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About these units
Megameter (Mm)
The megameter, measuring one million meters (1,000 kilometers), is useful for describing large geographical or geophysical distances on planetary scales. Earth's radius (approximately 6.37 Mm) and the thickness of atmospheric layers can be expressed effectively in megameters. This unit provides a clean and compact way to represent massive distances without resorting to scientific notation. The megameter also appears in discussions of communication signals, satellite orbits, and especially planetary science. While kilometers are still more common in public-facing contexts, megameters offer a more mathematically elegant representation for large calculations. They provide a middle ground between human-scale geography and astronomical units.
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.