Today, June 21, is traditionally the date of the summer solstice. On this day, at noon, at a certain latitude, a stick standing upright will cast no shadow at all–but at the same moment, at a different latitude, it will.
Thousands of years ago a man named Eratosthenes realized this, and decided he could use the discrepancy to measure how big the Earth was. The above video, a clip from the amazing Cosmos TV series by Carl Sagan, shows how he did it.
I thought this was very appropriate for today as we celebrate the passage of the Northern Hemisphere into summer and the Southern Hemisphere into winter. The ancients were very smart, and the clip is great!