Enigma 1318: Extended horizons
From New Scientist #2476, 4th December 2004
When Hurr was on her home planet Rizon she loved to stand at the edge of the sea and view the distant horizon. Her inbuilt range-calculator enabled her to work out how far she was from the horizon.
Now that Hurr has secretly landed on Earth and has mingled with the local population, she still slips down to the sea’s edge and views the horizon. But it turns out that on Earth the horizon is twice as far away from her as it was on Rizon.
The Earth’s radius is approximately 3960 miles. What, to the nearest hundred miles, is the radius of Rizon?