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What is a Parsec?

By R. Kayne
Updated May 21, 2024
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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.

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Discussion Comments
By anon264434 — On Apr 27, 2012

The answer for vctrphsr is a parsec is approx 31 trillion km as stated in the article and for your comment about relative vector: no.

By anon123076 — On Oct 30, 2010

@ anon29051: You have also mistakenly written the definition.

A parsec is the distance at which one astronomical unit (roughly, the radius, not the diameter) of the Earth's orbit, subtends an angle of 1 arc second.

By anon76946 — On Apr 12, 2010

@ anon16328. LOL! Parsec was way ahead of it's time. TI-99/4A

I always wondered what a parsec was! Thanks, very interesting!

By vctrphsr — On Jun 25, 2009

I read a book at one time, about 20 years ago, which presented a figure of 63,000 miles as the distance of a parsec.(at least I think that was the statement; it was 63 followed by zeros). Is the parsec a relative vector which is adapted to a conventional parameter?

By anon29051 — On Mar 26, 2009

A comment on Hank's definition of parsec:

A parsec is the distance at which the diameter of the Earth's orbit, (not of the Earth itself as Hank mistakenly writes) subtends an angle of 1 arc second.

By Hank — On Mar 24, 2009

When I was a freshman, we were required in physics to determine the value of a parsec. The definition of a parsec was: "That distance at which the diameter of the Earth subtends one second of arc"

Yes, indeedy, after all the mathematical fooling around to prove that we really knew trigonometry, it came out to exactly 3.26 light years.

Wonderful stuff, mathematics-I even still remember some of it after more than fifty years since graduation.

By anon28886 — On Mar 24, 2009

Parsec was a very cool video game that I played long, long ago on my TI 94A computer.

By anon16328 — On Aug 03, 2008

Excellent info. Thanks guys.

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