According to Yoder (1995), the radius of the earth is
6371.01 km. These functions are used to set the
default radis for functions that return a distance
or accept a distance as input
(e.g., s2_distance() and s2_dwithin()).
References
Yoder, C.F. 1995. "Astrometric and Geodetic Properties of Earth and the Solar System" in Global Earth Physics, A Handbook of Physical Constants, AGU Reference Shelf 1, American Geophysical Union, Table 2. doi:10.1029/RF001p0001