October 1, 2025 • 31 mins
And why does the day have 24 hours? Jorge explores the science behind the time units we use in our everyday lives.
See omnystudio.com/listener for privacy information.
Mark as Played
Transcript
Episode Transcript
Available transcripts are automatically generated. Complete accuracy is not guaranteed.
Speaker 1 (00:01):
Hey, welcome to Sign Stuff, a production of iHeartRadio. I'm
hoorheit him and today we're asking the question why does
the week have seven days? Why not six days or nine?
It's part of a larger episode in which we explore
the signs behind the time units we use in our
everyday lives. For example, we're gonna answer the question why
(00:22):
is a second called a second? Or is it day
actually one full rotation of the Earth. As we'll learn,
there are all kinds of surprises in the way we
keep track of time. So set your clocks, take a
minute with us, because the hour has come to answer
the question why does the week have seven days? Hi? Everyone,
(00:51):
Today we're going to explore the signs behind the time
units we use in our everyday lives. We're gonna learn
things like why does a minute have sixty second? Or
why are there twelve months to a year?
Speaker 2 (01:03):
Now?
Speaker 1 (01:04):
As it turns out, a lot of time units we
use have their origin in the movement of celestial bodies
in space, which is why our guide for today is
my friend, Professor Matt Siegler, a planetary scientist at the
University of Hawaii. Here's what he does. Well, Thank you,
doctor Siegler for joining us.
Speaker 2 (01:23):
Thank you for having me. My name is Matt Siegler.
I'm a professor at University Hawaii and I studied planets.
So I work on about ten different spacecraft missions that
are going to the Moon or Mars or Jupiter's moons.
Speaker 1 (01:38):
So you based out of Hawaii, but you work with NASA,
and yeah.
Speaker 2 (01:41):
So most projects are going to be through NASA projects.
And then I'm working on the Canadian rover that's going
to the south pole of the Moon and roughly twenty
twenty seven, and occasionally I've worked with some ISSA missions. Uh.
Speaker 1 (01:55):
Interesting, in ISSA means what ISSA is European Space Agency. Well,
the reason I thought you for this topic was that
you're expert on planets, and I imagine that time is
very important when you're thinking about the different planets, Like
you have to think about how long the rotation of
Mars takes or how long does the Moon go around
the Earth to be able to talk to it right.
Speaker 2 (02:16):
Oh, yeah, your spacecraft might be orbiting at a certain
rate around the Moon or you might be landed on
the Moon or Mars, and then you need to care
about what the day night cycle is. But then also
you're thinking about, you know, when Will has come to
view of Earth and when we'll be able to talk
to it, or maybe you're waiting for a satellite that's
going around Mars to come overhead that you talk to.
Speaker 1 (02:37):
It's.
Speaker 2 (02:38):
Yeah, there's a lot of complicated timekeeping, all right.
Speaker 1 (02:42):
So the first unit of time doctor Siegier and I
talked about was the day. Most people think that a
day is how much time it takes for the Earth
to spin and do a full rotation, but actually that's
not really true. Let's start with the day. Why did
you want to start with the day?
Speaker 2 (03:02):
So the day is kind of an obvious place to start,
because that is the most simple thing of you look
at the sun and you wanted to come back to
noon the next day, Right, that will happen in a
certain amount of time. But early on we realized that
that was actually different than if you looked at a
star and wanted that star to come back overhead. Those
(03:22):
two things were just a little bit different.
Speaker 1 (03:25):
Okay, so it turns out there are actually two days
here on Earth. The first is the time it takes
with the Earth to do a full rotation as it
spends relative to stay. The rest of the universe. That's
called this ideial day or the star day. And then
there's a time it takes for us to see the
Sun at its highest position or high noon. Those two
(03:47):
are different because the Earth is moving around the Sun.
Speaker 2 (03:54):
Those are a little different. And you have to imagine
how the system works. If you're spinning around on the Earth.
Speaker 1 (04:00):
Uh huh.
Speaker 2 (04:01):
If you're just the Earth spinning, it actually takes twenty
three hours and fifty six minutes roughly for you to
get back to the same place. The Earth spins on
its axis once every twenty three hours and fifty six minutes.
What yeah, what, And it's the fact that we're moving
around the Sun. It takes a little bit longer each
(04:22):
day for us to point back at the Sun again.
And so here's this combination of the Earth's motion around
the Sun and the spinning of the Earth that come
together to give us a day which is just the
solar day, which is a little bit longer than how
long it takes for us to spin.
Speaker 1 (04:38):
What wait wait wait, wait wait wait wait, you just
blew my mind and we just started this is crazy
wait wait.
Speaker 2 (04:46):
Yeah, we really need one of your animations.
Speaker 1 (04:49):
This is where being a cartoonist is extra frustrating. Okay,
here's what's going on to Earth actually takes twenty three
hours and fifty six minutes to spin around on its axis.
But because we're also going around the Sun, by the
time we spin around, the Sun is now a little
bit behind us, and so it takes a little bit
(05:11):
of extra time to catch it again at high noon.
Speaker 2 (05:16):
And so because at the same time, we move a
little way around the Sun, roughly a degree or so,
it takes a little bit of extra time for us
to come back and point to the Sun because we've
moved a little ways around, And so that extra time
is four minutes every day, no.
Speaker 1 (05:34):
Kidding, Wait, I guess it's sort of like the Sun
moved around relative to us in the solar system. Yeah.
Speaker 2 (05:41):
So let's say you're jogging down the street and you
see a crazy looking dog or something, and you're going
to turn your head backward a little to look because
you ran past. Right. Oh, you have to turn a
little more than if you're just standing right next to
that dog.
Speaker 1 (05:57):
I see, So we have to turn a little bit
more to complete the noon cycle. Yeah, and it's four minutes.
That seems like a lot.
Speaker 2 (06:04):
I know, and that's basically based on the ratio between
the day and the year. But it gets even crazier.
Speaker 1 (06:12):
Yes, it gets crazier. Not only is a day not
really a day, but it also depends on what time
of the year it is.
Speaker 2 (06:21):
Kepler was the one that did most of the math
behind it, and he figured out that the Earth's orbit
is not a perfect circle. So this adds an extra
complication of this. When we're going around the Sun, you
go at a little bit different speeds depending on if
you're at the near part of that oval or the
far part of that oval away from the Sun.
Speaker 1 (06:42):
So it turns out that the Earth's orbit around the
Sun is not a perfect circle. It's almost a perfect circle,
but not quite. It's a little bit oval shape. That
means the Earth is not always going at the same speed.
During the narrow parts of the oval, the Earth goes
a little faster, and doing the wide parts it goes
(07:02):
a little slower, which means the extra little bit of
rotation we need to catch the Sun again at the
high noon changes, so sometimes it's less than four minutes,
sometimes it's more than four minutes exactly.
Speaker 2 (07:15):
So it's basically you know again that analogy when we
saw that crazy dog when we were taking our jog
in the morning. If you're jogging at a little bit
faster pace, you're going to have to turn a little
more to see that same object. Or if you're jogging
at a slower pace, you're going to have to turn
a little less to see the dog behind you.
Speaker 1 (07:32):
Interesting, So high noon is always high noon, but it's
not always noon.
