A parsec (pc) is a unit of measurement used in astronomy to describe astronomical distances, particularly on cosmic scales. The term "parsec" is actually a contraction of "parallax of one arcsecond," which refers to the method by which this unit of distance is defined. A parsec is defined as the distance at which an object will appear to shift its position by one arcsecond (1/3,600th of a degree) as observed from Earth when the Earth is on opposite sides of its orbit (at an average distance of one astronomical unit, or about 93 million miles) from the Sun.
In more practical terms, a parsec is approximately equal to 3.09 × 10^16 meters or 3.09 × 10^13 kilometers.
To provide a sense of scale, some common astronomical distances expressed in parsecs include:
Parsecs are especially useful for describing distances between stars and galaxies, as astronomical distances can be incredibly vast, and using kilometers or even light-years can lead to unwieldy numbers.
The "archin" is an obsolete unit of length that was historically used in Russia and other parts of the former Soviet Union. The length of an archin varied by region and over time, but it was typically around 28 inches or approximately 71 centimeters. In some regions, an archin could be slightly shorter or longer.
The archin was used for measuring cloth, textiles, and sometimes land. Like many traditional units of measurement, it has largely been replaced by more standardized systems, such as the metric system, in modern times. While the archin is no longer commonly used for official measurements, it may still be encountered in historical or traditional contexts in Russia and other former Soviet states.