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ast2002 birth 1

The Birth of Modern Astronomy Part 1
Hi everyone. Welcome to module seven. In this module, we're going to look at the birth
of modern astronomy and how astronomy came out of the Dark Ages and developed into
the modern science of astronomy that we see today.
Essentially, in ancient times, there were two main models that grew up of the solar
system. One is called the geocentric model. That's the model that has the Earth at the
center of the solar system. The other model was called the heliocentric model. And that
has the sun at the center of the solar system rather than the Earth.
The geocentric model was a much more elaborate model. It was developed by the Greeks.
There's a whole long line of Greek philosopher, astronomer, mathematician guys that
developed this geocentric model.
We could start it off with the astronomer Hipparchus. He's recognized in history as being
the first true astronomer, the first astronomer, the first person who did astronomy for a
living. He spent a couple of decades systematically observing the sky.
He was convinced that the stars moved, and he recorded their positions very, very
accurately. In fact, he made up the very first map of the sky. We attribute the map of the
sky today to Ptolemy, but Ptolemy simply copied Hipparchus's work. Now, when I say he
copied it, he did attribute the work to Hipparchus. So it wasn't plagiarism. But
Hipparchus created the very, very first star map with 48 constellations in it, the original
map of the nighttime sky that we've mentioned earlier on.
He was convinced that the stars moved, so he recorded their positions very, very
accurately so that people later on could see that the stars moved. We've already talked
about procession earlier on in the course, and Hipparchus was the first person to notice
the procession of the equinoxes.
We can also attribute him with creating things like the sine, cosine, and tans of angles.
He's considered to be the inventor of trigonometry. He's certainly the first person that we
know of that had recognized trig tables.
Now, what Hipparchus did do-- he looked at the Greek model that had developed of the
solar system. And the Greeks were obsessed with geometry. They were obsessed with
circles because they considered the circle to be the perfect form. And so they were
convinced that the universe had to be made up of circles. Hipparchus was the first person
to prove that if the planets were moving in circles, then the Earth couldn't be at the center
of that circle.
So they started off with a model that was geocentric. It had the Earth at the center. It had
all the planets, including the sun the moon, orbiting around the Earth. And then
Hipparchus realized that the Earth couldn't be at the center of the system. So he made an
eccentric model of this planetary system. And the model developed over the course of
almost 2,000 years and became extremely complicated.
Now, the diagram at the top shows you how the model essentially works. We have
already seen how planets appear to move across the sky over the course of time. They go
prograde following the same direction as the sun. Then they slow down and stop, and
then turn around and go retrograde, and then slow down and stop and then go prograde
So that retrograde motion is built into the system. The planet doesn't actually orbit the
Earth. It orbits in a little circle called an epicycle. And by making that epicycle bigger or
smaller, you could get a planet to go backwards across the sky. Then the center of that
epicycle moves around the Earth in a bigger circle called a deferent. So it's a deferent and
epicycle model or an eccentric model of the solar system.
And over the course of a couple of thousand years, that model became extremely
complicated. Essentially what the ancient astronomers were trying to do was trying to
predict where the planets would be. It's all to do with protection. Because they were
convinced that the planets affected your life. And so if you could work out where they
were in the future, you could, in some sense, predict the future. And that was what it was
all about.
So Claudius Ptolemy is the end of this long line of Greek astronomers that contributed to
this model. Today, it's called a Ptolemaic system, the geocentric model of the solar
Now, here we have the Earth at the center of the model. The Earth is at the center for
several reasons. People were convinced, first of all, that the Earth didn't move. So the
Earth was referred to as terra firma, firm ground. The earth wasn't moving. Everything
else was moving.
It was also a beautiful idea in terms of philosophy, as well. Because when you look
around the Earth, you can see that it goes through changes. It goes through cycles of
seasons. It also has cycles of birth, life, and death. And in many senses, the Earth was
As you got away from the Earth, things became more and more perfect. The reason why
that was was because as you get further and further away from the Earth, things begin to
change less and less. So the closest object to the Earth, the moon, is the object which
moves around the sky the most. The farthest planet, Saturn, appears to move the least.
And then right around the outside edge, you have the fixed stars, the immutable heavens,
the unchanging realm of God or the gods.
And so you had this beautiful idea of the Earth at the center of this universe. And it also
would makes sense philosophically because if God has created the universe, and we're
the pinnacle of God's creation, then obviously we should be at the center of it.
That wasn't the only idea that was around. There was a famous astronomer by the name
of Aristarchus. And he was convinced that the Earth did not lay at the center of the solar
system. In fact, he developed heliocentric model. He was convinced that the sun lay at the
center of the universe. And he did that very, very cleverly.
First of all, he demonstrated that the moon was smaller than the Earth. Clearly during a
lunar eclipse, you can see the moon going into the Earth's shadow. And the moon is
smaller than the Earth's shadow. He was able to estimate that the moon was about two
and a half times smaller than the Earth. Through a very clever technique, which we can't
go into, he was able to estimate and show that the sun was at least about 20 times larger
than the moon. And that meant that the sun was larger than the Earth.