Speaker 2 (07:37):
Yes, it's going to happen at a little bit different time,
all right.
Speaker 1 (07:42):
So a day is definitely not a star day, which
is how much time it takes with the Earth to
do a full spin, and a solar day, which is
how much time passes between when the sun is at
its peak in the sky, changes throughout the year. So
what exactly is what we call a day? What does
(08:02):
the official day that we use in our calendars and
clocks and phones represent? So if it's always changing, how
do we pick what an official day is? I wonder
if what we picked us the official day will be
plus exactly four minutes and minus exactly four minutes, or
is it like plus two minus six Do you know
(08:24):
what I mean?
Speaker 2 (08:24):
So I think in the end we chose the average day.
Speaker 1 (08:27):
Oh fascinating, Wow, actually, we were wrong here. What we
use as the official day in our everyday lives is
not the average day. It's something a little different. When
we come back, we'll tackle what a day actually is
and we'll answer why a week is made up of
(08:48):
seven of those days. Stay with us, we'll be right
back and we're back, all right. We talked about what
a day actually is, and we learned a day is
(09:10):
not really a day. It's not how long it takes
to do a full revolution of the earth, and it's
not the amount of time between when the sun is
at its highest peak in the sky twice in a row.
That time changes throughout the year. But now let's go
small and look at the time units that are shorter
than a day, and let's start with the hour. Have
(09:32):
you ever wondered why there are twenty four hours in
a day? Where did the hour come from? Why does
the day have twenty four hours and not twenty three?
Why not thirty? Why not fifty?
Speaker 2 (09:44):
Yeah? So where did twenty four come from? It seems
like a pretty random number to us to divide something
into that cycle, And it came out of wanting twelve
hours in the day and twelve hours at night. This
is what the Egyptians and Babylonians wanted because it's obvious
if you count on your fingers, you count to twelve.
Speaker 1 (10:02):
Right, Wait, how many fingers do you have?
Speaker 2 (10:06):
Man, So it's not that the Babylonians had extra fingers.
It's basically a different way of counting, and you actually
can count higher if you use your hands in this
way that you count on your knuckles.
Speaker 1 (10:20):
Uh huh.
Speaker 2 (10:21):
So each finger has three knuckles, right, and so you
do that for your four fingers, and you can count
with your thumb.
Speaker 1 (10:28):
Okay, this is a little hard to explain on the radio,
but basically the idea is that you can count to
twelve using one hand. This is how it works. Hold
your hand out so that you're looking at your palm.
Now turn your thumb in so that it touches the
bottom joint that connects your pinky to your hand. That's one.
(10:49):
Then move your thumb up and touch the middle joint
of your pinky. That's two. Now move it up again
and touch the top joint of your pinky. That's three.
Then move on to your ring finger and touch the
bottom joint that connects to your ring finger to your hand,
and repeat.
Speaker 2 (11:09):
And you go down your pinky one, two, three, go
down your next finger one to three knuckles, four fingers.
That adds up to twelve. So you can keep track
of twelve pretty easy.
Speaker 1 (11:21):
Oh, I see, I see.
Speaker 2 (11:22):
So that was where the base twelve system came from,
and so that was how everyone used to count.
Speaker 1 (11:29):
Really.
Speaker 2 (11:30):
Yeah, this idea of us counting on ten fingers is
kind of a new thing that came basically from the
Romans when we think, oh, it comes naturally from our
ten fingers. But the original base twelve system was an
easier way to count things on your hand. So this
base twelve system is called the duo decimal.
Speaker 1 (11:47):
System, not the Dewey decimal system, yes, not.
Speaker 2 (11:50):
The Dewey decimal system, but the duo decimal system.
Speaker 1 (11:53):
So you can count to twelve with one hand.
Speaker 2 (11:55):
Yeah. And so this was the basis of bartering in
the market or counting, you know, things that you were
going to sell or whatever it might be. Started as
base twelve, and so it was natural to use that
system to count the hours of the day. And so
they split the day into twelve hours and then split
the night into twelve units.
Speaker 1 (12:16):
I see.
Speaker 2 (12:16):
And the night comes from the importance of the signs
of the zodiac. There's just twelve constellations that are pretty
equally spaced throughout the night sky, and so you could
use these zodiac signs over the course of an evening
to see how many hours you had gone.
Speaker 1 (12:32):
Wait what Yeah, so over the course of the night,
the constellations will kind of like parade across the sky.
Speaker 2 (12:40):
Yeah, and so you know when this constellation set that
an hour has passed. And then they decided, oh, we
need a little more precision, so they broke these up
into things called decans, which are a third of a
zodiac sign. So they took like, ah, this star from
that zodiac sign and this star from that zodiac sign
of this and broke them up in threes to give
(13:01):
them a little more accuracy over the night.
Speaker 1 (13:03):
I see, But wouldn't that differ depending on the season.
Speaker 2 (13:07):
Well, which zodiac signs are up in the night sky
changes with the seasons, but not the number of them
that will pass.
Speaker 1 (13:15):
There will always be twelve of them parading across the
sky evenly space about one hour each.
Speaker 2 (13:20):
Yeah.
Speaker 1 (13:21):
Oh fascinating. And so it was like this merger of like,
we like twelve because we can count with our fingers,
and also there's these things, these markers in the sky. Yeah,
I always thought that it was twelve because twelve have
so many factors to it.
Speaker 2 (13:36):
Well, that was the other convenient thing of the base
twelve system is you could divide by three and by four,
and all these things were natural units. So three, four,
there were these easily countable numbers that were the base
of all these and so early mathematicians got very excited
about these kind of patterns.
Speaker 1 (13:54):
Fascinating. So that's why we split the day into twenty
four hours.
Speaker 2 (13:58):
Yeah, was because of the obvious counting of twelve on your.
Speaker 1 (14:01):
Hand, and so an hour is one twenty fourth of
a day.
Speaker 2 (14:08):
Yeah. And why they chose to space twenty four rather
than twelve, that I think was just kind of a
random choice. They wanted the unit to be twelve for
the day and twelve at.
Speaker 1 (14:19):
Night because otherwise that Keefer Sutherland show from the nineties
would have been called twelve hours, not twenty four hours. Yes,
all right, so then that's the hour. So the hour
is dependent on what we define as a day. Yes,
so that's an hour, it's one twenty fourth of a day.
And now we get to minutes and seconds But first,
it's interesting to think about why we would want to
(14:40):
split the hour in the first place.
Speaker 2 (14:44):
As time went on, people cared about smaller and smaller
time increments. Right When back in the olden days, you
didn't care too much about an hour. It was just
good enough to say, is it morning or afternoon? Right?
But now when you had to go to work, because
now there was a building that you had a job
at and you had to get to that building, you
(15:05):
needed to know roughly what hour it was.
Speaker 1 (15:07):
Right, because we needed to start coordinating between humans, Like
I needed to know when I should go to work
because you were expecting me.
Speaker 2 (15:14):
Right.
Speaker 1 (15:14):
It's like complication that comes from a society.
Speaker 2 (15:18):
Yeah, and so when we didn't have society, we didn't
have those complications. Yeah, an hour's so complicated. You need
to know everything down to the mill sector.
