A megaparsec (Mpc) is a unit of measurement used in astrophysics and cosmology to express vast distances on cosmic scales. It is equal to one million parsecs. A parsec (pc) is a unit of length used in astronomy to describe astronomical distances, and it is approximately equal to 3.09 × 10^16 meters or 3.09 × 10^13 kilometers.
So, a megaparsec is equivalent to:
1 megaparsec (Mpc) = 1,000,000 parsecs (pc)
Megaparsecs are typically used to describe distances between galaxies and galaxy clusters in the universe. Since astronomical distances are extremely large, the parsec and its multiples, such as the megaparsec, provide a convenient way to express these distances without dealing with unwieldy numbers of kilometers or light-years. Cosmologists often use megaparsecs when discussing the large-scale structure of the universe, the size of galaxy clusters, and the expansion of the cosmos.
A "barleycorn" is a small unit of length that was historically used for measuring small lengths or sizes of objects, particularly in some English-speaking countries. It is based on the length of an average grain of barley.
In the traditional English system of measurement, one barleycorn is equal to approximately 1/3 of an inch or about 0.8467 centimeters. This unit was used for measuring short lengths, such as the length of a person's foot for shoe sizing.
While the barleycorn is not commonly used in modern times for scientific or everyday measurements, it has historical significance, especially in the context of footwear sizing. Some traditional shoe sizing systems still reference barleycorns, where each size increase or decrease corresponds to the length of a barleycorn. However, most modern countries have adopted standardized shoe sizing systems based on centimeters or inches for more precise and consistent measurements.