In this case we are dealing with the pythagorean theorm involving right angled triangles. This theorm states that a^2 + b^2 = c^2 which means the square of the hypotenuse (side c, opposite the right angle) is equal to the square of the remaining two sides.
In this case we will say that a = 3963 miles which is the radius of the earth. c is equal to the radius of the earth plus the additional altitude of the space station which is 250 miles; therefore, c = 4213 miles. We must now solve for the value b which is equal to how far an astronaut can see to the horizon.
