Why is my personal procedure for calculating the radius of the earth incorrect? Details are below.?

Suppose that, while lying on a beach near the equator watching the sun set over a calm ocean, you start a stopwatch just as the top of the sun disappears. You then stand, elevating your eyes by a height "H=1.70m" , and stop the watch when the top of the sun again disappears. If the elapsed time is "t=11.1s" , what is the radius of the earth? Answer- 5.2E6m. My question is why did the formula "Omega(W)=velocity(V)/radius(R)" failed?