Tumblr User Explains Why Elves’ Eyes In Lord Of The Rings Shouldn’t Look The Way They Do, but in fact there is no problem.
In The Two Towers Legolas claims to clearly see the horsemen of Rohan at a distance of 5 leagues. How is this possible when you consider the curvature of the Earth? You can come up with some bizarre ideas about Elven anatomy which are not suggested in the books or seen in the movies. However, there is a an elegant answer based on the nature of Arda, Tolkien’s world. It was flat, not round, until the end of the second age. At that time the Valar made it round so that mortals could not access Valinor. But Elves could still go there. For them the Earth was still flat and they could sail the straight path to Valinor. Hence for Legolas the curvature of the Earth and the horizon did not exist and hence he could accurately see the horsemen 5 leagues away.