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.
A "handbreadth" is a unit of length that is approximately equal to the width of an adult human hand, excluding the fingers. It is often used in various historical and cultural contexts as a convenient way to estimate small lengths or distances. The actual measurement of a handbreadth can vary somewhat depending on the size of an individual's hand, but it is typically around 3 to 4 inches (about 7.6 to 10.2 centimeters).
In some historical systems of measurement, particularly in the ancient Near East and the Bible, the handbreadth was used as a standardized unit for measuring length. It was one of the smaller units within a larger system of measurement, often used for small-scale applications like measuring cloth, the dimensions of objects, or architectural details.
While the handbreadth is not a commonly used unit of measurement in modern times, it remains a useful and intuitive way to approximate small lengths informally, especially when a quick estimate is needed.