# The (almost) universal constant pi

The constant pi, which we celebrated the other day, is far more versatile than is known to most scientists. For example, within 10 %,

π = 3 = e = √10

It can be shown that the equality is exact, given a small correction.

π is useful in physics as well as mathematics. The number of seconds in a year is π×107. The temperature of the earth is 100π K. The speed of light is 100,000π km/s. Any oπticist knows that Wien’s law can be expressed as

λmT = 1000π μm⋅K

Additionally, πies are called pies because they are circular. Philologists do not know the origin of the trailing “e,” and they do not know why some pies are called pizzas.

And, finally, π is precisely equal to 3, with no corrections, for very small circles. Indeed, it was equal to 3 in Biblical times but has been increasing ever since.