From Stargazers to Starships: The Round Earth and Christopher Columbus
The Flat Earth Society could learn a lot by reading about the many calculations made over several centuries determining the spherical size of the earth. Early Greek philosophers and mathematicians proposed different ways of calculating the circumference. Christopher Columbus was surely aware of the the conjectures and used the information in his presentation to Isabella and Ferdinand to get funding for his soon-to-be historic voyage. This site links to two lesson plans, one which explores the concept of the horizon, and the other which teaches students how to derive the formula for the distance of the horizon from a given height. What a great way to apply math to history!
http://www-spof.gsfc.nasa.gov/stargaze/Scolumb.htm