Speaker 1 (15:27):
Yeah, it's all because of people. Yes, okay, let's get
to a minute. The reason the hour is split into
sixty increment also goes back to counting with your hands.
If you use one hand to count to twelve with
your thumb and finger joints, you can use your other
hand to count groups of twelve.
Speaker 2 (15:47):
And then you have your other hand free because you're
not counting any of those fingers.
Speaker 1 (15:50):
Uh huh.
Speaker 2 (15:51):
And so then you could say, oh, I counted one twelve,
I counted two twelves, I counted three twelve.
Speaker 1 (15:55):
Here doctor Ziegler is counting to twelve with his right hand,
and each time he reaches twelve, he opens one full
finger with his left hand, just like you would count
to five with your fingers.
Speaker 2 (16:07):
It's I kind of four twelves. I counted five twelves, huh,
which is sixty? Wait?
Speaker 1 (16:11):
Wait, so on one hand I use twelve, but on
the other hand you do use your five fingers.
Speaker 2 (16:16):
Yeah, so you can count to twelve five times before
you've used up all your fingers on both hands.
Speaker 1 (16:22):
But why wouldn't I use twelve times twelve? If I
can count to twelve in the one hand, it's just
too much.
Speaker 2 (16:27):
Then you have to be ambidextrious the other.
Speaker 1 (16:31):
So you can count to twelve with one hand, but
then you keep track of how.
Speaker 2 (16:35):
Many twelves you count on it with the other.
Speaker 1 (16:37):
Other hand, which and you use your five fingers, and
that's where we get sixty.
Speaker 2 (16:41):
Yeah. And so the natural ways of counting to Babylonian
were base twelve and base sixty. So why sixty minutes
to an hour was basically because we got used to
this base sixty system. They decided it was sensible to
use this system rather than a base twelve minutes system.
Speaker 1 (17:00):
Like, the word minute comes from the word sort of
first first division.
Speaker 2 (17:04):
Yeah, first division, and then second is obviously the second division.
Speaker 1 (17:10):
And that's why the second is called the second.
Speaker 2 (17:12):
Yeah, it's because it was a first division of an
hour and the second division of an hour.
Speaker 1 (17:18):
Man, I'm forty nine years old, and it's taking me
this long to figure that, to learn this fact. Yeah,
that's where the word second comes from.
Speaker 2 (17:28):
Yeah.
Speaker 1 (17:30):
Yeah, So that, according to doctor Siegler, is where the
word second comes from. It's the second division of an hour.
And we divide the hour and the minute into sixty
because that was the preferred number system of the ancient Egyptians,
except that in the nineteen sixties we decided to redefine
the second, which in turn change what exactly a minute
(17:53):
is and an hour and even a day. We'll dig
into that and we'll finally answer why the week has
seven days. After the break, they'll go anywhere, we'll be
right back, welcome back.
Speaker 2 (18:17):
Okay.
Speaker 1 (18:18):
So the reason we have twenty four hours to a
day and sixty minutes to an hour and sixty seconds
to a minute. Goes back to traditions we've had since
ancient Egypt, which makes you wonder why haven't we changed it.
Speaker 2 (18:35):
So it's a lot of just heritage and being stuck
with what works. One of the reasons it's stuck in
our culture is because it's always going on, right. You
can't just stop and say, okay, we're all going to
switch to the metric system of time.
Speaker 1 (18:49):
People are like, forget it, I'm too used to hours
and minutes and seconds. Don't make me think too much.
Speaker 2 (18:55):
Yeah, And you can see that those traditions are hard
to shake in even things like measurement right where we're
worried in America about oh, can we ever switch to
the metric system or not? But like if you switched
time suddenly, now everyone's off.
Speaker 1 (19:10):
Okay. So for a long time, a second was a
subdivision of a subdivision of a subdivision of a day,
which you can measure using the average solar day. But
as we said, the solar day is a bit squishy,
it's not super precise. So in the nineteen sixties people
decided to standardize the second and ground it in a
(19:33):
precise physical phenomenon.
Speaker 2 (19:38):
So basically, by the nineteen sixties, we really were carrying
about smaller time scales than the second, and we need
to keep track of things really precisely. Someone in France
having a different second than someone here, it just wouldn't work.
So in the nineteen sixties we started carrying about these
smaller units of time more precisely. So they got together
(19:59):
internationally and said, okay, what are we going to do
to develop the second? And they decided, here is this
vibration that happens in the caesium atom.
Speaker 1 (20:08):
Uh huh.
Speaker 2 (20:08):
So we take this caesium atom and it turns out
that nine trillion, one hundred and ninety two million, six
hundred and thirty one thousand, seven hundred and seventy vibrations
of the caesium atom happen every second.
Speaker 1 (20:23):
Huh.
Speaker 2 (20:23):
They said, from now on that will be the second.
Speaker 1 (20:26):
Whatever I was before, I forget it. This is what
it is now exactly.
Speaker 2 (20:30):
So now the second is very precisely calibrated to this
transition of the caesium.
Speaker 1 (20:35):
Ad why the casion atom, why not the carbon atom
or oxygen atom.
Speaker 2 (20:41):
It was just an atom that had this property of
a very regular vibration.
Speaker 1 (20:47):
All right, So we redefined the second to be the
precise vibrations of a caesium atom, and this technically redefines
the minute, and the hour and even the day. Now
here's my question. If I take the second according to
the season atom I multiplied by sixty, I now get
an official minute, right, Yes. If I multiply that by sixty,
(21:10):
I get a new hour, which is now the official hour. Yes.
And if I multiply that by twenty four, I now
get the official day, right yeah. So now how does
that day match to the solar day that we talked
about before, Like, I wonder if it's really the average
now or fit's deviated from the average.
Speaker 2 (21:28):
Yeah, so it doesn't match. Well, that gets into why
in the nineteen seventies we started adding these leap seconds
into the calendar to correct ourselves back to this caesium
atom standard of time.
Speaker 1 (21:44):
Yes, you might have heard of leap years, where every
four years we add February twenty nine to the calendar
to keep us synchronized with the orbit of the Earth
around the Sun. Well, we also have leap seconds, and
that is when we add a second to the official
time of the human race, called coordinated Universal Time or UTC,
(22:05):
to make up the difference between the official day, which
comes from multiplying the official second that comes from the
caesium atom, and the actual solar day, which again is
the time between when the sun reaches high noon. In
nineteen seventy two, when we first realized we needed to
do this, we added ten seconds to everyone's clock, and
(22:27):
since then, according to the National Institute of Standards and
Technology or NIST, we've added one second to the world
clock about every year and a half. So about every
year and a half, without most of humanity knowing it,
our clocks repeat one second right at midnight on either
New Year's Eve or the end of June. But everyone
(22:52):
has to remember to do that, do you know what
I mean? Like that is a huge logistical burden for everybody.
Like whatever you do, just remember everyone in the world.
We need to add one second this year, not next year,
not the even before this year. We need to everyone
at a second.
Speaker 2 (23:07):
Yeah, and you know, right now, at least in history,
we don't care about those super small time scales that much.
But maybe in another hundred years, being off by a
few milliseconds is going to be an issue societal, right.
