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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⊕.
The yoctometre (International spelling as used by the International Bureau of Weights and Measures; SI symbol: ym) or yoctometer (American spelling) is an SI measure of length that is equal to 10−24 (yocto) of a metre. It is the smallest named subdivision of the metre in the SI base unit of length.