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About these units
Inch (in)
An inch is defined as exactly 25.4 millimeters, a precise metric-based definition that preserves its usefulness within imperial systems. Historically, the inch was based on the width of three barleycorns placed end-to-end, a charming relic of medieval measurement practices. Today, the inch is vital in manufacturing, woodworking, consumer electronics (e.g., screen sizes), and tooling standards across the US and partially in the UK. Its size is small enough to offer usable precision yet large enough to avoid unwieldy fractions for many everyday objects. Even in predominantly metric industries, certain products—such as plumbing parts, bicycle rims, and camera mounts—retain inch-based standards for compatibility. This persistence shows how technological ecosystems can outlive their measurement origins.
Parsec (pc)
A parsec is equal to 3.26 light years, or approximately 3.0857 × 10¹⁶ meters. It is defined based on the method of stellar parallax, the apparent shift in a star's position caused by Earth's orbit around the Sun. Specifically, a star at a distance of one parsec exhibits a parallax angle of one arcsecond (1/3600 of a degree). Because its definition emerges directly from geometric measurement techniques, the parsec became the standard astronomical unit for professional research. Parallax-based distances are foundational to the cosmic distance ladder — the sequence of methods by which astronomers measure distances from nearby stars to the farthest galaxies. The parsec is widely used in astrophysics because calculations involving gravitational dynamics, luminosity, or galactic structure often become more intuitive in parsecs than in light years. Although less familiar to the public, it is the preferred unit in scientific publications, planetary catalogs, and distance mapping of the Milky Way.