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