How to determine the circumference of the Earth?

How to determine the circumference of the Earth?
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The eighth- and ninth-grade students were able to show through their own measurements that the Earth is round, and with their resulting value of the Earth’s circumference, they are only 0.3% above the so-called “polar circumference” of the Earth of 40,009 km.
The method used for this purpose was developed more than 2000 years ago in Greece by Eratosthenes. Under the guidance of Mr. Weinert (geography teacher) and Mrs. Kraus (math and physics teacher of the classes) the young people of the CDSC took advantage of the fact that in Chiang Mai twice a year the sun is exactly vertically above us at noon – in the zenith.
So while they were sweating at the CDSC on May 16, 2022, setting up a perpendicular meter stick that for a brief moment cast no shadow at all, far away in Freiburg in southern Germany, Ms. Kraus’ father helped with a comparative measurement: his perpendicular stick formed a shadow even at the highest position of the sun that day. So the earth cannot be a flat disk!
With a few assumptions (the earth as a perfect sphere, the sun infinitely far away), with the north-south distance between the two places and with knowledge of trigonometry, our students were able to calculate the circumference of the Earth to 40,150 km – and then cool down contentedly in the shade of the school building: mission accomplished!