How do we measure the radius of earth back in the 15th century?
The previous answerer gave one method that was actually used in antiquity and gave a very good answer. Other methods involved looking at the horizon from the top of a mountain and trying to measure the angle down to the horizon from a level. That can be used to figure out the size of the earth, but the angle tends to be too small to give much accuracy. Many scholars in the 15th century, though relied on this figure more than the one done by the stick method (which was much more accurate). This is part of why they thought the earth was so much smaller than it really is.
