On the 14^{th} of March every year, we celebrate Pi Day. It’s a time to marvel at miraculous math (and maybe enjoy some tasty baked goods…) But what is “Pi”, other than a delicious treat?

Pi, written with the Greek symbol π, is a special number that has fascinated mathematicians for millennia. Pi is the circumference of a circle divided by the diameter, a ratio of 3.14. We can’t say what pi is *exactly*, because its decimal places stretch on forever with no pattern. The closest we’ve gotten was last year, where computers calculated pi’s decimal places up to 13.3 trillion digits!

Today, scientists and engineers use pi for an amazing range of calculations involving circles and curves, from planning the routes of airliners to analyzing sound waves.

That’s a pretty weird and wonderful number. But what other incredible measurements are there? In honour of Pi Day 2016, we’ve put together a list of the best and most bizarre units of measure from Johnny Ball's book *Why Pi?*. How many have* you* heard of?

In 1938, mathematician Edward Kasner invented the googol. His 8-year-old nephew came up with the name. it’s a REALLY, REALLY BIG NUMBER: 1 followed by 100 zeroes. The Google search engine was named after it.

In the 19^{th} century, they didn't have the paper sizes that we have today. Instead, paper came in sizes such as “foolscap” (42 x 37cm) to “elephant” (71 x 58cm). And if you really wanted to impress, you could write your essay on the largest size of paper available at the time: the double elephant.

Some scientific Australians use this unit of volume to measure water. One sydharb is the amount of water in Sydney Harbour, which is about 500 billion litres.

In medieval times, the Latin word *atomus* meant “a twinkling of the eye” – the smallest amount of time imaginable. Nowadays, it’s defined as precisely ^{1}/_{376} of a minute, or about 160 milliseconds. See you in an atomus!

One beard-second is the length a man’s beard grows in one second: 5 nanometres (0.000005 mm). This not-entirely serious unit is used only by atomic physicists to describe the tiny distances that atoms and subatomic particles move in.

Andy Warhol once said that “in the future everyone will be famous for fifteen minutes.” So, a warhol is a measure of fame. 1 kilowarhol means being famous for 15,000 minutes, or approximately 10 days.

When you were born you were given an Apgar score. It’s the first test you took! The Apgar score evaluates the health of newborns immediately after birth, based on their Appearance, Pulse, Grimace, Activity, and Respiration. It ranges from 0 to 10.

Want to learn more about magnificent math and neat numbers? Johnny Ball’s book *Why Pi?* has got puzzles to solve, conundrums to crack and incredible tricks to show to friends. You’ll find out why math isn't just numbers and calculations, it's a fundamental, incredible, magical way to discover how everything works.

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