Originally Posted by
Lord Torath
The distance to horizon method is kind of tricky. You're right that you're going to need some very precise instruments (you need that for the distance to the sun, as well). You're also going to need a perfectly flat spherical bit of land to get an accurate measurement. What that generally means is a lake or sea, and it's very hard to measure distance across the surface of a liquid. You could measure the height of a tree just visible over the horizon across a lake. But you can run into air distortion effects in your line of sight, and the lake needs to be at least a couple of miles across. And you need to be able to tell exactly how far away that tree is, which can be tricky with primitive measuring tools.