Parallaxes are measured in units of arc seconds, where one arc second equals 1/3600 degrees. On the sky, the image of the moon is about 0.5 degrees, or 1800 arcseconds, wide. If a star has a parallax of one arc second, it is stated to have a distance of one parsec. As all the nearest stars have parallaxes below to well below one arcsecond, it has become a custom to express the parallax in milliarcsecond (abbreviated as mas), or 0.001 arcsec. It takes light 3.26 years to travel one parsec, so distances are also sometimes expressed in light-years, In more conventional units, one parsec equals 30.857 trillion km. Similarly, the average distance between the Earth and the Sun is 149 million km, equivalent to 8 light-minutes.
The Hipparcos satellite mission made it possible to measure the parallax displacements with an accuracy of up to 0.1 mas, or 1 part in 2 billion. Gaia will do the same to an accuracy of 0.01 mas, one part in 20 billion. To put this in perspective, 0.1 mas is about the size of Neil Armstrong's footprint on the Moon as observed from the Earth, and 0.01 mas is the size of a golf ball at the same distance.
Page last updated: 23 December 2013