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By Pythagoras the square of the distance to the horizon on a planet of radius r from an altitude of h is:

And on Earth (radius = R) the horizon is twice as far away as it is on Rizon (radius = r). So the square of the distance is 4 times as much:

Unless Hurr is hundreds of miles tall (which would make mingling with the local population difficult) the R/4 term will be hugely greater than the 3h/8 term.

We are told the Earth has a radius of 3960 miles, so the radius of Rizon is 990 miles (less ⅜ of Hurr’s eye height).

So, to the nearest 100 miles, Rizon has a radius of 1000 miles.

Solution:The radius of Rizon is approximately 1000 miles.