The radius of the Earth = 8000/2 = 4000 miles
We can figure the distance to the horizon using a right triangle
The distance from the center of the Earth to any object above the Earth's surface = the hypotenuse = 4000 + h where h is the height of the object [in miles ] above the Earth's surface
And the radius of the Earth is equal to one leg of the right triangle
And the distance from the object to the horizon, D, is the other leg
So...we have this general formula to determine D
D = √ [ (4000 + h)^2 - 4000^2]
So....
(a) D = √ [ (4000 + 1)^2 - 4000^2 ] ≈ 89.4 miles
(b) D = √ [ (4000 + 6)^2 - 4000^2 ] ≈ 219.2 miles
(c) D = √ [ (4000 + 100)^2 - 4000^2 ] = 900 miles
