Consider a rod placed vertically into the ground with exactly one meter above the ground sticking up. assume you are someplace like bonneville, ut (famous for auto racing) where the earth will be assumed perfectly level and smooth without hills or valleys or trees to worry about. how far must the stick be from you so that it becomes invisible from a vantage point of being on the ground (face or eyeball at ground level as if you're an ant)? consider the curvature of the earth in this case. earth's radius is 6.37 * 10^6 m. do the calculation again for an eyeball height of 1.5m above the ground.