Pi Day: A picture of the Earth’s hemisphere to celebrate math nerd day

# Here’s 255,032,236 Square Kilometers of Pi Day. More or Less.

The entire universe in blog form
March 14 2013 1:33 PM

# Happy Pi Day!

In the United States, today is the nerdiest of unofficial holidays: Pi Day (or really, π Day), because as we write dates here, today is 3/14, the first three digits of the eponymous number.

Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!

Pi is one of the most important of mathematical numbers, popping up everywhere in physics. Most people know it because it’s how you get the circumference of a circle:π x the diameter (or 2 x π x the radius).

It’s also how you get the surface area of a sphere: 4 x π x radius2. In honor of that, NASA released a photo showing half the surface area of our planet today, about 255,032,236 square kilometers (98,034,392 square miles) worth of it:

Pretty, isn’t it? To be fair, this is slightly less than half the Earth, since the satellite is about 36,000 kilometers (22,000 miles) above the Earth, and you'd need to be infinitely far away to see exactly half (due to the horizon problem). Still, close enough.

I like how they chose an image taken today by GOES 13, an Earth-observing weather satellite, which looks down upon the western hemisphere, including the U.S. In other parts of the world, dates are written in the form DAY-MONTH-YEAR, so to them today is (ignoring the year) 14-3, which doesn’t help. I guess for everyone else, π Day would be on the 31st of April.

Um.

π really is amazing. It's irrational, which means it cannot be exactly represented as the ratio of two other numbers. It goes on forever after the decimal point, never ending and never repeating (some numbers, like 1/9, are just repeating numerals when expressed as a decimal). That seems so counterintuitive, but it’s just the way numbers are. The Universe is under no obligation to conform to our common sense. More likely, our brains didn’t evolve to grasp such things easily. Good thing we invented math to figure this stuff out!

Otherwise, we might never have discovered the elegance that is the greatest equation of all time:

eiπ + 1 = 0

where e is the base of the natural logarithm, and i is the square root of -1. This is called the Euler Identity, and the more you think about it the more your brain will twist into a Möbius strip.