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to welcome you >> Jim Lattis: I'd like
Our presentation tonight is on a to Space Place tonight.
just lately, in the news a lot topic that's been of interest
business about a 13th sign of just latethe zodiac.t is this
So what's all this about a 13th sign of the zodiac?
Turns out there's been a lot of discussion about this, a lot of
which fails to actually define
using, and so I have found that evethe explanations of this's
have been lacking everywhere. So I've tried to take us
through it here.
way, well a bit about, We talk a lot about, along the
And so this is not a talk about the validity of astrology, but
just for the record astrology is
about it some other time. nonsense an[LAUGHTER]ave a talk
But that's not what we're going to talk about tonight.
But these things do come into the foundations of astrology,
which is certainly important in the history of our culture, but
also, and just as much, in the
And we'll see that as we go history along.ronomy.
So what's all this about
The claim that we've been hearing, as I refer to these
news stories that we hear, is that the precession of the
equinoxes has caused there to be
And then we also hear that the a 13th sign of the zodiac.
Ophiuchus, or is named new sign is the constellation
constellation that few enough people have heard of that just
the very mention of it often
But it's a perfectly good causesconstellation.irth.
It's easily as old as all the rest of them that we've all
heard of, and we'll talk about
But we need to get a little bit of background, which is exactly
where many of the news stories
We need to get a little bit of have been failing.
I'm going to use this software background, and so to do that,
called Stellarium, that I like to use and recommend
of the sky that we need to talk so thabout here in order toparts
understand the basic ideas. Sign of the zodiac suggests that
we need to know about the zodiac
Here we have the sky, shown actually in the daytime but with
the atmosphere turned off so
there and planets and the stars tall conveniently the way wet
never see them unless we live on
During the course of the year as the earth orbits the sun, the
sun appears to move through the sky, makes a complete track
through the sky, 360 degrees
during the course of the year. all the way around
particularly is that changes in latitude in the sky cause our
seasons and that kind of thing. But for our purposes, what's
important is if you could see the sun moving through the sky,
as well as the planets, which of
the sky, against the background coof stars, if you tracked itugh
during the course of the year you would find that it follows
this path.
course, a section of it which goes all the way around the sky.
And the path of the sun during
as it's labeled there, the course of the year is,
That's the path of the sun. We always find the planets,
for example Mercury and Venus,
visible planets, and the moon balways in the general vicinitye
of the ecliptic.
perfectly on the ecliptic. You can see they're not
little bit. So the ecliptic defines this
region of the sky that is where we always find all the solar
system activity, the motion of
The ecliptic passes through, the sun, moon, and planets.
if you like, constellations. we say, or in front of
across which the ecliptic Anpasses, so the background ofns
stars here, that set of constellations is called the
zodiac. But now I just used the term
that we actually have to talk about just a little bit and that
is a constellation. So what is a constellation?
Most everybody has an idea of what we mean by a constellation,
and most of us probably think of
mention the word, as a star a constellatpicture.n you first
And Stellarium is perfectly good
Stellarium will fill-in these star pictures for us, which are
generally pretty closely related
constellation. to the name of the
them, though, is by connecting The way most of us learn to see
or using the stars as dots to connect together lines to make
star pictures and little
can remember. chThe star picture idea of ae
constellation is quite ancient. It's probably the original idea
of what a constellation is, although not necessarily,
it might be that people thought of regions of the sky
as dedicated to one concept or
together a picture. anotheI don't know.ter put
But right now, our idea is
of thing represented up there. usually that there's some kind
the constellation of Scorpius, For example, right over here is
Scorpius was our friend that the scorpion, and right above
about, Ophiuchus right up here. weAnd I'll just turn theng
constellation art back on for a moment.
I'll turn on the star labels
So now you Ophiuchus and there too.
In the oldest star catalog that we have, we have a star catalog
that's a couple thousand years
who's going to come up again, the ancient Greek astronomer
Ptolemy, compiled a star catalog
but we don't have those. based on earlier star catalogs
star picture idea as the basis Ptolemy star catalog uses the
In the star catalog it's the of its organization.
example. star constellation Ophiuchus for
head. That's actually how this stare
which we have a name for, Rasalhague, an Arabic
name, we can also call it alpha --, that star in Ptolemy's star
catalog is the head.
within Ophiuchus. It jAnd then he gives theaning
coordinates of it and estimates its magnitude.
