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, the observations are the same as those of the first Eratosthenes experiment between Alexandria and Aswan, 2200 years ago.