Circumference and Pi

$$\text{Carmen Sandiecho}$$

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A spy is using her satellite phone to call her headquarters on the exact opposite side of the Earth. As they are discussing her mission, they have her on speakerphone, and she hears her own echo.

The radius of the Earth is about 6 million meters, and the phone signal routes through a network of secret communications satellites which relay the signal via radio waves over a distance equal to exactly half the circumference of the Earth. The signal travels at the speed of light (300 million meters per second).