(narrator) Hey, smart people.

Joe here.

I was looking at my calendar recently, and two questions popped into my head: one, why are "Lambies in Jammies" so stinking cute; and two, why are weeks a thing?

Like, why do we divide the calendar into seven-day chunks and not eight or five?

Most of the units we divide our calendar with come from natural astronomical cycles.

A year is the time it takes the Earth to orbit the Sun once, months, roughly the length of the Moon's full cycle of phases, days, from the Earth's rotation on its axis.

But the week... there's no natural earthly or astronomical cycle that measures seven days.

Despite that, almost every culture on Earth today divides its calendar this way.

Some historians think the seven-day week is so old, it may be the oldest known human institution still functioning without a break.

It seems like a week is actually a pretty strong idea.

(Speaker) Boo.

(narrator) But everything comes from somewhere.

Every invention has an inventor.

So why is a week?

[upbeat music] Seven is an odd choice for the number of days in a week, and not just because it isn't even.

It's also a prime number, and we can't evenly divide it into 365 days in a year.

But seven has been regarded as a significant number by countless cultures for thousands of years in religion, mythology, superstition, and folklore.

The seven deadly sins.

The seven virtues.

The seven days of creation.

The seven samurai.

The seven heavens.

The seven chakras.

The seven lucky gods.

And the Mercury Seven.

The Mercury Seven were the first astronauts selected for NASA's human space flight program.

Each spacecraft in Project Mercury was given a name ending in the number seven.

And Project Mercury laid the groundwork that led to humans landing on the Moon.

Incidentally, counting Apollo 13, NASA originally planned seven missions to land on the Moon.

And the Moon is where the story of the seven-day week begins.

There are roughly, though not exactly, 12 Moon cycles in a solar year.

And that was precise enough for a farmer in 5,000 B.C.

As a fundamental division of time, 12 is a convenient number for a few reasons.

For one, it's pretty small.

You can probably even count to it.

It can also be divided into two parts or three parts or four or six parts, which makes it a good basis for measuring things, like circles.

Let's draw a circle with radius r. Then, for the edge, mark an intersecting circle with the same radius, and again on the other side.

These points of intersection divide a circle into six equal portions.

Just subdivide each of those in half, and you can split a circle into 12 equal parts without complicated measuring tools, which is handy for dividing the sky into signs of the zodiac.

Now, once a month, ancient sky watchers watched the Moon's cycle through the 12 slices of the zodiac, a seemingly unchanging backdrop of stars.

To an observer on Earth, the stars do rotate slowly once per day, but their positions don't change relative to each other.

But, like the Moon, a few other objects bright enough to be seen with the naked eye do seem to move on their own, not following the background stars.

These were known as wanderers, or planetes in Greek.

And they were: the Moon, Mercury, Venus, Mars, Jupiter, and Saturn.

And if you add the biggest, brightest wanderer, the Sun, you get seven of them, originally thought to be each embedded in rotating spheres with the Earth at their center.

These are not all planets by today's definition, and that's not how the solar system works, like, at all, but ancient sky watchers from the Babylonians to the Greeks and Romans, even India and China, put the planets in this order based on how fast they wandered through the sky, with Saturn, the planet with the longest cycle, at the top.

And this is where astronomy become astrology.

Now, in many ancient cultures, each planet represented a god whose position in the sky could influence the lives of us puny, little mortals.

Seven planets, seven gods, seven days.

Can you see where this is going?

Except things aren't quite in order yet.

Around the 3rd century BC, Greek astrologers writing horoscopes in Egypt decided that each god was only in charge for one hour at a time.

So Saturn would rule for an hour, and then Jupiter, Mars, and so on.

But with 24 hours in a day, and only seven planets, this cycle spilled into the following day, each day moving four planets down the list, and finally, after 168 hours, repeating on the eighth day.

This is the Planetary Week.

And in this astrological system, each day was named for its first hour.