And that's how the whole star catalog works.
So then there'll be the right hand under Ophiuchus, or the
left hand, the right leg, the left leg, whatever.
All the constellations are
The head of the eagle, the tail of the eagle and so on.
So the star picture idea is intrinsic to this organization
that comes down to us from
constellation has meant for most antiquity, and that's what a
However, there is another way of of astronomical history.
find more often in star catalogs thinking about them that you'll
artwork. and things these days than the
belongs to one constellation or another these days, they're
usually referring to these
the red lines. boundary lines that you can see,
enough for you to see them. These are the constellation
boundaries that were identified,
word, by the International set up to find, I guess is the
And sort of like setting up Awhere Utah ends and Nevada
begins or whatever, the boundary
constellations bud up against each other, and they don't leave
any unmapped space in the sky.
where there can be gaps and where if Ptolemy doesn't mention
a particular star you don't know
far as a constellation, the boundary lines don't have that
problem.
entire sky, and so it's unambiguous whether a given star
is part of Scorpius or part of
So down here, for example, is Ophiuchus which kind of extends
down in that direction. You can sort of see the boundary
lines.
boundary lines can extend down I hope you can see that the
Scorpius begins. there, and Ophiuchus ends and
which are sort of in between And these stars right here,
here, those stars are designed Scorpius and Sagittarius right
modern definition. to be part of Ophiuchus by the
That's the modern idea of what But that's only since 1922.
We're going to talk mostly about it means to be a constellation.
how people have seen them for a the star pilong time.use that's
So let's turn the star pictures back on, and you might be able
to notice here a little bit better that there's Scorpius,
once again, and there's Sagittarius, the well-known
teapot asterism right there, and
kind of a straggling bit of stars which, it turns out by the
modern definition, fall on the
of them up here and then some of Ophiuchus side of the line, some
actually falls, some of them them fall wherever the line
They're kind of right in there fall in Scorpius.
happens, the ecliptic runs right in the middle, and, as it
So by both the modern definition through there.
Ptolemy's ancient definition, it's possible that the ecliptic
moves through the constellation Ophiuchus.
If you decide that that's part
By the star picture idea it depends on whether you want the
star picture's leg to extend
the sky. that far down into that part of
definitely passes right through By the modethere.ndaries it
Well, one of the things that's
of the equinoxes causes this to beebe a relatively new thing.ion
We're going to see that that's
In fact, the ecliptic runs right not the case.
least what might be a part of through OpOphiuchus.ere, or at
By the way, in Ptolemy's star catalog, when he gets to those
stars down there in that sort of, this end, I guess it's the
end of Ophiuchus' leg or
of trails off, when Ptolemy's star catalog in modern editionsd
gets down there you'll find that
by all of those stars because they're not actually sure whichs
stars Ptolemy is talking about. There's some ambiguity in, we
get this ultimately out of
centuries old, there's some ambiguity about just which stars
Ptolemy is talking about. So it's possible that Ptolemy
thought of Ophiuchus as ending
there. considerably farther north
fuzzy, fuzzy enough that we'll But that's all a little bit
possible that Ptolemy understood just have to say that it's
Ophiuchus as well. the ecliptic to run through
that's the zodiac, why are there Okay, so that's constellations,
There are 12, as you know, signs 12 of them?
Why isn't Ophiuchus a sign of othe zodiac?c.
Well maybe because ancients didn't consider those stars to
be part of Ophiuchus, but maybe it's because 12 is a convenient
number and a 13th one would be
There are 12 full moons during unwelcome and inconvenient.
example. the course of the year, for
to divide into 360, and the idea 12 is a very convenient number
parts is a very old idea. 12 would have been a much more
convenient number if you wanted to divide up the ecliptic.
And there's good reason to think
wanted to do. thThey wanted a good scheme tole
divide up the ecliptic. And I say that because now we're
going to talk about the idea of a sign.
What is a sign of the zodiac? Since antiquity, let's just
slide this over a little bit, since antiquity, astronomers and
astrologers both have divided up
ecliptic really, have divided up the zodiac, I should the
divisions of 30 degrees each, adding up then to 360 degrees
all the way around. You don't have to look very long
at the constellations here to see that they don't all occupy
the same angular width. Only a short bit of the ecliptic
passes through Scorpius here. More of it passes through
Capricornus here and Aquarius.
ways depending on where you Piscwould decide to draw thee a
boundaries. Aries is where the coordinate
system has always started,
but those aren't always the same either in name or in actuality,
Aries is where the system of thing.
