  In the Tuscan Archipelago (Italy) it's possible to make an approximate
measurement of the Earth's radius from the Elba Island using the Pianosa Island
as a ship that appears and disappears on the horizon depending on the
observer's height above sea level. The necessary calculations require the
mathematical skills of the first two years of high school. The measurement
described here refers to a particular geographical location, but it can
probably be repeated in similar situations around the world.

Physics Education