It seems so messy, yeah, I mean, and it's messy
when we're trying to match this traditional system and.
Speaker 1 (23:26):
To match it to like giant physical processes like the
rotation of the Earth and whether it sinks to the
orbit around the Sun. I mean, these are huge things, right, Yeah.
Speaker 2 (23:37):
And so to get to the month, we start to
throw in the moon cycle, which is another bizarre excerp.
Speaker 1 (23:47):
Okay, let's get to the main question we set out
to answer, which is why does the week have seven days?
What do you know about why the week has seven days?
Speaker 2 (23:57):
So the original week back in ancient Egypt was actually
ten days a week, and this came from trying to
get this daily cycle to match the lunar cycle. So
the moon appears to go from it's noon that we
see the full moon to the full moon happens in
about twenty nine and a half days, Okay, And so
that was close to thirty days.
Speaker 1 (24:17):
Uh huh.
Speaker 2 (24:17):
So the original idea was that thirty days are important.
And then for some reason, because thirty couldn't be broken
up by twelve, they said, ah, let's break it up
into three weeks and so there were three weeks a
month that added to thirty days, and that roughly, you know,
at twelve months of thirty days, you got to three
hundred and sixty, which fit the nice base sixty system,
(24:40):
and that's how it was. Then you have this other
cultural tradition of the Jewish tradition and where that became
the Christian tradition of the seven days a week.
Speaker 1 (24:51):
Okay, before we get to that, I feel like you're
saying like maybe the original concept of the week was
somehow related to the month, and the concept of the
month was sort of to the moon cycle. So what's
the moon cycle?
Speaker 2 (25:03):
So if you look at the moon, for it to
go from full moon to full moon, that cycle is
going to be about twenty nine and a half days.
But the rotational cycle of the moon is actually different.
How often the moon goes around the Earth and gets
to the same place in the sky, uh huh is
actually different. It's like twenty seven point three days. And
(25:26):
so that gets really complicated as well. Do we count
where the moon comes back to the same spot or
do we count where the moon goes through its whole cycle?
Speaker 1 (25:35):
Okay?
Speaker 2 (25:35):
And so it was a little bit of a which month,
do you choose develop which way you want to break
that month apart into weeks?
Speaker 1 (25:44):
All Right, What doctor Siegler is saying is that the
reason we have a week is to break up the month,
and the month is related to the cycle of the moon.
Except there are two moon cycles, just like there are
two day cycles. There's how long it takes for the
moon to do a full rotation around the Earth, and
there's how long it takes for the moon to go
(26:04):
from full moon to full moon. One it's about thirty days,
which lends itself to have a ten day week, and
the other is about twenty eight days, which lends itself
to have a seven day week.
Speaker 2 (26:18):
And so you could split the month into four seven
day weeks if it was a twenty eight day month,
or you could split it into three ten day weeks
if it were a thirty day But when the Romans
had to choose, huh, they basically went with the seven
day system because that worked better with whatever traditions they
(26:38):
were adopting.
Speaker 1 (26:40):
Well, from what I read, nobody really cared about the
week basically, like nobody really kept track of like, oh,
this important event happened on a Tuesday. But the Jewish
people obviously did back then. You know, for them, it
was a command from God not to work. And then
what historians seem to have found is that at the
same time, the Roman people had this seven day cycle
(27:03):
that was just based on astrology. So they looked at
the sky. They could see five planets, the Sun, and
the moon. I imagine they couldn't like really see Neptune or Urinus.
Speaker 2 (27:12):
Right, yeah, those weren't discovered until late seventeen hundreds. We
found Urinus and then Neptune was even later, so.
Speaker 1 (27:19):
Right, right, but the others you can see in the sky,
right yeah, So they like each God must be in
charge of the day. So that's why we have this
astrological keeping track of seven days at a time. So
then it's sort of like this nebulous process of we
sort of have this planetary astrological seven day week. The
Jewish people are keeping track every seven days, and then
(27:39):
Christianity adopted this Sunday as the day of worship and
so let's make it a seven day week.
Speaker 2 (27:46):
So it all kind of came out of this conglomeration
of cultures.
Speaker 1 (27:50):
All right, here's the basic scenario for the month. We
seem to have based it on the lunar face cycle,
the period of time between two full moons, which is
about thirty days. But for the week, we opted to
base it on a multiple of the orbital lunar cycle.
How long it takes for the moon to go around
to Earth, which.
Speaker 2 (28:11):
Is twenty eight days, probably.
Speaker 1 (28:13):
Because of the cultural traditions of the Jewish Sabbath and
Roman astrology, which had seven days. But here's a twist,
how long it takes for the moon to go around
the Earth has been changing.
Speaker 2 (28:29):
The month wasn't always this twenty nine and a half
day cycle. We have this whole idea of the moon
formed from a giant impact with the Earth and you
form the moon. The original moon in that idea would
have had a four day month, and then due to
the interactions with tides on the Earth. Uh huh, the
moon has in the last several billion years been moving
(28:49):
outward towards where it is today, and the month is
slowly getting longer all the time. So this hasn't happened
over the course of human history, right in the last
million years or so that you could say humans were around.
But if you go back in the geologic record, you
can see times when the month was actually shorter.
Speaker 1 (29:06):
What you mean, like, if we had lived in a
different time in the history of Earth and created the
month like a billion years ago, there would be more
months in the year exactly. Okay, So basically the reason
we have twelve months a year is that we just
happened to be living in his time in the history
of Earth, where the moon goes around the Earth twelve
(29:29):
times roughly a year.
Speaker 2 (29:30):
Exactly, and we picked twelve rather than thirteen because twelve
was nice to count on our fingers.
Speaker 1 (29:38):
And because of the month. That's how we picked the week,
because a week, if we really probably just picked it
as a division of the month. Yeah, well, there you
have it. The reason the week has seven days is
partly cultural and religious, partly due to the physics of
the Moon's orbit, and partly coincidence. If human said come
(30:00):
around earlier or later in the history of the Earth,
the week might have been shorter or longer. So if
you're having a long week, just think it could have
been even longer. Thanks for joining us, See you next week. Hey,
please take a second and leave us a review on
(30:22):
Apple Podcasts, Spotify, or wherever you listen to the podcast.
Speaker 2 (30:26):
Thanks a lot.
Speaker 1 (30:28):
You've been listening to Science Stuff the production of iHeartRadio,
written and produced by me Or Hitcham, edited by Rose Seguda,
executive producer Jerry Rowland, and audio engineer and mixer Casey Peckram.
And you can follow me on social media. Just search
for PhD Comics and the name of your favorite platform.
Be sure to subscribe to Sign Stuff on the iHeartRadio app,
(30:49):
Apple Podcasts, or wherever you get your podcasts, and please
tell your friends We'll be back next Wednesday with another episode.
Hey, welcome to Sign Stuff, a production of iHeartRadio. I'm
hoorheit him and today we're asking the question why does
the week have seven days? Why not six days or nine?
It's part of a larger episode in which we explore
the signs behind the time units we use in our
everyday lives. For example, we're gonna answer the question why
(00:22):
is a second called a second? Or is it day
actually one full rotation of the Earth. As we'll learn,
there are all kinds of surprises in the way we
keep track of time. So set your clocks, take a
minute with us, because the hour has come to answer
the question why does the week have seven days? Hi? Everyone,
(00:51):
Today we're going to explore the signs behind the time
units we use in our everyday lives. We're gonna learn
things like why does a minute have sixty second? Or
why are there twelve months to a year?