Or translated: Saturn-day, Sun-day, Moon-day, Mars-day, Mercury-day... wait a second.

That was sounding really good for a minute, but then Mars-day and Mercury-day don't really sound like Tuesday and Wednesday.

Unless you speak Latin or any of the many languages that descended from it, like Spanish or French.

As the Greek astrological seven-day week was adopted by the Romans and spread across their empire, we can clearly see the planetary roots in the languages' names for the days.

As Christianity spread throughout Europe, days were renamed to align with Christian religious traditions.

And the first day of the working week was moved to Moon-day-- I mean, Monday, like it is today.

I mean, I love the Moon, but man, do I hate Moon-days.

Now, right now, I can hear you asking, "What the heck is up with English?

Only three of these sound like their Latin root."

Well, Northern European folks-- Germanics, Anglo-Saxons, and my Norsemen ancestors-- well, they adopted the seven-day week, but they translated some of the day names.

And for others, they inserted their favorite local gods in place of the Roman planets, maybe as a way to send a message to their Roman conquerors.

By 1,500 years ago, the seven-day week had even reached India and China.

And the names for the days of the week in Hindi and some older Chinese dialects are named for the same astronomical bodies and space gods as the Greek and Roman system, in the same order.

Even in indigenous American cultures, like the Navajo, with no native concept of a week, the seven-day system was immediately adopted following contact with the Spanish and adapted to the local language.

So the seven-day week is used basically everywhere today, but the naming system isn't completely universal.

A few odd day names do still remain.

Icelandic, German, and Finnish call Wednesday literally the middle of the week.

In the Gaelic languages of Ireland and Scotland, Wednesday through Friday are all named for days of fasting.

The Icelandic Saturday means "the day to bathe in a pool," which sounds pretty nice, though I try to bathe every day.

In some cultures, a seven-day week is used, but the names of the days are simply represented by numbers, which, after all this, seems like a much easier way to do it.

It is amazing how quickly and widely the planetary seven-day week spread around the world in the footsteps of armies and trading and religion.

And we still use it today.

Stay right where you are, because this does not completely explain why the number seven is significant across so many disconnected cultures, in so many ways.

I mean... hold on.

Okay, okay, okay.

For one thing, the planetary week-- well, that's not the only seven-day week that we find in history.

You see, the Jewish week dates to at least 500 B.C., and it's drawn not from the planets, but from the seven days of creation in the Hebrew Bible with the first six days numbered in order.

And the seventh day, a day of rest, given a special name.

Huh?

That name, Shabbat, was almost certainly borrowed from the Babylonian Shabattu, a festival of the full moon.

Huh?

Huh?

Okay, so it's obvious.

There's more than one reason that seven was significant.

And if we really want to uncover more of the theories of why seven is so significant, well, the Moon is a pretty good place to start.

Now a month--on our calendar, well, it traces its origin to the cycle of the Moon from full to crescent back to full.

That big, bright thing in the night sky is one of humanity's oldest and ancient ways of tracking the passage of time.

And it conveniently, but only coincidentally, matches up with the time period between female ovulations and menstruation, which happens to share the same word root as the word month.

Huh?

Which we can trace back to an even older root, meaning to measure, as in measuring the Moon's phases.

And when we measure a lunar cycle, each of the four quarter moon phases is separated by about seven days.

Now, one new moon to full moon-- that's approximately 14 days, or 2 sevens.

And the average time between one new or full moon is pretty close to, but not exactly, 4 sevens, or 28 days.

This is a very special set of numbers.

The factors, or the numbers that evenly divide into 28, are 1, 2, 4, 7, and 14.

And those happen to add up to 28.

Numbers with this coincidental property-- their factors add up to the number itself-- are called perfect numbers.

Not to be confused with perfect letters, of which there is only one: you.

Now, perfect numbers are rare.

The ancient Greeks--well, they just knew about four of them.

