Do you like pi? I’m not talking about fruit or custard in a round pastry shell like those made with apples, cherries, or lemon meringue.

I am talking about a mathematical constant that shows up almost everywhere in our understanding of the universe, symbolized by the Greek letter “Ï€”. This is pronounced “pi.” It is the ratio of the diameter of a circle to its circumference, or more plainly, the ratio of how tall a circle is to the distance traveled when it rolled exactly once around.

Every year, Pi Day is observed on 3/14, since 3.14 are the first three digits of pi. But twice today, 3/14/2015, we’ll get to see the clock hit the first 10 digits of pi — 3.141592653 — an occurrence that won’t happen again for another 100 years.