Physics: Principles with Applications (7th Edition)

Circumference $4\times10^7 m$ Radius $6.37 \times 10^{6} m$
The meter was originally defined so that the distance from the Earth’s equator to either pole was 10 million meters. Assuming a spherical Earth, the circumference would be 4 times that, or 40 million meters. Use the circumference to find the radius of $r = \frac{40,000,000 m}{2 \pi} = 6.37 \times 10^{6} m$.