You Already Celebrate τDay

Today is πDay, at least according to some subset of nerdy Americans. People outside of the US may claim that 31-4 (April 31st) or 22/71 (July 22nd) is the “right” πDay. Of course, the enlightened few among us have no love for π (despite any love of pie) and instead prefer to celebrate τDay. But how few, really?

You see, π represents the ratio of a circle’s circumference to its diameter, while τ represents the ratio of its circumference to its radius. Now, there is a long history of this ratio, and it was actually (kind of) right before it was wrong. But, at some point we realized “radius” was generally more useful than “diameter”, but we kept the diameter ratio as our constant, which meant that π, applied to the radius, now only represented half of a circle (as the radius is half of the diameter). So we had to go around using 2π everywhere (aka, τ).

Anyway, Earth’s orbit around the sun is nearly circular, and so, rather than picking dates that look something like the digits in π, we can choose a date that represents π applied to that orbit. That means halfway through the year – roughly July 2nd. There’s a new πDay option for you (since I’m sure three wasn’t enough). But wait: if we take this logic a step further, that means τDay should celebrate a complete revolution around the sun. And you (probably) already do that! Whether it’s the Western New Year, China’s Spring Festival, or some other version of “hooray, we survived another trip around the sun,”2 that is your τDay.

And, in fact, isn’t that what the new year is already about? The completion of yet another cycle, an ouroborian end and new beginning, a quite literal circle traced by the movement of our planet.

So, during the next local celebration of the new year, you can smugly watch your πDay-observing friends inadvertently honor the truly transcendental holiday – τDay: Day of the Circle.

  1. July 22nd is particularly clever, as both the US 7/22 and the non-US 22/7 are reasonable representations of the relationship between the diameter and circumference.

  2. We can get into a lot of semantical arguments about solar vs sidereal years, solstices vs apsides, the shape of our orbit, or whatever, but it’s all splitting hairs compared to the truly arbitrary selection of 3-14 as πDay, merely because the numbers look like an obsolete constant.

Greg Pfeil 14 March 2015
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