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An zeptometer is a unit of length, a combination of the metric prefix zepto (symbol z) and the SI unit of length meter (symbol m), equal to 10−21 meter or 1.0E-21 meter and its symbol is zm. Plural name is zeptometers.
Earth radius is the distance from Earth's center to its surface, about 6,371 km (3,959 mi). This length is also used as a unit of distance, especially in astronomy and geology, where it is usually denoted by R⊕.