Related Terms:

• Annual Parallax

Term and definition status: This term and its definition have been approved by a research astronomer and a teacher

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