Pi is a mathematical constant that expresses the ratio of a circle's circumference to its diameter. It is part of many formulas used in physics, astronomy, engineering and other fields, dating back thousands of years to ancient Egypt, Babylon and China. In 2009, U.S. Congress designated every March 14 to be the big day — to hopefully spur more interest in math and science. Today, using sophisticated computers, researchers have come up with trillions of digits for pi