# How have we measured the radius of the Earth (old and new)?

I'd imagine ancient methods are fun exercises in geometry. how has the measurement has evolved over time?

The first method is credited to Eratosthenes. He knew that during the summer solstice in Syenne the angle of the Sun overhead at noon was $0^\circ$. Following this he made the same measurement in Alexandria and measured the angle of the Sun overhead using shadows and calculated it to be $\approx 7^\circ$ which is approximately $1/50$ of a circle. Hence, both the circumference and the radius are easily calculated and quite accurate for such crude methods.

If you really want to break this down into a simple number, the WGS model, at the crudest level, considers the Earth to be an oblate spheroid with an equitorial radius of $R_{eq} = 6\:378\:137\:\mathrm{m}$ and a polar radius of $R_{p} = 6\:356\:752.3\:\mathrm{m}$.