The disproofs of a round earth are so plentiful and readily available that we can show its absurdity with ease at the beck and call of any globularist - just don't expect such a man (or woman!) to accept their defeat but instead you will be privy to the greatest show of mental acrobatics this side of the plane. I am sure we will see some such acrobats visit this very thread.
Procure the following items, and keep them securely in a map-case should the need arise to dumbfound those whose ideas are founded in dumbness. The rational man will have to reject any round earth slumgullion immediately upon seeing the results.
- A candle.
- A ball of twine, 1320 feet. This should cost approximately 61 dollars.
- Two good sized, sturdy sticks approximately half a meter in length.
- A box of matches
In preparation, take out your pocket knife - which any good field experimenter should have readily on hand at all times - and make notches at equal heights on both wooden sticks. Do the same on both ends to allow yourself the ability to plant these on a level surface at an even height, accounting for both where the string will be tied as well as the amount of stick that will be thrust into our flat earth - preferably with gusto. The top line should be at such a height that it extends past the bottom line plus two inches - including the height of an average flame from said candle.
Next, find the center of the twine, and mark it appropriately with a permanent black marker. This will let you know at what point your candle should lie beneath the twine.
Now you are prepared. When questioned about the perceived absurdity of a flat earth, smile your largest grin while opening your map-case. Procure the sticks and plunge one of them into the ground at the prescribed height.
Be ready for confusion at this point, but take no note of it. The round earther is religious beast and is not often accustomed to seeing real science at work. He may mistake the entire ordeal for a ritual and in these cases you will be unable to convince him or her that they are in actuality wankers.
Next, tie one end of the string to this pole. Walk until the string is taught, to the point that the string is level and the stick sturdy. Plunge the second stick into the ground here, and fasten the other end of twine to the pre-marked location on your rod. Travel back your course, and place the candle underneath the pre-marked black line.
Now, light the candle while explaining that should the earth indeed have such a curvature - the candles flame would be touching said black line. Unfortunately for those globularist, the flame will not touch the twine, showing the predicted drop in curvature of two inches is not observed. If necessary, repeat this experiment a number of times and localities to rule out local variances skewing the results.
As a one-two-punch you can then note that the shadows are at the same angle on these sticks - showing that the charlatan Eratosthenes was a fool.
At this point yell in triumph: "Sockdolager!" for the matter has been suitably settled. The earth is not some whirlidirly ball dancing about the heavens in a celestial race - no it is flat as a cupboard shelf.