So the first sign was said to be signs traditionally started.
it up into 36, I'm sorry, into Aries, and then you just divide
divisions into 30 degrees each 30, I'm sorry, into 12 equal
And the precise reference point was the vernal equinox, which is
the point where the sun crosses the celestial equator.
So here we are, this has shifted
about precession, this is where it is now because we're set for
2011 here.
what divides the northern sky from the southern sky, and this
is the path of the sun. And when the sun crosses the
celestial equator headed
equinox, which happens around March 21st, and it's that point
in the sky which was considered to be the origin of the system
of signs. So the sign of Aries started
there and then went for 30
after Aries and so on all the way around until you'd done all
12. So already using a system like
that it's not really very important whether you're talking
about the stars of Aries because
is the position of an object in whthe sky and how many degreesut
past the vernal equinox it is.
system of signs was designed to That's whatdo.e coordinate
And astronomers and astrologers both use that system for quite a
long time.
necessarily, in fact the signs So the signs are not
constellations. aYou could think of them as ae
series of 30-degree segments
constellations which were nearby that are named after the
defined, whenever that was. aWell, so I mentioned that thiss
point has moved, and what I'd like to do is talk about why
this point, the vernal equinox, has moved.
This, what we call, precession
I'm going to switch back over to of the equinoxes.
little bit about the history of the other precession. and talk a
Actually, this has been known for quite a while.
The precession of the equinoxes was discovered by a guy who
lived around 150 BC, the Greek
Hipparchus was looking at records of positions of stars
that were already quite a bit older than him, at least a
couple hundred years old at the time that he was studying them,
and he compared the positions of
the positions of the stars in his day and found that the stars
had shifted eastward with respect to the vernal equinox.
I'm going to go back to Stellarium and show you this in
just a minute.
drifted eastward with respect to He found that the stars had
He estimated that they had the vernal equinox.
in 100 years. So it's 360 degrees all the waye
around, so one degree during a
certainly, that a casual hundred years is not something,
And unless you were making high precision measurements, not
something that you would care about even over generations.
But over a number of generations
that they had shifted. Hipparchus was able to establish
of the stars. He thought of the equinox as the
fixed thing because he was, like
The earth was stationary and most people, a geocentrist.
was the stars that were central in shifting.s, and so it
The stars were doing a shift
degree per one hundred years. Hipparchus' astronomical worke
was continued, I'm skipping over
not like these were the only two lots of other astronomers, it's
were sparse, but Hipparchus' guys in antiquity, although they
by Ptolemy, the guy I mentioned worka little while earlier.inued
Notice he lives about 300 years later than Hipparchus, so this
was not an ongoing research program or something.
But Ptolemy came to gather up Hipparchus' work, redo
Hipparchus' star catalog into the one that we have inherited,
and Ptolemy's star catalog, the one in the Almagest, is the
reason why he had to deal with
stars were shifting because of precession because he knew the
figure how much they had shifted Hipparchus' work, he had to
their coordinates into his star during that catalog.order to put
He wanted to correct for this precession.
He decided, on the basis of observations going back to
Hipparchus and perhaps even before, that Hipparchus had had
the right number of about one degree per century, which means
that the whole cycle of precession would take 36,000
years.
stars, which were imagined as Dufixed to a sphere out at thee
outer edge of the universe, and we'll see some diagrams of that
in a minute too, would rotate around the earth one complete
revolution in 36,000 years. Now, that's not the way we see
it now. So just to make sure that we are
talking about modern physics for
precession of the equinox is not a minute, the way we understand
stationary, of course, but that the earth is central and
orbits the sun annually and rotates on its axis daily, the
axis of the earth also has
Usually we will think from another motion.
think too much about precession, day-to-day because we don't
us, we think about the earth's it takes a long time to affect
the same direction in space axwhich is basically up to then
north and south celestial poles.
get to look up to Polaris. In the northern hemisphere, we
here you would find the star So from directly above the pole
But, in fact, the earth's axis Polaris up there.
So it's carried around in a circle just the way a gyroscope,
if set on a table, or a top will
It spins very rapidly, wobbles precess.
