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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.
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.