Quote Originally Posted by Lord Torath View Post
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.
Hm... Excluding having a perfect spherical planet, I wonder how well a buoy could work. I mean, it won't get me the perfect value but I wonder if it will work better than the shadows (if I don't have a camel to perfectly measure dozens of miles)

Also, any better way to get the size of other planets than knowing the distance and having a really good telescope?
I guess I can get their mass if they have visible moons with a bit of patience and clever math (well, most is clever math)