College students, rocket scientists and math enthusiasts around the world are celebrating Pi Day. The holiday is on Thursday, March 14 since 3/14 correlates to the first three digits of the infinite number. It’s also Albert Einstein’s birthday, making the pie eaten at Pi Day gatherings all the sweeter. Pi is a mathematical constant that expresses the ratio of a circle’s circumference to its diameter. It is part of many formulas useful in physics, astronomy, engineering and other fields, dating back thousands of years to ancient Egypt and Babylon.