Its axis makes a conical motion which is called precession, the
earth does that as well.
measurements put that rate about And the modern a rate, modern
So a little faster than Hipparchus and Ptolemy thought.
That comes out to about 26,000
axis to make one swing around here and come back to pointing,
in this case, at Polaris. So 26,000 years from now, the
earth will have done one
back up there at Polaris, if complete wobble and be pointing
Polaris could have wandered over Polaris is still there.
Polaris didn't, then we have that amount of time, but if
but only after 26,000 years. Polaris as the north star again
All righStellarium. back to
All right.
did this precession look like to So what is this precession, what
One of the great things about this software is that we can
find that we can set Stellarium back to dates long past.
Here we are in 2011, but
said, around 150 BC. Hipparchus lived, as we just
So let's putOops.egative 150.
There we go.
Here we are and we're looking at 159 will do.
seen it. the sky as Hipparchus would have
the vernal equinox is, so here's And when we look to see where
crosses the equator headed the ecliptic and where it
equinox, and here we see the northward, that's the vernal
the middle of Pisces. Here's the constellation Pisces
right here. If we go back further, let's go
back another thousand years or
more clear. so just tThere we are.fect even
We'll go back about a thousand
here's the vernal equinox yeaoccurring clearly in theBC
vicinity of the constellation of
Now it would make sense to start Aries.
You'd say, oh, okay I'm going to your coordinate system in Aries.
I'm going to call that 30 degrees Aries, and the next oned
is going to be Taurus and so on around there.
By Hipparchus' time, where we just were a minute ago, go a
thousand years ahead of time, the vernal equinox had moved
this far. By Ptolemy's time, the vernal
equinox had moved from over here
So you see what's happening. to over there.
westward or, the way they were The equinox seems to be going
going eastward. The stars are drifting eastward
kind of slowly. Let's go a thousand years after
Ptolemy.
here. You see that the vernal equinox
is now over on this side of Pisces.
So at 1,000 BC it was over here,
for Hipparchus and then Ptolemy and a thousand years later it's
drifted this far, and if we come
we'll find the vernal equinox back to toddown here.se enough,
So look how far it's gone.
3,000 years ago, and in that It started out over here about
Aries and most of the way across amount of time it's gone from
and you've all heard about how Pisces and headed for Aquarius,
when the equinox gets there into great things are going to be
That's what that's all about, the vernal equinox moving into
Aquarius. And that's, sure enough, where
precession is taking the vernal equinox.
So let's just take a quick look at what this, so this is what
Hipparchus saw because the
that he could measure. vernal equinox was something
about was looking at the pole, But what we were just talking
So if we go up here to get a up at the north celestial pole.
pole, we'll turn on some look at the north celestial
Here's the north celestial pole right there, and if we go
backwards in time, we'll go back, the easiest thing for me
to do is just slip a negative sign in there and we'll jump
back 4,000 years. All right, there's Polaris right
there.
pole. AnSo if you go out tonight andal
look up, look for the Big Dipper
the Little Dipper right there, right over there and look for
pyramids were brand new, the 4,000 years ago when the
halfway in between these two north celestial pole was sort of
the bowl and handle of the Big stars and the Little Dipper and
star in Draco called Thuban. Dipper right out there near a
a thousand years, oops, there, a And if we creep forward in time
the pole, our pole star seems to thousand years, we'll see that
What's actually happening is the have moved closer.
around because of precession and pole of the earth is swinging
we're seeing, again, is the comotion of the stars becauseat
Stellarium wants to keep the
there. coordinate system centered
here's the pole star. It's approaching right here, or,
you can think of it as the pole approaching the pole star.
And then we'll go another thousand years, and then we'll
just do the other thousand
Oops, I better take out that years.
The other thousand years and negative sign, sorry.
looking situation where the pole here we are back to the familiar
there. is very close to Polaris right
very same motion that's moving So that's the same motion, the
direction that the pole points. It's just that we tend to think,
in the modern idea, we tend to
points. That's sort of the Newtonian
mechanics idea because we think about the earth as a gyroscope,
but the astronomical idea, the
idea was the precession of the same motion, the astronomical
ancient, their older positions. All right, now we want to go
back here to the area of the
vernal our slides here.ack to
All right, so what's this got to
signs and the astronomical do now with the astrological
Well, about 3,000 years ago the astronomical/astrological sign
system started in Aries, and
to specify where they saw things astronomers used that as a way
that they did. in the sky, at least we presume
point, was pretty much But that sign system, at that
It doesn't matter anymore whether they're attached to the
constellations or not.
started at the vernal equinox, And the sign system always
equinox was shifting. This is just a generalized idea,
a generalized graphic to show that the signs here, this is
from the 16th century astronomy textbook, this was how the
author illustrated the idea of
This is an astronomy book, and the signs.
