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Stellar parallax is the apparent shift of position of any nearby star (or other object) against the background of distant stars. By extension, it is a method for determining the distance to the star through trigonometry, the stellar parallax method .
A parsec is the distance from the Sun to an astronomical object that has a parallax angle of one arcsecond (not to scale). The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to 3.26 light-years or 206,265 astronomical units (AU), i.e. 30.9 trillion kilometres (19.2 trillion miles).
Stellar parallax motion from annual parallax. Half the apex angle is the parallax angle. Parallax is an angle subtended by two lines crossing a point. In the upper diagram, the Earth (blue-filled circle) in its orbit sweeps the parallax angle subtended on the Sun (yellow-filled circle).
This number is wrong; originally announced in 1891, the figure was corrected in 1910 to 40 ly (60 mas). From 1891 to 1910, it had been thought this was the star with the smallest known parallax, hence the most distant star whose distance was known. Prior to 1891, Arcturus had previously been recorded of having a parallax of 127 mas.
The lower diagram shows the equal angle swept by the Sun in a geostatic model. A similar diagram can be drawn for a star except that the angle of parallax would be minuscule. The most important fundamental distance measurements in astronomy come from trigonometric parallax, as applied in the stellar parallax method. As the Earth orbits the Sun ...
Aberration is distinct from parallax, which is a change in the apparent position of a relatively nearby object, as measured by a moving observer, relative to more distant objects that define a reference frame. The amount of parallax depends on the distance of the object from the observer, whereas aberration does not.
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The measurement of stellar parallax of nearby stars provides a fundamental baseline in the cosmic distance ladder that is used to measure the scale of the Universe. Parallax measurements of nearby stars provide an absolute baseline for the properties of more distant stars, as their properties can be compared.