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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⊕.
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.