30-degree segments that divide the signs are these equal
ecliptic. Notice that the stars are shown
here, but they're not constellations, it's just that's
how we divide up the stars. The signs always started at the
vernal equinox, 30-degrees each,
but they were not coincident named after the constellations,
and farther away from them. Always begin with Aries, that's
the conventional place. And the precession moves it in
such a way that the signs, for
completely arbitrary. How is that different from
astrological signs? Well not very.
In fact, the astrological signs were almost the same.
This slide is intended to show
the same period, a different you the precession from about
intended to show the precession textbof the equinoxes.de is
Here's the earth at the center
going to talk about all the planets, but there are all the
planets orbiting the earth
out here, you get out to the including the sun and moon, and
sphere of the stars, what was then called the eighth sphere.
That was considered the to be the last visible part of the
universe out here or the
stuck. firmament where the stars are
are represented by the signs. AThere are the symbols for thee
signs, and what I really wanted to point out here and I'm going
to talk about how they imagined this working in just a minute, I
wanted show that there's another
These are in the ninth heaven. set of signs out here.
any stars. The ninth heaven doesn't have
it's out there, and it has what were thought of as the realt
signs back in the background
front. with the stars shifting in
look like the stars, they're So these are not, although they
look like signs here but they're really intended, sorry, these
the real constellations because actuallthey've shifted.present
See that shift? That's the precession.
That shift, that distinction
firmament and the signs back between the stars in the
that start with Aries. there in the celestial sphere
uses, the signs, even though Now they had really practical
stars, and here's just an they don'texample.p with the
This is a replica, astrolabe, but you would always find on the
back of an astrolabe, this is just convenient maybe for
obvious reasons, on the back of an astrolabe there's a scale
that allows you to convert dates
So, for example, today, February into signs of the zodiac.
a line from the center out, it 8th there, ends up, if you draw
date is in about the 19th degree tells you that the sun on that
That's what that's telling you. So the sun is in the 19th degree
of Aquarius, and Aquarius doesn't mean the constellation,
it means the sign.
degrees for every constellation And soalong the way. up 30
So from Aries, Taurus and so on,
these constellations, plus the 30 degrees for every one of
Aquarius, and that's how many degrees away from the vernal
equinox the sun had moved.
longitude, and we just give you We,a number for God's sake.ial
It's 272 degrees or whatever it is.
But the way it was traditionally
Aquarius. So it's a very practical thing.
This is not unique to astrology in any way.
So the astrological signs, how
astronomical signs? aWell actually not at all.e
They're just exactly the same.
equinox, always have 30 degrees Alwall the way around.rnal
The only difference is that astrologers attach significance
to the signs. So Pisces, because the
constellation Pisces is a watery
tell you what that means, don't ask me, but it means something
to the astrologers. That's the only difference.
They're the same. It's the same set of signs, they
start the same way. They divide up the sky in the
same way. So, the question then really is,
is there anything to talk about in terms of a new sign?
I'm going to go back to Stellarium for a minute here.
Let's see I have to click over
All right. here.
look back over at the situation with Scorpius, Sagittarius, and
Ophiuchus right here, and let's ask ourselves whether the
precession of the equinoxes has changed anything over here.
We're going to do the same
vernal equinox and with the pole experiment that we did with the
in this part of the sky to see except we're going to be looking
its relationship to Ophiuchus in whether the ecliptic has changed
So I'm just going to slip in a the last 4,000 years.
Now the angle has tipped a minus sign in front of there.
Scorpius. There it is passing through
Ophiuchus, and there is Sagittarius.
4,000 years ago the ecliptic
modern Ophiuchus at any rate, wjust the way it does today.,
And if I went back a thousand years at a time, you wouldn't
see any significant difference
So the claim that the precession at all.