Speaker 2 (01:03):
Now?
Speaker 1 (01:04):
As it turns out, a lot of time units we
use have their origin in the movement of celestial bodies
in space, which is why our guide for today is
my friend, Professor Matt Siegler, a planetary scientist at the
University of Hawaii. Here's what he does. Well, Thank you,
doctor Siegler for joining us.
Speaker 2 (01:23):
Thank you for having me. My name is Matt Siegler.
I'm a professor at University Hawaii and I studied planets.
So I work on about ten different spacecraft missions that
are going to the Moon or Mars or Jupiter's moons.
Speaker 1 (01:38):
So you based out of Hawaii, but you work with NASA,
and yeah.
Speaker 2 (01:41):
So most projects are going to be through NASA projects.
And then I'm working on the Canadian rover that's going
to the south pole of the Moon and roughly twenty
twenty seven, and occasionally I've worked with some ISSA missions. Uh.
Speaker 1 (01:55):
Interesting, in ISSA means what ISSA is European Space Agency. Well,
the reason I thought you for this topic was that
you're expert on planets, and I imagine that time is
very important when you're thinking about the different planets, Like
you have to think about how long the rotation of
Mars takes or how long does the Moon go around
the Earth to be able to talk to it right.
Speaker 2 (02:16):
Oh, yeah, your spacecraft might be orbiting at a certain
rate around the Moon or you might be landed on
the Moon or Mars, and then you need to care
about what the day night cycle is. But then also
you're thinking about, you know, when Will has come to
view of Earth and when we'll be able to talk
to it, or maybe you're waiting for a satellite that's
going around Mars to come overhead that you talk to.
Speaker 1 (02:37):
It's.
Speaker 2 (02:38):
Yeah, there's a lot of complicated timekeeping, all right.
Speaker 1 (02:42):
So the first unit of time doctor Siegier and I
talked about was the day. Most people think that a
day is how much time it takes for the Earth
to spin and do a full rotation, but actually that's
not really true. Let's start with the day. Why did
you want to start with the day?
Speaker 2 (03:02):
So the day is kind of an obvious place to start,
because that is the most simple thing of you look
at the sun and you wanted to come back to
noon the next day, Right, that will happen in a
certain amount of time. But early on we realized that
that was actually different than if you looked at a
star and wanted that star to come back overhead. Those
(03:22):
two things were just a little bit different.
Speaker 1 (03:25):
Okay, so it turns out there are actually two days
here on Earth. The first is the time it takes
with the Earth to do a full rotation as it
spends relative to stay. The rest of the universe. That's
called this ideial day or the star day. And then
there's a time it takes for us to see the
Sun at its highest position or high noon. Those two
(03:47):
are different because the Earth is moving around the Sun.
Speaker 2 (03:54):
Those are a little different. And you have to imagine
how the system works. If you're spinning around on the Earth.
Speaker 1 (04:00):
Uh huh.
Speaker 2 (04:01):
If you're just the Earth spinning, it actually takes twenty
three hours and fifty six minutes roughly for you to
get back to the same place. The Earth spins on
its axis once every twenty three hours and fifty six minutes.
What yeah, what, And it's the fact that we're moving
around the Sun. It takes a little bit longer each
(04:22):
day for us to point back at the Sun again.
And so here's this combination of the Earth's motion around
the Sun and the spinning of the Earth that come
together to give us a day which is just the
solar day, which is a little bit longer than how
long it takes for us to spin.
Speaker 1 (04:38):
What wait wait wait, wait wait wait wait, you just
blew my mind and we just started this is crazy
wait wait.
Speaker 2 (04:46):
Yeah, we really need one of your animations.
Speaker 1 (04:49):
This is where being a cartoonist is extra frustrating. Okay,
here's what's going on to Earth actually takes twenty three
hours and fifty six minutes to spin around on its axis.
But because we're also going around the Sun, by the
time we spin around, the Sun is now a little
bit behind us, and so it takes a little bit
(05:11):
of extra time to catch it again at high noon.
Speaker 2 (05:16):
And so because at the same time, we move a
little way around the Sun, roughly a degree or so,
it takes a little bit of extra time for us
to come back and point to the Sun because we've
moved a little ways around, And so that extra time
is four minutes every day, no.
Speaker 1 (05:34):
Kidding, Wait, I guess it's sort of like the Sun
moved around relative to us in the solar system. Yeah.
Speaker 2 (05:41):
So let's say you're jogging down the street and you
see a crazy looking dog or something, and you're going
to turn your head backward a little to look because
you ran past. Right. Oh, you have to turn a
little more than if you're just standing right next to
that dog.
Speaker 1 (05:57):
I see, So we have to turn a little bit
more to complete the noon cycle. Yeah, and it's four minutes.
That seems like a lot.
Speaker 2 (06:04):
I know, and that's basically based on the ratio between
the day and the year. But it gets even crazier.
Speaker 1 (06:12):
Yes, it gets crazier. Not only is a day not
really a day, but it also depends on what time
of the year it is.
Speaker 2 (06:21):
Kepler was the one that did most of the math
behind it, and he figured out that the Earth's orbit
is not a perfect circle. So this adds an extra
complication of this. When we're going around the Sun, you
go at a little bit different speeds depending on if
you're at the near part of that oval or the
far part of that oval away from the Sun.
Speaker 1 (06:42):
So it turns out that the Earth's orbit around the
Sun is not a perfect circle. It's almost a perfect circle,
but not quite. It's a little bit oval shape. That
means the Earth is not always going at the same speed.
During the narrow parts of the oval, the Earth goes
a little faster, and doing the wide parts it goes
(07:02):
a little slower, which means the extra little bit of
rotation we need to catch the Sun again at the
high noon changes, so sometimes it's less than four minutes,
sometimes it's more than four minutes exactly.
Speaker 2 (07:15):
So it's basically you know again that analogy when we
saw that crazy dog when we were taking our jog
in the morning. If you're jogging at a little bit
faster pace, you're going to have to turn a little
more to see that same object. Or if you're jogging
at a slower pace, you're going to have to turn
a little less to see the dog behind you.
Speaker 1 (07:32):
Interesting, So high noon is always high noon, but it's
not always noon.
Speaker 2 (07:37):
Yes, it's going to happen at a little bit different time,
all right.
Speaker 1 (07:42):
So a day is definitely not a star day, which
is how much time it takes with the Earth to
do a full spin, and a solar day, which is
how much time passes between when the sun is at
its peak in the sky, changes throughout the year. So
what exactly is what we call a day? What does
(08:02):
the official day that we use in our calendars and
clocks and phones represent? So if it's always changing, how
do we pick what an official day is? I wonder
if what we picked us the official day will be
plus exactly four minutes and minus exactly four minutes, or
is it like plus two minus six Do you know
(08:24):
what I mean?
Speaker 2 (08:24):
So I think in the end we chose the average day.