And so he was convinced that the sun lay at the center of the solar system, the Earth
orbited around the sun, and then the moon orbited around the Earth. Unfortunately, his
ideas were not widely accepted. The ideas of the Ptolemaic school, the philosophy of
Aristotle held sway for several thousand years.
Now, the ideas that the Greeks had were almost lost to us today because after the fall of
the Roman Empire-- the first sacking of Rome was around 410, I think. After the gradual
decline of the Roman Empire, the knowledge of the Greeks went into decline. Common
Greek was a common language throughout the whole of the Roman Empire. And as it
went into decline, the quest and desire for knowledge slipped away.
And we read the works of the likes of Hipparchus and Ptolemy today not through their
original Greek, but through their Arabic translations. With the decline of the Roman
Empire, the Islamic world started to dominate around the Middle East. And they grew to
prominence around about the ninth and 10th centuries.
And they created a new center for learning in the city of Baghdad, which is in Iraq today.
And they sent out scholars all into the known world to gather knowledge from wherever
they can find it. And if you were around at that time, you could actually make a lot of
money if you could bring a book to their famous House of Wisdom in Baghdad. If they
didn't have the book, they would have weighed the book and give you the weight of the
book in gold as a purchase price.
The desire and quest for knowledge was insatiable. So they looked up at the ideas of the
Greeks. They incorporated mathematics from India. They incorporated medicine from
China. They just gathered knowledge from all over the world and translated it into
Arabic. That is the main reason today why the vast majority of the stars in the sky have
Arabic names. Now, we get these Greek ideas from Arabic astronomers.
So essentially, the Greek works of the likes of Ptolemy were translated into Arabic. And
then, with the fall of Constantinople in 1453, the great library of knowledge of
Constantinople was transported back into Europe. And then people that we know of
today, people likes of Copernicus, they rediscovered the works off people like
Aristarchus. And the ideas came back to the forefront today.
So we attribute Copernicus with the heliocentric model, this model that has the sun at the
center of the solar system. But really, we know for certain that Copernicus read the works
of Aristarchus. And he wrote a book early in the 1500s auspicing this heliocentric model.
He didn't elaborate on it in any more detail. He was being employed by the Roman
Catholic Church at the time. And certainly, Church doctrine dictated that the geocentric
model was the correct model of the universe. And Copernicus certainly didn't want to get
into any trouble.
And so he had stated in a very early work called the "Little Commentary--" he put
forward this heliocentric idea. And he stated that he would elaborate on it in a later work.
And he never did. In fact, he came to the last year of his life, and he'd still not published
it until a man called Rheticus visited him and got him to go ahead and publish the work
that we attribute to him today.
The work was called De Revolutionibus. It was a work on the revolution of the heavenly
spheres. And in that work he states, "finally, we shall place the sun himself at the center
of the universe. All this is suggested by the systematic procession of events and the
harmony of the whole universe if only we face the facts, as they say, with both eyes
So Copernicus, in this work, declared that if we could only look at the universe, we could
see that the geocentric model wasn't the correct model. It was the heliocentric model
which was correct. Unfortunately, nobody really believed him. And for the next 50 years,
Rheticus was the only publisher of Copernicus's work. It wasn't until Galileo came along
and with the invention of the telescope was able to indeed look at the heavens with eyes
open and see that the heliocentric model was the correct one.
So essentially, during the Renaissance there was a battle between these two models, one
that has the Earth at the center of the universe and the other that has the sun at the center
of the universe. The major issue that there was with Copernicus's model was not that it
wasn't correct. The major issue with it was it wasn't very accurate. It was a lot simpler
than Ptolemy's model. Ptolemy's model had become very complicated. Lots of epicycles
were added. But it was pretty good at predicting the positions of the planets in the sky.
Copernicus's model had one major flaw to it, which we'll look at when we talk about
Kepler's laws. But the major flaw meant that although Copernicus's model was very, very
simple, it wasn't as accurate as the geocentric model. There are two diagrams here which
show you how the heliocentric model could make predictions which were different from
the geocentric.
In the Ptolemaic universe, we have the Earth at the center of the universe, and everything
in the solar system orbits around the Earth. So the sun orbits around the Earth. And it was
pretty certain that Venus and Mercury were orbiting in little epicycles, which were linked
between the Earth and the sun. And here's a model showing you how that would actually
Now because the sun is the only thing that produces light, if Venus is actually orbiting
around in a circle like this, you can see the phases that Venus should have. Notice that
there are no full phases. There are only crescent phases in that model. Whereas in
Copernicus's model, the planet Venus is actually orbiting around the sun. And so when
you look at Venus, it shows all the different phases that you can see.
And indeed, when Galileo pointed his telescope at Venus, he could see that it was going
through all the phases, and he was convinced that the Copernican system was correct. In
fact, that is the beauty of science. True science should make predictions. And if it is
falsifiable, if you can test it and show whether it is true or not, then it is considered to be