Ophiuchus to be different somehow today than it was 4,000
years ago is clearly not the
There have been shifts in where the pole points, shifts in the
location of the vernal equinox, but the path of that red line
through the sky is defined by the plane of the earth's orbit
which is much more stable than
And the background of the stars, most of those other things.
variation, but the background of although there is a slight
essentially the same. So Ophiuchus is not new to this
game. Ophiuchus was just never
included from the beginning for whatever reason.
Either because those stars
Ophiuchus or, more likely, weren't considered to be part of
convenient number and who needs a little chunk thrown in there.
Whatever the reason was,
and things haven't changed right Ophiuchus was never in the game,
So to talk about a change is now.
need to introduce Ophiuchus in wrong and to suggest that we
because the signs have been divorced from the constellations
for millennia anyway. Well, what I think is
interesting about this, so my answer is I can't see what all
the fuss is about, there's no need for is 13th sign, nobody
uses the signs anyway except
weren't connected to the real asconstellations even inns
antiquity.
because I think there's actually Let's go back to the slides
talk about. And I'd like to close with that.
What's interesting here is the
understand the precession of the way people have tried to
Let's go back to this slide here equinoxes.
If you go back and look at the from Peter Apian.
sphere here, and I mentioned the sphere of the stars, the eighth
displacement between the visible little deviation here, the
out here, the ninth sphere, what stars and this invisible sphere
here, what are these funny you'll see if you come around
what's that? little wheels right there,
And they're right there. And there's another one.
This funny little wheel is stuck That's Aries.
Aries. right here at the beginning of
the middle ages astronomers have Those are in there because in
than Hipparchus and Ptolemy did. a different idea of precession
astronomer al-Battani was checking Ptolemy's numbers, and
he decided, and of course he
connection for precession just would have had to make the
decided that precession seemed as Ptolemy did before him, he
Ptolemy's day than it had from to have gone faster since
He measured the precession Hipparchus' day to Ptolemy's.
it was approximately one degree din 66 years, which would givet
you a full cycle of a little bit
Quite a bit faster than the 36,000 years that Ptolemy had
talked about. So al-Battani had thought that
precession had speeded up, not a
and Ptolemy had both imagined steady rate tit.way Hipparchus
And in order to explain this, his contemporary, Thabit ibn
Qurra, created a theory called
sometimes called variable precession or sometimes also, in
Latin texts they'll call it the kind of awkwardly named access
and recess of the stars.
sphere was not just creeping It really means that that eighth
forth over a long period of time around but was going back and
Al-Battani thought that's what so that its rate varied.
the speed. he had seen was the varying of
standard picture in the west. It's interesting that oriental
astronomers, successors of these
didn't really pick up on the guys, of Thabit and al-Battani,
They didn't like it for whatever idea.
We don't see it in star tables reason.
but it became popular in the that were composed in the east,
Spain, and became the standard west, in particular in Muslim
some astronomical tables that way owere very influential.on in
The Toledan tables from Toledo
trepidation built into them. around the year 1,000 have
Alfonsine tables also created in And the later ones, the
century also have trepidation Spainbuilt into them.13th
But by the time we get to the Alfonsine tables, the precession
correction was so much, just the
correction, that they had to add observable precession
So what you end up with is a another motion.
So in order to simplify it I've fgotten rid of everything frome.
the stars in, and we're looking on the outside and this solid
sphere here is the sphere of the
Since we're on the outside you stars from the outside.
This is the thing which is can't see the stars, of course.
is accessing and recessing. The way that works is the eighth
sphere here is connected to two little circles, those are those
little gears, two little circles
in the surface of the ninth hsphere, and they rotate aroundd
in such a way that the eighth sphere, where the stars are,
wobbles.
back. It actually does a little bit of
this, too, which is one of the
but it goes ahead and back. Those circles carry the vernal
equinox up and back and up and
And then the ninth sphere here back.
And with respect to the tenth sphere, the whole ninth sphere
creeps steadily ahead.
superimposed on that is the So there'strepidation.tion, and
So it's gotten fairly complicated, one might say
fairly sophisticated, by this time.
Now, this idea was really influential, so influential that
even Copernicus took it for granted.
