Yes, basic math.
If a circle is 1 in diameter (a line through the circle) the circumference is Pi : 3,14... half the circumference would be from pole to pole (where the diameter line is drawn) Pi/2 or Pi rad.
Problem though - the flat earthers doesn't have a south pole. Their Antarctica circumnavigates the earth, so from their point of view the distance from the North Pole to the south pole is one radiusian. Using our model from above the distance between pole to "pole" is 0,5 ... so you can measure this in real life by travelling from our pole (North of Spitsbergen) and go directly south over Europe, Africa and end up at the ice cap of the south. This distance should be multiplied by 2 and then you plant a flag and start walking along the ice cap and for reference you should be able to walk a distance which is approximately 2Pi rad where rad would be the distance from the Arctic to the ice cap of Antarctica.
Say the distance measured was around 10 000 km. To account for the whole circumference you need this formula 2Pi x 10 000 km = 62 831,85 km
So you shouldn't encounter your planted flag until you have walked (traveled) 62k+ km. certainly not only after about 18k km
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