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About these units
Exameter (Em)
An exameter, equal to 10¹⁸ meters, reaches into the interstellar and even intergalactic scale. It is useful for describing the dimensions of star clusters, distances between major astronomical features, or large-scale cosmological structures. Only a handful of physical phenomena require expression in exameters, and even then, astronomers typically prefer parsecs or light-years. The unit sees more use in theoretical physics, cosmology, and data modeling where SI uniformity is necessary. For example, mathematical simulations of the universe's evolution may employ exameters in their internal computations. Although too large for everyday use, the exameter highlights the vastness of the universe and the adaptability of the metric system to describe phenomena across incomprehensible scales.
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.