And the next one isn't until 33,550,336.

The precise length of an average lunar cycle is actually slightly more than 29 days, but seven is the nearest whole number of days between each of its quarter phases.

And the near-perfect number pattern of the Moon's phases, centered around seven-- well, that would've been known to mathematicians at least as far back as ancient Greece.

And speaking of math... [chuckles] Speaking of math, we can trace the mystical, mathematical, nature of the number seven back to, well, the very first math that was ever written down, ancient clay tablets dating to nearly 2,000 BC.

Unlike our modern, decimal base-10 system, well, the Babylonian and Sumerian scribes who wrote these--they used the base-60 number system.

Now, each place in our base-10 system tells us the number of hundreds and tens and ones, tenths, hundredths, and so on in a given number.

And when any place fills up with ten, we just roll it over to the next highest place.

The number 2,107.3 would be: Two thousands, one one hundred, I have 0 tens, plus seven ones and three tenths.

Huh?

Well, a base-60 system--well, that works just the same way.

The number 59.

Well, that would be written using a single symbol meaning 59 or 59 ones.

The number 63-- well, that would be written with a one in the 60s place... [chuckles] and, well, 3 ones.

To make this easier, we can write this using our modern numbers, too.

The number, well, 72.

Well, that would be one in the 60s place plus 12 ones, and we put a little comma in there just to make it easier to read.

But what about fractions?

Well, let's take one-half.

We know in our decimal system, that simplifies to zero ones and five tenths.

Well, in a base-60 system, that would be written as 0 ones and 30 one-sixtieths.

And we put a semicolon because you obviously didn't have a decimal point.

[distressed groan] One-third--well, that would be 0 ones and 20 sixtieths.

And one-fourth--well, that's 0 ones and 15 sixtieths.

One-fifth, 0 ones and 12 sixtieths.

One-sixth, zero and ten.

And one-seventh-- [chuckles] Well, things are nice and tidy until we get to one-seventh.

We try to divide one into seven parts in a base-60 system, this simplifies into a repeating sexagesimal fraction.

8, 34, 17, 8, 34, 17, 8, 34, 17.

More than 4,000 years ago, to the Samarian and Babylonian mathematicians who invented the earliest mathematics, seven would've been the first number whose fraction is infinite.

And some scholars-- well, they believed that this helped give birth to the mystical and superstitious nature of seven, a number that's still associated with luck and superstition today, more than 4,000 years later.

I mean, perhaps when a superstitious Sumerian astrologer noticed that there were seven heavenly bodies that moved across the stars, he saw it as a sign.

And there are seven stars in the Big Dipper, and in Orion and in other Asterisms.

Oh, what's an asterism, you ask?

Well, that is a well-known pattern of stars that is visible to the naked eye within a larger constellation, the way that the Big Dipper is actually part of Ursa Major.

And, of course, the arrangements and brightnesses of these stars--well, they're just a coincidence thanks to Earth's particular position in space, but still, our minds love to insert meaning into coincidences, and to certain ancient eyes, the universe did seem to be screaming that seven was special.

Of course, we know that those eyes-- well, they just couldn't see far enough because there are, in fact, eight planets.

Don't fight me about Pluto today.

I've got to wrap this video up.

And Earth is just one of them.

We orbit the sun.

And the Moon is not a wandering star goddess.

It's actually a big freaking rock that slammed into Earth billions of years ago, and then it got stuck there.

[clears throat] But by the time that humans figured all of that out, there were already seven days in a week, and nobody felt much like changing it if they even wondered where it came from in the first place.

It is one of the rare ideas that is simply so old, no records remain of exactly who first invented it or exactly why.

And it shows us that even ideas that we take for granted, that seem like they've always just been there-- well, even those came from somewhere.

Of course, now you know.

[mouse clicks] What are you going to do with all of this knowledge?

[chuckles] Please get rid of Mondays.

Stay curious.