In Copernicus' de revolutionibus
well clearly it's not the stars doing all this motion, because
in Copernicus' picture the stars
earth doing all the moving, are fixearth and planets.t's the
So the earth is orbiting the
It's spinning on its axis. sun.
as it's precessing, it's It's doing this precession, and
also doing what we call today smutation, bobbling up and downd
during its precession motion. So he actually gave the earth
two independent motions.
independent motions here that were superimposed upon all the
others ones, and that's because he needed to account for
trepidation which was accepted
Of course, as you know, aCopernicus' ideas were not.
immediately taken up as a great cause by the world.
It took a little while for
And even a half century later ppeople still didn't like.
Copernicus' idea, at least some didn't, but some of the foremost
astronomers of that period
solution was really good, that decided that Copernicus'
the traditional medieval it actually worked better than
Copernicus' mechanisms but they trbelieved that the earth wasted
central and stationary. But you could get those motions
built into the celestial sphere if you just introduced another
sphere. So here is this latest
development here where once again we have the firmament
right here, and it's doing C to D, and it's mounted, this is the
eighth sphere, the firmament is mounted inside the ninth sphere,
which is doing this A to B
sorry I lost count of the motion, anspheres.t's being,
The eighth spheres is on the
It's on the inside. The ninth sphere is doing one of
those.
another, and the 11th is doing The tenth sphere is doing
was about 49,000 years by that the steady, what they thought
motion. So this is actually Copernican
astronomy then adapted to the geocentric idea still trying to
make sure that they've got
variable precession built in trepidation built in there,
Now, we don't teach trepidation today in astronomy classes, and
that is because Tycho Brahe, the
was the one who got rid of it. famous astronomer Tycho Brahe,
published the first original Tycho Brahe compiled and
Isn't that astounding? star catalog since Ptolemy.
star catalog, and the next Ptolemy in 150 AD published a
the early 1600s, about 1601. original one was published in
him, if you're going to do a Obviously, like everybody before
with precession. star catalog, you have to deal
problem that others did not, and What Tycho brought to the
all, Tycho was a geocentrist, what Tycho brought to this was
an appreciation for what happens
He is really the first guy that we see who deals with what we
today call observational error. Whenever you see data these
days, if it's responsibly presented, they tell you how
well-known the data is. That's the error bars on some
measurement.
today to be plus or minus however many tenths of a degree
our thermometer allows us to
That idea that there's an uncertainty in a measurement is
what Ptolemy brought to this, a concept which was relatively,
well was new as far as I know.
Al-Battani had looked at New with Tycho.
idea how accurate they actually Ptolemy's nwere.s and had no
It turns out these were difficult measurements to make,
the longitude of stars. Al-Battani didn't know how to
estimate the uncertainty of his own measurements, so he could
only take these numbers and not realize that the accuracy would
have allowed you to draw many lines through the rate of
precession, and so instead of changing, you could draw
multiple lines through them, the
up with, was about one degree per 71 years or very, very close
to the modern number. He did that just by averaging
all the historical numbers but
precise number that indicated a not tchange in the rate of very
precession. And I have just a little
quotation here because he says
You want the Latin or the thisEnglish?ell.
I only have the English,
[LAUGHTER] never mind.
irregularity of the rate of the change of the longitudes of
the stars is not so considerable
His erroneous ideas on this matter are a consequence of the
incorrect observations of the ancients, as well as those of
more recent times.
the equinoctial point during Consequently the precession of
asserted. For in our times the fixed stars
do not take a hundred years to
71 and a half years. move a degree but only
the case, as appears when observations of our predecessors
are carefully checked.
irregularity appears, which is In fact, only a small
So Ptolemy, I'm sorry, due to accidental causes."
an "A" in the astronomy lab because he knew how to lookn
at his data and estimate
Well, I stuck this on to this discussion of the 13th sign of
the zodiac because I think that it's really interesting, these
two issues are very closely coupled for astronomers.
Trepidation, as silly as it kind
eventually 11 spheres all rocking each other around and
stuff, that's a theory that was a response to evidence that they
had before them.
theory, an attempt to construct something that would allow you
to make predictions like the
like that. positions of stars and things
completely rational ground Tycho disproved it on a
the nature of observational because of hierror.rstanding of
It didn't make good predictions,
Tycho, as we said, showed that and so he rejected it.
which theory had to explain. there was just one simple motion
It's all about making predictions and finding out
whether they work or not and
observational data. Just compare that to astrology.
And I think I'll leave it right there.
Thanks a lot. [APPLAUSE]