What is a Parsec?

A parsec is an astronomical unit of measurement that is equivalent to 3.26 light years distance, or the distance photons will travel in vacuum over the period of 3.26 years. Light travels at an approximate speed of 186,000 miles per second (300,000 kilometers per second), so this distance is just over 19 trillion miles (about 31 trillion kilometers).

By comparison, the average distance to the Sun from Earth is only 93 million miles (150,000,000 km). This distance is referred to as 1 astronomical unit (AU). A person would have to make 103,000 round trips to the Sun to cover the distance indicated by a single parsec. Earth's solar system, defined for example by Pluto's orbit, is only 1/800ths of a light year across. It would have to be 2,608 times larger to equal 1 parsec across.

This distance is calculated using the parallax of 1 arc second, leading to the shorter term, parsec. To understand what this means, it will be helpful to define the terms parallax and arc second.

In a spherical plane or a simple circle bisected evenly by 180 lines that form 360 equal sections, the distance between two adjacent lines equals 1° of arc. All arcs added together equal 360° or the entire circle. If each degree of arc is bisected further into 60 more equal sections, each of these sections equals 1 arc minute, so 60 arc minutes equals 1° of arc. Each arc minute can be divided into 60 more equal sections, representing arc seconds. An arc second is therefore an angular measurement that equals 1/60th of an arc minute, or 1/3600 of a single degree of arc.

Parallax refers to the apparent motion of a fixed object along an angular trajectory due to a change in the observer's position. For example, if a person uses one eye to gaze at a computer monitor and then switches eyes, the monitor will seem to "jump" horizontally in reference to the background. Scientists make use of parallax to measure the distance to stars.

To achieve the parallax effect, an object is photographed against background stars from a fixed position on Earth. Six months later, when the Earth has traveled halfway around its orbit at a relative distance of 186 million miles (2 AU) from the first position, a second photograph is taken. By measuring the distance the object "jumped," scientists can calculate the arc seconds of the parallax to reveal the distance. (As an aside, a third photograph is taken in one full year from the original position to calculate and subtract any effects from natural seasonal shift.) If a star generated 1 parallax arc second annually, scientists would know the distance to that star is 1 parsec, though no stars lie neatly at this distance.

The further the object, the less parallax it has, while the closer the object, the more parallax. This means that distance is inversely proportional to parallax: an object with a parallax of 0.5 arcsecond would be twice the distance of an object with 1 arcsecond of parallax. Conversely, if a star were close enough to have 2 arcseconds of parallax, it would be twice as close as an object with 1 arcsecond of parallax.

In reality, there are no stars located so near to Earth, aside from the Sun. Parallax is therefore measured in fractional increments corresponding with greater distances. Scientists also use milliarcseconds (mas), or 1/1000 of an arcsecond to indicate parallax in whole numbers. For example, the Sirius system lay at a distance of about 2.6 parsecs, (0.37921 arcsecond), or 379.21 mas.

Parsecs are more convenient to indicate astronomical distances than light years. One thousand of them is known as a kiloparsec, or kpc, while a megaparsec is equal to 1 million, abbreviated as Mpc. A trip from Earth to the center of the Milky Way Galaxy would be a lengthy trip at just over 8.5 kpc.

Although the units kpc and Mpc come in handy, to actually measure very distant stars of more than 100 parsecs or over 400 light years away, parallax is no longer viable. In that case, scientists use other methods involving the calculation of brightness, sometimes referred to as spectroscopic parallax.