![]() These distances give a sense of the scales involved when discussing astronomical phenomena and underscore the importance of the parsec as a unit of measure in these contexts. For example, the Milky Way galaxy is about 30 kiloparsecs in diameter, and the Andromeda Galaxy, our nearest spiral galaxy neighbor, is located about 780 kiloparsecs away. Parsecs are particularly useful in astrophysics and cosmology when discussing distances within a galaxy or between galaxies. Larger Unitsįor even greater astronomical distances, multiples of the parsec are used, including the kiloparsec (kpc), the megaparsec (Mpc), and the gigaparsec (Gpc), which are, , and parsecs respectively. ![]() In practical terms, if we observe a nearby star from two points in Earth’s orbit six months apart, and that star appears to move by one arcsecond compared to more distant stars, then that star is said to be one parsec away. ![]() The term is a portmanteau of “parallax” and “arcsecond”, reflecting its definition. The term parsec comes from the method of parallax measurements used to determine the distances of nearby stars. These relationships can be represented by the equations: In terms of light-years, a commonly used unit in astronomy, one parsec is approximately equal to 3.26 light-years. Conversion to Other UnitsĪ parsec is approximately equal to 3.09 trillion kilometers, or 1.92 trillion miles. An astronomical unit is the average distance between the Earth and the Sun, approximately 149.6 million kilometers. If two objects are apart by some fraction of the sky, treating the sky as a sphere, then the angle is that fraction times 360 degrees or 2 2 radians. ![]() The sky can be divided up into 360 degrees or 2 2 radians. The parsec (pc) is defined as the distance at which one astronomical unit (AU) subtends an angle of one arcsecond. How astronomers measure the parallax angle and how it relates to an actual length are really two separate questions. ![]()
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