Dolmen of Sorginetxe, Basque Country, Álava, Spain

Pi in the sky. Celebrating Pi Day

Calculating the ratio of a circle's circumference to its diameter is as easy as pi. Today is Pi Day, an event that celebrates this mathematical constant. This date was chosen because 3, 1, and 4 are the first three significant figures of pi (π) when it is written out. Pi is an irrational number, which means that it can't be written as a fraction. When written out, its decimal representation goes on infinitely without repeating. The oldest attempts to calculate pi date back to ancient Babylon and Egypt, around 1900 BCE.

Pi Day, however, has more recent origins. In the 1980s, physicist Larry Shaw organized the first celebration, where staff at a San Francisco science museum walked around in circles and then ate the number's delicious homophones: pies. Today's image takes us to Álava, northern Spain. The circular motion of the stars appears above the Dolmen of Sorginetxe, an ancient megalithic monument dating back to 3000 BCE. Pi has long played a key role in the study of the stars, and some of the most accurate early approximations of the number were calculated by Indian and Persian astronomers.