Speaker 1 (08:27):
Oh fascinating, Wow, actually, we were wrong here. What we
use as the official day in our everyday lives is
not the average day. It's something a little different. When
we come back, we'll tackle what a day actually is
and we'll answer why a week is made up of
(08:48):
seven of those days. Stay with us, we'll be right
back and we're back, all right. We talked about what
a day actually is, and we learned a day is
(09:10):
not really a day. It's not how long it takes
to do a full revolution of the earth, and it's
not the amount of time between when the sun is
at its highest peak in the sky twice in a row.
That time changes throughout the year. But now let's go
small and look at the time units that are shorter
than a day, and let's start with the hour. Have
(09:32):
you ever wondered why there are twenty four hours in
a day? Where did the hour come from? Why does
the day have twenty four hours and not twenty three?
Why not thirty? Why not fifty?
Speaker 2 (09:44):
Yeah? So where did twenty four come from? It seems
like a pretty random number to us to divide something
into that cycle, And it came out of wanting twelve
hours in the day and twelve hours at night. This
is what the Egyptians and Babylonians wanted because it's obvious
if you count on your fingers, you count to twelve.
Speaker 1 (10:02):
Right, Wait, how many fingers do you have?
Speaker 2 (10:06):
Man, So it's not that the Babylonians had extra fingers.
It's basically a different way of counting, and you actually
can count higher if you use your hands in this
way that you count on your knuckles.
Speaker 1 (10:20):
Uh huh.
Speaker 2 (10:21):
So each finger has three knuckles, right, and so you
do that for your four fingers, and you can count
with your thumb.
Speaker 1 (10:28):
Okay, this is a little hard to explain on the radio,
but basically the idea is that you can count to
twelve using one hand. This is how it works. Hold
your hand out so that you're looking at your palm.
Now turn your thumb in so that it touches the
bottom joint that connects your pinky to your hand. That's one.
(10:49):
Then move your thumb up and touch the middle joint
of your pinky. That's two. Now move it up again
and touch the top joint of your pinky. That's three.
Then move on to your ring finger and touch the
bottom joint that connects to your ring finger to your hand,
and repeat.
Speaker 2 (11:09):
And you go down your pinky one, two, three, go
down your next finger one to three knuckles, four fingers.
That adds up to twelve. So you can keep track
of twelve pretty easy.
Speaker 1 (11:21):
Oh, I see, I see.
Speaker 2 (11:22):
So that was where the base twelve system came from,
and so that was how everyone used to count.
Speaker 1 (11:29):
Really.
Speaker 2 (11:30):
Yeah, this idea of us counting on ten fingers is
kind of a new thing that came basically from the
Romans when we think, oh, it comes naturally from our
ten fingers. But the original base twelve system was an
easier way to count things on your hand. So this
base twelve system is called the duo decimal.
Speaker 1 (11:47):
System, not the Dewey decimal system, yes, not.
Speaker 2 (11:50):
The Dewey decimal system, but the duo decimal system.
Speaker 1 (11:53):
So you can count to twelve with one hand.
Speaker 2 (11:55):
Yeah. And so this was the basis of bartering in
the market or counting, you know, things that you were
going to sell or whatever it might be. Started as
base twelve, and so it was natural to use that
system to count the hours of the day. And so
they split the day into twelve hours and then split
the night into twelve units.
Speaker 1 (12:16):
I see.
Speaker 2 (12:16):
And the night comes from the importance of the signs
of the zodiac. There's just twelve constellations that are pretty
equally spaced throughout the night sky, and so you could
use these zodiac signs over the course of an evening
to see how many hours you had gone.
Speaker 1 (12:32):
Wait what Yeah, so over the course of the night,
the constellations will kind of like parade across the sky.
Speaker 2 (12:40):
Yeah, and so you know when this constellation set that
an hour has passed. And then they decided, oh, we
need a little more precision, so they broke these up
into things called decans, which are a third of a
zodiac sign. So they took like, ah, this star from
that zodiac sign and this star from that zodiac sign
of this and broke them up in threes to give
(13:01):
them a little more accuracy over the night.
Speaker 1 (13:03):
I see, But wouldn't that differ depending on the season.
Speaker 2 (13:07):
Well, which zodiac signs are up in the night sky
changes with the seasons, but not the number of them
that will pass.
Speaker 1 (13:15):
There will always be twelve of them parading across the
sky evenly space about one hour each.
Speaker 2 (13:20):
Yeah.
Speaker 1 (13:21):
Oh fascinating. And so it was like this merger of like,
we like twelve because we can count with our fingers,
and also there's these things, these markers in the sky. Yeah,
I always thought that it was twelve because twelve have
so many factors to it.
Speaker 2 (13:36):
Well, that was the other convenient thing of the base
twelve system is you could divide by three and by four,
and all these things were natural units. So three, four,
there were these easily countable numbers that were the base
of all these and so early mathematicians got very excited
about these kind of patterns.
Speaker 1 (13:54):
Fascinating. So that's why we split the day into twenty
four hours.
Speaker 2 (13:58):
Yeah, was because of the obvious counting of twelve on your.
Speaker 1 (14:01):
Hand, and so an hour is one twenty fourth of
a day.
Speaker 2 (14:08):
Yeah. And why they chose to space twenty four rather
than twelve, that I think was just kind of a
random choice. They wanted the unit to be twelve for
the day and twelve at.
Speaker 1 (14:19):
Night because otherwise that Keefer Sutherland show from the nineties
would have been called twelve hours, not twenty four hours. Yes,
all right, so then that's the hour. So the hour
is dependent on what we define as a day. Yes,
so that's an hour, it's one twenty fourth of a day.
And now we get to minutes and seconds But first,
it's interesting to think about why we would want to
(14:40):
split the hour in the first place.
Speaker 2 (14:44):
As time went on, people cared about smaller and smaller
time increments. Right When back in the olden days, you
didn't care too much about an hour. It was just
good enough to say, is it morning or afternoon? Right?
But now when you had to go to work, because
now there was a building that you had a job
at and you had to get to that building, you
(15:05):
needed to know roughly what hour it was.
Speaker 1 (15:07):
Right, because we needed to start coordinating between humans, Like
I needed to know when I should go to work
because you were expecting me.
Speaker 2 (15:14):
Right.
Speaker 1 (15:14):
It's like complication that comes from a society.
Speaker 2 (15:18):
Yeah, and so when we didn't have society, we didn't
have those complications. Yeah, an hour's so complicated. You need
to know everything down to the mill sector.
Speaker 1 (15:27):
Yeah, it's all because of people. Yes, okay, let's get
to a minute. The reason the hour is split into
sixty increment also goes back to counting with your hands.
If you use one hand to count to twelve with
your thumb and finger joints, you can use your other
hand to count groups of twelve.
Speaker 2 (15:47):
And then you have your other hand free because you're
not counting any of those fingers.
Speaker 1 (15:50):
Uh huh.
Speaker 2 (15:51):
And so then you could say, oh, I counted one twelve,
I counted two twelves, I counted three twelve.
Speaker 1 (15:55):
Here doctor Ziegler is counting to twelve with his right hand,
and each time he reaches twelve, he opens one full
finger with his left hand, just like you would count
to five with your fingers.
