# What exactly is a parsec? A parsec is a unit of length in astronomy. It’s the equivalent of 30.9 trillion kilometres (19.2 trillion miles), or about 3.26 light years. The term was coined by British astronomer Herbert Hall Hoover in 1913, and it’s the distance from the Sun to an object (like a star) that has a parallax angle of one arc-second. The word ‘parsec’ is derived from ‘parallax’ and ‘arc-second’. Imagine a right-angle triangle in space. The angle opposite the right angle is one arc-second. An arc-second is one-60th of an arc-minute, which is one-60th of a degree in a 360-degree circle. The opposite side is one astronomical unit – the distance from Earth to the Sun. The adjacent side is a parsec long. Astronomers have long calculated the distance of stars by measuring their positions above Earth at two different points, six months apart. The difference in angle between the measurements is twice the parallax angle, an angle formed by drawing imaginary lines from the Sun and Earth that meet at the star. With this data, we can work out the distance of stars.