On the Earth's spherocity....

So how do we know that the Earth is "spherical?"

http://howdoweknowthat.blogspot.com/2009/07/how-do-we-know-that-earth-is-spherical.html

And for how we measured its size.....

http://howdoweknowthat.blogspot.com/2009/07/so-how-big-is-earth.html

And for how we got the distances to lots of things we can't visit....

http://howdoweknowthat.blogspot.com/2009/07/how-far-away-is-that.html