### February 7, 2013 - “e” Day

Is it irrational to celebrate a number today?

Like “Pi Day” (March 14), “e Day” celebrates one of the most famous and useful irrational numbers. “Irrational” means “not rational,” and in math it refers to numbers that cannot be expressed as simple fractions, or ratios.

The irrational number e starts out 2.7182818284590452353602874713527...but it goes on and on without stopping or repeating a pattern. Because the first two numbers are 2 and 7, we celebrate e on 2/7, or February 7.

I bet you're wondering how on earth such a cumbersome number can be famous and useful! Well, this number is the base of the natural logarithms, and it is also used in compounding interest, which is how your bank account or other investment can increase. More money = useful!

The constant e also shows up in such places as graphs of growth or decay, the statistical bell curve, and the shape of a hanging cable or a standing arch.

(By the way, the logarithm of a number is the exponent by which the base must be raised to produce that number. Logarithms are useful in science, engineering, calculus, and computer science.)

Also on this date: