The longest baseline we have for parallax is the distance between the Earth at one point in its orbit and the Earth six months later. This distance is 2 AU. If we measure the parallax angle in units of arc-seconds ( 1 arc-second = 1/3600 th of a degree) then we can use the simples equation from the data booklet to calculate the distance of that object in parsecs. One parsec is therefore equal to the distance at which the parallax angle is one arc-second. This is about 3.26 light-years.

Limitations of the parallax method

An arc-second is not a large angle, and the closest stars outside our solar system have a parallax of slight less than one arc-second (our nearest extra-solar star, Proxima Centauri, has a parallax angle of about 0.7687 ± 0.0003 arcsec. More distant objects have much smaller parallax angles. Measuring very small angles on Earth is difficult due to atmospheric effects. The best current measurements are taken by the European Space Agency's Gaia mission. This can measure parallax angles as small as 10 micro arc-seconds, equivalent to a distance of 10 000 parsecs. This is the distance limit of the parallax method using current technology.