What is the simplest way to prove the Earth is round?
Simplest, you say? There are two that strike me as being simple to demonstrate. Luckily someone on the internet has already spent some time to help us here to make these easy to illustrate:
1. Shadows differ from place to place
Eratosthenes carried out this experiment to determine the circumference of the Earth, already assuming its spherical shape; incidentally, the proof of such being consequential of the procedure.
However, a demonstration can be achieved by a simple, local experiment (as opposed to having a party venture to a distant enough point):
Take a piece of card (A3, or so), attach two obelisks to the card by their bases and, with a light source, produce shadows - now, slowly bend the card so that it becomes convex (that is, the side with obelisks attached bulging out) and watch the effect.
2. You can see farther from higher
There are numerous other ways of demonstrating that the Earth is round, or curved, at least, from analysing the center of gravity to simply observing the other round objects that are visible in space; but I believe these illustrations to be the simplest to comprehend.
Images sourced from SmarterThanThat
The shadow of the Earth on the Moon during an eclipse and the way masts of ships are still visible when the hulls are out of sight are the classical reasons.
Another way is the triple-right triangle:
- You move in a straight line for a long enough distance
- Turn right 90° degrees, walk in that same direction for the same distance
- Turn again to the right 90° degrees and walk again the same distance
After this you'll end up at the starting point. This is not possible on a flat surface since you'd just be "drawing" an incomplete square.
Source: http://www.math.cornell.edu (add /~mec/tripleright.jpg to find the image)