Parallax Definition - Astronomy 101

The term parallax refers to the change in position of an object when viewed from two different positions. A good example to help demonstrate this would be to hold a finger at arm's length, and watch as it jumps from right to left when viewed alternately with each eye. This jump from side to side is termed a parallactic shift.

In astronomy, nearer stars will show a parallactic shift against the backdrop of the more distant stars when they are viewed from opposite sides of the Earth's orbit. The change from the star's mean position is called its parallax. The amount of parallax is a measure of the star's distance, meaning that the nearer stars will show the largest parallax.

The inverse, or reciprocal of the star's parallax gives its distance in Parsecs. The first star to actually have its parallax measured was the "Flying Star" 61 Cygni, by Friedrich Bessel in 1838. Nearly simultaneously, the parallax of Alpha Centauri was measured by the Scottish astronomer Thomas Henderson at the Cape of Good Hope, as well as Vega being measured by Friedrich Struve.

The parallax of our nearest star Proxima Centauri, is 0.762 arc seconds. Astrometry concerns itself greatly with the measurements of these tiny parallax shifts. Surprisingly, even our own Sun, Moon, and planets reveal a parallactic shift when viewed from different positions on Earth. This method was the first one used to determine their distance from us.

When seen from opposite sides of the Earth, the Moon has a parallactic shift of almost 1°. The rotation of the Earth carries an observer from one side to the other each day. The daily shift that nearby objects show, is termed a diurnal parallax.

By: Andrew Duley

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