Earth flat as a pancake, page-57

I listened to a guy on the net giving his explanation to prove a globe earth. He said if the earth were flat then you could sail from Seattle to Hawaii using geometry, because geometry would work on a flat plane. But using geometry you would never reach Hawaii. Trigonometry is required to navigate on a curved surface.

My explanantion is more simple. The earths shadow on the moon is always curved. If the earth were flat, then at some point the shadow would be a straight line. The only shape that always throws a curved shadow is a sphere.