Pick two locations that are some distance apart (at least a couple hundred miles from each other and on the same meridian). Grab two sticks or dowels (or other objects) of equal length, two tape measures, and a friend. Each of you will take one stick/dowel/object and one tape measure to your location, stick the object into the ground, and measure the shadow. (For accuracy, you should both take your measurements at the same time of day.)
On a flat Earth, the shadow that is cast by each would be of the same length. However, if you and your friend compare notes, you'll find that one shadow was longer than the other. That's because, due to the curvature of Earth, the sun will hit one part of Earth at one angle and another part of Earth at a different angle even at the same time of day.
This experiment has been around since about 240 B.C., when Greek mathematician Eratosthenes compared the shadows cast in both Syeneโnow Aswan, Egyptโand Alexandria on the summer solstice. Eratosthenes had learned of a well in Syene where once a year on the summer solstice, the sun would illuminate the entire bottom of the well and tall buildings and other objects would not cast a shadow. However, he noticed that shadows were being cast on the summer solstice in Alexandria, so he measured the angle of the shadow and found it to be an angle of about 7.2ยฐ.
