The summer solstice occurs exactly when the Earth's axial tilt is most inclined towards the sun at its maximum of 23° 26'. Though the summer solstice is an instant in time, the term is also colloquially used like Midsummer to refer to the day on which it occurs.The summer solstice occurs is on June, 21st in the Northern Hemisphere and on December 21st in the Southern Hemisphere.
On the Summer Solstice we can reproduce Eratosthenes experiment to measure Earth's perimeter.
Eartosthenes was chief librarian of the Libarary of Alexandria and he read many documents and found out that at the Ancient Egyptian city of Swenet (known in Greek as Syene, and in the modern day as Aswan) that is located near the tropic of Cancer on June there is a well where on certain day of the year the sunlight goes down to the bottom of the well. He knew that in Alexandria there was no day that the great Obelisk did not produce shadow and he measured the shadow angle on the day the Sun was directly above the well in Aswan. He needed to know the distance from the well in Aswan to Alexandria and there several different versions of how he found out its value. The most popular one is that he send a slave to measure it in footsteps. The value that he used in his calculations was 8000 stadia (1 egiptian stadium is about 157.5 m, though the exact size of the stadium is often a theme of discussion).
With this information he measured the circumference of the Earth without leaving Egypt.
Tomorrow teachers from all over the world will make measurements allowing them to reproduce this historical experiment. Learn more about this on the website of the project.