Equinoxes, solstices and zero shadow days

The changes of seasons are astronomical events when the sun is directly above:

• The Equator on march and september equinoxes
• The Tropic of Cancer on june solstice
• The Tropic of Capricorn on december solstice.

Between the 2 tropics, there are special days, special places , when and where the sun casts no shadow at noon.

On those zero shadow days, in Medellín on April 05th, Hyderabad on May 09th and Aug 03rd , Guarne on Sept 06ththe observations are the same as those of the first Eratosthenes experiment between Alexandria and Aswan, 2200 years ago.