Happy Solstice!

A shadow is a region where light from a light source is obstructed by an opaque object. It occupies all of the three-dimensional volume behind an object with light in front of it. The cross section of a shadow is a two-dimensional silhouette, or reverse projection of the object blocking the light.
-https://en.wikipedia.org/wiki/Shadow

A gnomon ([ˈnoʊmɒn], from Greek γνώμων, gnōmōn, literally: “one that knows or examines”) is the part of a sundial that casts the shadow. The term has come to be used for a variety of purposes in mathematics and other fields.
-https://en.wikipedia.org/wiki/Gnomon

A solstice is an astronomical event that occurs twice each year as the Sun reaches its highest or lowest excursion relative to the celestial equator on the celestial sphere. The solstices and the equinoxes are connected with the seasons. In many cultures the solstice marks either the beginning or the midpoint of winter and summer.
-https://en.wikipedia.org/wiki/Solstice

Eratosthenes calculated the circumference of the Earth without leaving Egypt. Eratosthenes knew that at local noon on the summer solstice in the Ancient Egyptian city of Swenet (known in ancient Greek as Syene, and now as Aswan) on the Tropic of Cancer, the Sun would appear at the zenith, directly overhead. He knew this because he had been told that the shadow of someone looking down a deep well in Syene would block the reflection of the Sun at noon off the water at the bottom of the well. Using a gnomon, he measured the Sun’s angle of elevation at noon on the solstice in Alexandria, and found it to be 1/50th of a circle (7°12′) south of the zenith. He may have used a compass to measure the angle of the shadow cast by the Sun. Assuming that the Earth was spherical (360°), and that Alexandria was due north of Syene, he concluded that the meridian arc distance from Alexandria to Syene must therefore be 1/50th of a circle’s circumference, or 7°12’/360°.
-https://en.wikipedia.org/wiki/Eratosthenes

http://www.youtube.com/watch?v=sSJwohj-v_w