Measuring the radius of the Earth at the beach

We begin by lying down on the ground.  When the sun hits the horizon, we start the stopwatch and stand up, making the sun appear above the horizon again.  When it hits a second time, we stop measuring.  Among our measurements, all were around 10^1 seconds.  Using this value, we calculate the angle to be 7.2e-4 radians.  Setting our hight at 6 feet, we calculate the radius of the Earth to be 2.3e7 feet, or about 6900km.  The actual radius of the Earth is about 6400km.  Error was likely due to human reaction time, as well as the atmospheric effects that made it difficult to see when the sun hit the horizon.