Why Are There 7 Days In a Week? EXPLAINED

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Hey, smart people, Joe, here, I was looking at my calendar recently and two questions popped into my head.

One, why are Lambis in jammies so stinking cute?

And two, why are weeks a thing?

Like why do we divide the calendar into seven day chunks,

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.

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

But everything comes from somewhere.

Every invention has an inventor. So why is a week?

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 divided into the three hundred sixty five 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 spaceflight program.

Each spacecraft in Project Mercury was given a name ending and 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. And there

are roughly, though not exactly twelve moon cycles in a solar year.

And that was precise enough for a farmer in five thousand B.C.

as a fundamental division of time.

Twelve 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 are then from the edge market

intersecting circle with the same radius. And again on the other side,

these points of intersection divide a circle into six equal portions

to subdivide each of those in half and you can split a circle

into twelve equal parts without complicated measuring tools,

which is handy for dividing the sky into signs of the Zodiac.

Once a month, ancient sky watchers watch the moon cycle through

the twelve slices of the Zodiac, a seemingly unchanging backdrop of

stars. It's 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 planets 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.

Of these are not all planets by today's definition,

and that's not how the solar system works 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 becomes 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 third century BC, Greek astrologers writing horoscopes in

Egypt decided that each God was only in charge for one hour at a time,

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

But with twenty four 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 one hundred and sixty eight

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 Saturday, Sunday, Monday, 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 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 these 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 Monday.

I mean, Monday like it is today.

I mean, I love the moon then. 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 they're Latin root.

Well, northern European folks, Germanic, Anglo-Saxon and

Norsemen ancestors, well, they adopted the seven day week,

but they translated some of the day names and for others,

the of their favorite local gods in place of the Roman planets,

maybe as a way to send a message to their Roman conquerors. By fifteen

hundred 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 an 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 four 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 70 week spread

around the world in the footsteps of armies of 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, because so many disconnected

cultures in so many ways. I mean, hold on. OK, ok, OK.

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 five hundred 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, Har, that name Shabat was almost certainly borrowed

from the ancient Babylonian shabi to a festival of the full moon. Huh. OK,

so it's obvious there's more than one reason that seven was significant.

And if we really want to uncover more theories why seven is so significant.

Well, the moon is a pretty good place to start now.

A month on our calendar, what 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 populations and menstruation, which

happen to share the same word root as the word modern, which we can trace

back to the 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 two sevens.

And the average time between one new or full moon is pretty close to,

but not exactly four sevens or twenty eight days.

This is a very special set of numbers. The factors or the

numbers that evenly divided into twenty eight are one, two, four,

seven and 14 and those happen to add up to twenty eight numbers.

With this coincidental property there,

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 or they just knew about four of them.

And the next one is until thirty three million,

five hundred and fifty thousand three hundred and thirty six.

The precise length of an average lunar cycle is actually slightly

more than twenty nine days. But seven is the nearest whole number of days

between each of its quarter phases and the near perfect number pattern.

The Moon's phases centered around seven that would have been known

to mathematicians at least as far back as ancient Greece.

And speaking of math all.

Speaking of math, we can trace the mystical mathematical nature of the

No.7 back to, well, the very first math that was ever written down,

ancient clay tablets dating to nearly 2000 B.C.,

unlike our modern decimal base ten system. Well, the Babylonian and

Sumerian scribes who wrote these used to be basically no system.

Now, each place in our base ten system tells us the number of

hundreds and tens and ones, tens, hundreds 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 two

thousand one hundred and seven point three would be two thousand one, one

hundred at zero tens plus seven ones and three tenths of a base 60 system.

What works just the same way, the number fifty nine.

Well that would be written using a single symbol,

meaning fifty nine or fifty nine once the number. Sixty thirty.

Well that would be written with a one in the 60s place and

well three wants to make this easier.

We can write this using our modern numbers to the number seventy two.

Well that would be one in the 60s place plus 12 once.

And we put a little comma there just to make it easier to read.

But what about fractions? Let's take one half. We know in our

decimal system that simplifies to zero ones and five cents or a base

60 system that would be written as zero ones and thirty one sixty FS.

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

one third. Well that would be zero ones in twenty six. Yes.

And one fourth or that zero one in fifteen sixty one zero one

twelve sixty is one six zero and ten and one seventh hole.

Well things are nice and tight. We get them one step.

We tried to buy one to seven parts in a base 60 system.

This simplifies into a repeating six a decimal fraction. Eight.

Thirty four. Seventy eight. Thirty four. Seventy eight.

Thirty four. Seventy more than 4000 years ago to the Sumerian

Babylonian mathematicians who invented the earliest mathematics,

seven would have been the first number whose fraction is infinite.

And some scholars really believe that this helped give birth to the

mystical and superstitious nature,

a number that's still associated with luck and superstition.

Today, more than four thousand 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. Oh, right.

And in there, pastoralism. Oh, what's an astronaut?

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 because there are, in fact, eight planets.

Don't fight me about Pluto today.

I've got to wrap this video up and it's 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.

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 ever wondered where it came from in the first place.

It is one of the rare ideas. It 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.

What are you going to do with all of this knowledge,

please get rid of Monday's stay curious.

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