Between the 2 tropics, there are special days, special places , when and where the sun casts no shadow at noon.
Those conditions are the same as those of the first Eratosthenes experiment between Alexandria and Aswan, 2200 years ago.
Eratosthenes, at the time Director of the Great Library of Alexandria in Egypt, proposed a purely geometrical method to measure the length of the Earth’s meridian (circle passing through the poles).
He started by using the observation of shadows made at two different places, Alexandria and Syene (now Aswan) distanced approximately 800 km apart (distance estimated in relation to the time taken by a caravan of camels to connect the two towns !) at the time of the Summer solstice and at noon local solar time.
2012-2017 previous nO NooN shadow days on Eratosthenes 2017 Twinspace
2018 nO NooN shadow days on Eratosthenes 2018 Twinspace
2019 nO NooN shadow days on Eratosthenes 2019 Twinspace