Speaker 2 (16:07):
It's I kind of four twelves. I counted five twelves, huh,
which is sixty? Wait?
Speaker 1 (16:11):
Wait, so on one hand I use twelve, but on
the other hand you do use your five fingers.
Speaker 2 (16:16):
Yeah, so you can count to twelve five times before
you've used up all your fingers on both hands.
Speaker 1 (16:22):
But why wouldn't I use twelve times twelve? If I
can count to twelve in the one hand, it's just
too much.
Speaker 2 (16:27):
Then you have to be ambidextrious the other.
Speaker 1 (16:31):
So you can count to twelve with one hand, but
then you keep track of how.
Speaker 2 (16:35):
Many twelves you count on it with the other.
Speaker 1 (16:37):
Other hand, which and you use your five fingers, and
that's where we get sixty.
Speaker 2 (16:41):
Yeah. And so the natural ways of counting to Babylonian
were base twelve and base sixty. So why sixty minutes
to an hour was basically because we got used to
this base sixty system. They decided it was sensible to
use this system rather than a base twelve minutes system.
Speaker 1 (17:00):
Like, the word minute comes from the word sort of
first first division.
Speaker 2 (17:04):
Yeah, first division, and then second is obviously the second division.
Speaker 1 (17:10):
And that's why the second is called the second.
Speaker 2 (17:12):
Yeah, it's because it was a first division of an
hour and the second division of an hour.
Speaker 1 (17:18):
Man, I'm forty nine years old, and it's taking me
this long to figure that, to learn this fact. Yeah,
that's where the word second comes from.
Speaker 2 (17:28):
Yeah.
Speaker 1 (17:30):
Yeah, So that, according to doctor Siegler, is where the
word second comes from. It's the second division of an hour.
And we divide the hour and the minute into sixty
because that was the preferred number system of the ancient Egyptians,
except that in the nineteen sixties we decided to redefine
the second, which in turn change what exactly a minute
(17:53):
is and an hour and even a day. We'll dig
into that and we'll finally answer why the week has
seven days. After the break, they'll go anywhere, we'll be
right back, welcome back.
Speaker 2 (18:17):
Okay.
Speaker 1 (18:18):
So the reason we have twenty four hours to a
day and sixty minutes to an hour and sixty seconds
to a minute. Goes back to traditions we've had since
ancient Egypt, which makes you wonder why haven't we changed it.
Speaker 2 (18:35):
So it's a lot of just heritage and being stuck
with what works. One of the reasons it's stuck in
our culture is because it's always going on, right. You
can't just stop and say, okay, we're all going to
switch to the metric system of time.
Speaker 1 (18:49):
People are like, forget it, I'm too used to hours
and minutes and seconds. Don't make me think too much.
Speaker 2 (18:55):
Yeah, And you can see that those traditions are hard
to shake in even things like measurement right where we're
worried in America about oh, can we ever switch to
the metric system or not? But like if you switched
time suddenly, now everyone's off.
Speaker 1 (19:10):
Okay. So for a long time, a second was a
subdivision of a subdivision of a subdivision of a day,
which you can measure using the average solar day. But
as we said, the solar day is a bit squishy,
it's not super precise. So in the nineteen sixties people
decided to standardize the second and ground it in a
(19:33):
precise physical phenomenon.
Speaker 2 (19:38):
So basically, by the nineteen sixties, we really were carrying
about smaller time scales than the second, and we need
to keep track of things really precisely. Someone in France
having a different second than someone here, it just wouldn't work.
So in the nineteen sixties we started carrying about these
smaller units of time more precisely. So they got together
(19:59):
internationally and said, okay, what are we going to do
to develop the second? And they decided, here is this
vibration that happens in the caesium atom.
Speaker 1 (20:08):
Uh huh.
Speaker 2 (20:08):
So we take this caesium atom and it turns out
that nine trillion, one hundred and ninety two million, six
hundred and thirty one thousand, seven hundred and seventy vibrations
of the caesium atom happen every second.
Speaker 1 (20:23):
Huh.
Speaker 2 (20:23):
They said, from now on that will be the second.
Speaker 1 (20:26):
Whatever I was before, I forget it. This is what
it is now exactly.
Speaker 2 (20:30):
So now the second is very precisely calibrated to this
transition of the caesium.
Speaker 1 (20:35):
Ad why the casion atom, why not the carbon atom
or oxygen atom.
Speaker 2 (20:41):
It was just an atom that had this property of
a very regular vibration.
Speaker 1 (20:47):
All right, So we redefined the second to be the
precise vibrations of a caesium atom, and this technically redefines
the minute, and the hour and even the day. Now
here's my question. If I take the second according to
the season atom I multiplied by sixty, I now get
an official minute, right, Yes. If I multiply that by sixty,
(21:10):
I get a new hour, which is now the official hour. Yes.
And if I multiply that by twenty four, I now
get the official day, right yeah. So now how does
that day match to the solar day that we talked
about before, Like, I wonder if it's really the average
now or fit's deviated from the average.
Speaker 2 (21:28):
Yeah, so it doesn't match. Well, that gets into why
in the nineteen seventies we started adding these leap seconds
into the calendar to correct ourselves back to this caesium
atom standard of time.
Speaker 1 (21:44):
Yes, you might have heard of leap years, where every
four years we add February twenty nine to the calendar
to keep us synchronized with the orbit of the Earth
around the Sun. Well, we also have leap seconds, and
that is when we add a second to the official
time of the human race, called coordinated Universal Time or UTC,
(22:05):
to make up the difference between the official day, which
comes from multiplying the official second that comes from the
caesium atom, and the actual solar day, which again is
the time between when the sun reaches high noon. In
nineteen seventy two, when we first realized we needed to
do this, we added ten seconds to everyone's clock, and
(22:27):
since then, according to the National Institute of Standards and
Technology or NIST, we've added one second to the world
clock about every year and a half. So about every
year and a half, without most of humanity knowing it,
our clocks repeat one second right at midnight on either
New Year's Eve or the end of June. But everyone
(22:52):
has to remember to do that, do you know what
I mean? Like that is a huge logistical burden for everybody.
Like whatever you do, just remember everyone in the world.
We need to add one second this year, not next year,
not the even before this year. We need to everyone
at a second.
Speaker 2 (23:07):
Yeah, and you know, right now, at least in history,
we don't care about those super small time scales that much.
But maybe in another hundred years, being off by a
few milliseconds is going to be an issue societal, right.
It seems so messy, yeah, I mean, and it's messy
when we're trying to match this traditional system and.
Speaker 1 (23:26):
To match it to like giant physical processes like the
rotation of the Earth and whether it sinks to the
orbit around the Sun. I mean, these are huge things, right, Yeah.
Speaker 2 (23:37):
And so to get to the month, we start to
throw in the moon cycle, which is another bizarre excerp.
Speaker 1 (23:47):
Okay, let's get to the main question we set out
to answer, which is why does the week have seven days?
What do you know about why the week has seven days?
Speaker 2 (23:57):
So the original week back in ancient Egypt was actually
ten days a week, and this came from trying to
get this daily cycle to match the lunar cycle. So
the moon appears to go from it's noon that we
see the full moon to the full moon happens in
about twenty nine and a half days, Okay, And so
that was close to thirty days.
Speaker 1 (24:17):
Uh huh.
Speaker 2 (24:17):
So the original idea was that thirty days are important.
And then for some reason, because thirty couldn't be broken
up by twelve, they said, ah, let's break it up
into three weeks and so there were three weeks a
month that added to thirty days, and that roughly, you know,
at twelve months of thirty days, you got to three
hundred and sixty, which fit the nice base sixty system,
(24:40):
and that's how it was. Then you have this other
cultural tradition of the Jewish tradition and where that became
the Christian tradition of the seven days a week.
Speaker 1 (24:51):
Okay, before we get to that, I feel like you're
saying like maybe the original concept of the week was
somehow related to the month, and the concept of the
month was sort of to the moon cycle. So what's
the moon cycle?
Speaker 2 (25:03):
So if you look at the moon, for it to
go from full moon to full moon, that cycle is
going to be about twenty nine and a half days.
But the rotational cycle of the moon is actually different.
How often the moon goes around the Earth and gets
to the same place in the sky, uh huh is
actually different. It's like twenty seven point three days. And
(25:26):
so that gets really complicated as well. Do we count
where the moon comes back to the same spot or
do we count where the moon goes through its whole cycle?
Speaker 1 (25:35):
Okay?
Speaker 2 (25:35):
And so it was a little bit of a which month,
do you choose develop which way you want to break
that month apart into weeks?
Speaker 1 (25:44):
All Right, What doctor Siegler is saying is that the
reason we have a week is to break up the month,
and the month is related to the cycle of the moon.
Except there are two moon cycles, just like there are
two day cycles. There's how long it takes for the
moon to do a full rotation around the Earth, and
there's how long it takes for the moon to go
(26:04):
from full moon to full moon. One it's about thirty days,
which lends itself to have a ten day week, and
the other is about twenty eight days, which lends itself
to have a seven day week.
Speaker 2 (26:18):
And so you could split the month into four seven
day weeks if it was a twenty eight day month,
or you could split it into three ten day weeks
if it were a thirty day But when the Romans
had to choose, huh, they basically went with the seven
day system because that worked better with whatever traditions they
(26:38):
were adopting.
Speaker 1 (26:40):
Well, from what I read, nobody really cared about the
week basically, like nobody really kept track of like, oh,
this important event happened on a Tuesday. But the Jewish
people obviously did back then. You know, for them, it
was a command from God not to work. And then
what historians seem to have found is that at the
same time, the Roman people had this seven day cycle
(27:03):
that was just based on astrology. So they looked at
the sky. They could see five planets, the Sun, and
the moon. I imagine they couldn't like really see Neptune or Urinus.
Speaker 2 (27:12):
Right, yeah, those weren't discovered until late seventeen hundreds. We
found Urinus and then Neptune was even later, so.
Speaker 1 (27:19):
Right, right, but the others you can see in the sky,
right yeah, So they like each God must be in
charge of the day. So that's why we have this
astrological keeping track of seven days at a time. So
then it's sort of like this nebulous process of we
sort of have this planetary astrological seven day week. The
Jewish people are keeping track every seven days, and then
(27:39):
Christianity adopted this Sunday as the day of worship and
so let's make it a seven day week.
Speaker 2 (27:46):
So it all kind of came out of this conglomeration
of cultures.
Speaker 1 (27:50):
All right, here's the basic scenario for the month. We
seem to have based it on the lunar face cycle,
the period of time between two full moons, which is
about thirty days. But for the week, we opted to
base it on a multiple of the orbital lunar cycle.
How long it takes for the moon to go around
to Earth, which.
Speaker 2 (28:11):
Is twenty eight days, probably.
Speaker 1 (28:13):
Because of the cultural traditions of the Jewish Sabbath and
Roman astrology, which had seven days. But here's a twist,
how long it takes for the moon to go around
the Earth has been changing.
Speaker 2 (28:29):
The month wasn't always this twenty nine and a half
day cycle. We have this whole idea of the moon
formed from a giant impact with the Earth and you
form the moon. The original moon in that idea would
have had a four day month, and then due to
the interactions with tides on the Earth. Uh huh, the
moon has in the last several billion years been moving
(28:49):
outward towards where it is today, and the month is
slowly getting longer all the time. So this hasn't happened
over the course of human history, right in the last
million years or so that you could say humans were around.
But if you go back in the geologic record, you
can see times when the month was actually shorter.
Speaker 1 (29:06):
What you mean, like, if we had lived in a
different time in the history of Earth and created the
month like a billion years ago, there would be more
months in the year exactly. Okay, So basically the reason
we have twelve months a year is that we just
happened to be living in his time in the history
of Earth, where the moon goes around the Earth twelve
(29:29):
times roughly a year.
Speaker 2 (29:30):
Exactly, and we picked twelve rather than thirteen because twelve
was nice to count on our fingers.
Speaker 1 (29:38):
And because of the month. That's how we picked the week,
because a week, if we really probably just picked it
as a division of the month. Yeah, well, there you
have it. The reason the week has seven days is
partly cultural and religious, partly due to the physics of
the Moon's orbit, and partly coincidence. If human said come
(30:00):
around earlier or later in the history of the Earth,
the week might have been shorter or longer. So if
you're having a long week, just think it could have
been even longer. Thanks for joining us, See you next week. Hey,
please take a second and leave us a review on
(30:22):
Apple Podcasts, Spotify, or wherever you listen to the podcast.
Speaker 2 (30:26):
Thanks a lot.
Speaker 1 (30:28):
You've been listening to Science Stuff the production of iHeartRadio,
written and produced by me Or Hitcham, edited by Rose Seguda,
executive producer Jerry Rowland, and audio engineer and mixer Casey Peckram.
And you can follow me on social media. Just search
for PhD Comics and the name of your favorite platform.
Be sure to subscribe to Sign Stuff on the iHeartRadio app,
(30:49):
Apple Podcasts, or wherever you get your podcasts, and please
tell your friends We'll be back next Wednesday with another episode.
Popular Podcasts
On Purpose with Jay Shetty
I’m Jay Shetty host of On Purpose the worlds #1 Mental Health podcast and I’m so grateful you found us. I started this podcast 5 years ago to invite you into conversations and workshops that are designed to help make you happier, healthier and more healed. I believe that when you (yes you) feel seen, heard and understood you’re able to deal with relationship struggles, work challenges and life’s ups and downs with more ease and grace. I interview experts, celebrities, thought leaders and athletes so that we can grow our mindset, build better habits and uncover a side of them we’ve never seen before. New episodes every Monday and Friday. Your support means the world to me and I don’t take it for granted — click the follow button and leave a review to help us spread the love with On Purpose. I can’t wait for you to listen to your first or 500th episode!
Stuff You Should Know
If you've ever wanted to know about champagne, satanism, the Stonewall Uprising, chaos theory, LSD, El Nino, true crime and Rosa Parks, then look no further. Josh and Chuck have you covered.
Dateline NBC
Current and classic episodes, featuring compelling true-crime mysteries, powerful documentaries and in-depth investigations. Follow now to get the latest episodes of Dateline NBC completely free, or subscribe to Dateline Premium for ad-free listening and exclusive bonus content: DatelinePremium.com