Ever since the dawn of humanity, humans have
looked up to the heavens in hopes of finding
answers about the gods, the universe, and
life.
We have carefully observed the stars, we’ve
calculated their movements, discovered their
patterns, and slowly we have begun to understand
the secrets of the universe.
If you’re new here, I’m Darius Cosden
and this is Vlogs of Knowledge.
Make sure to subscribe and hit the notification
bell because we do have brand new episodes
every single Wednesday.
And today, we’re talking about Copernicus,
the Heliocentric Theory and the history of
Astronomy.
The sky has always been visible from earth.
I mean the ancient civilizations called it the “heavens”
and have looked at it for millennia.
This vast unknown, filled with bright stars
and galaxies, has been the place of observation
ever since the dawn of humanity.
The first evidence of this that we have comes
from the year 35,000 BC where we find remnant
bone sticks marked in ways indicating a link
with the different phases of the moon.
The evidence for that comes from the fact
that the bones have 29 markings on them, as
if they were counting the days it took for
the Moon to go from new to full, which is
about 27 days.
Now a woman’s menstrual cycle is also linked
to the moon and scientists think these bones
could have also been used to keep track of
menstruations, making women the very first
astronomers.
But we’re still not sure.
It’s hard to tell when a bone is this old.
Plus, some parts of the bone are missing,
suggesting that maybe there were more than
29 markings on it originally, which would
break down this whole argument.
Nonetheless, while this discovery doesn’t
tell us too much about what the pre-historic
people thought of the moon and the sky, it
does at least give us confidence that very
early on, humans have used the stars to live
their life.
From pre-history we now move into history.
In Mesopotamia, the land between the rivers
where the ancient kingdoms of Sumer, Assyria,
and Babylon existed, we find what we can truly
call the very first astronomers.
We have found documents dating to the year
1200 BC, that catalogued the different stars,
and gave each one its own name.
The interesting thing is that these names
are mostly written in Sumerian, which indicates
that the practice of astronomy probably existed
long before 1200 BC, since Sumer is believed
to have been the very first civilization,
dating as far back as the year 5,000 BC.
The Sumerians were the first ones to associate
their gods to the stars, to divide the circle
in 360 degrees, and to use a base 60 numerical
system, which allowed them to represent large
and small numbers with greater ease.
Now although they had made these great discoveries,
they had not yet involved mathematics with
astronomy.
They were still simple observers.
For the next step in our history of astronomy,
we have to go to Babylonia itself.
The Babylonians, who inherited much of their
knowledge from the Sumerians, were the first
ones to have used mathematics in relation
to the stars and astronomy, according to the
clay tablets we have found.
It is believed that they used mathematical
calculations to track the variation in the
length of each day as the year goes by.
Amongst the centuries of documented celestial
phenomena that we have discovered, perhaps
the most interesting one was the documented
risings of Venus in the sky for about 21 years.
This is an important discovery because it
is the earliest place in recorded history
where we have evidence of humans having clearly
distinguished between planets and stars.
Now this is still very, very early in the
history of astronomy.
I mean we still haven’t even talked about Copernicus
and the Heliocentric theory.
But don't worry, we will.
First, it's important to understand the steps that humanity took
to get there.
Now when you do something repeatedly for centuries,
like the Babylonians were doing looking at the
sky, you tend to become very good at that said thing.
The Babylonians got very good at astronomy
and mathematics.
Towards the end of their civilization, they
had made perhaps the greatest discovery of
the time: the moon operated in an 18 year
lunar cycle.
This is not talking about the Moon’s orbit,
which takes approximately 27 days.
This is talking about the inclination of Moon’s
orbit, relative to Earth.
Which simply means that the moon changes the
inclination of its orbit relative to earth’s
equator, every 18 years.
This discovery was so important, that later
on the famous astronomer Ptolemy used it to
fix the beginning of a new era, since he considered
that moment the first time humans had made
proper usable observations about the stars.
From the Babylonians we then pass briefly
to the Greeks, where we find a man by the
name of Hipparcus.
Hipparcus was perhaps the greatest astronomer
of Antiquity.
He used the Babylonian observations to create
quantitive models of the sun and the moon’s
movements.
His most famous discovery is that of Earth’s
precession, which is the fact that Earth not
only spins on itself, but also spins at the
north and south pole in a small circle over
a period of about 26,000 years.
This is called axial precession.
To make this example a little bit clearer, think of a spinning top.
You spin it, and it spins on itself, but the
top and bottom also wobble a little bit, increasing
in rotation until the spinning comes to a
full stop.
The faster the spinning top spins, the slower
the top and bottom parts move.
It’s the same with the Earth, only on a
much larger scale.
The spinning top is affected by gravity.
Gravity is trying to pull it to one side or
another, but the fact that it is spinning
so fast, the rotational gravity keeps it straight.
The Earth is the same.
It's rotation wants to keep it straight, but
the gravity of the moon and the sun make it
wobble at a slight angle.
This discovery was huge, and was perhaps what
inspired our next astronomer, Ptolemy, to
come up with his own models of the stars.
Now by that point, astronomers had already
discovered 5 planets of our solar system:
Mercury, Venus, Mars, Jupiter, and Saturn.
And they had already created a model of the
solar system.
The most obvious thing about this model, is
that the Earth is at the centre of the solar
system.
They did this because to them, from Earth’s
perspective looking at the stars, it seems
like Earth is stationary and that everything
else is moving in a circle around the planet.
Which, I mean if you don’t know any better,
makes total sense.
This model also put every planet on a circular,
not elliptical, orbit around Earth.
They used circles because they interpreted
these planets as heavenly bodies, and that
anything heavenly should be perfect, thus
it should be a circle.
Because a circle is perfectly round, it symbolized
perfection.
This model was a good start, but there were
some problems.
It couldn’t accurately account for some
of the planets appearing to move backwards
in the sky at some points in the year.
Take Mars for example, we see a natural orbit,
but then suddenly, it looks like Mars is moving
backwards, doing a sort of small loop, only
to continue moving forward again.
This is strange, but is something that we
have discovered to be totally normal.
We call it retrograde motion.
The model of simple circular orbits wasn’t
enough to explain such a phenomenon.
Ptolemy, was faced with a problem.
How could he explain these retrograde motions?
It was a hard thing to explain, but he came
up with a clever idea.
He modified the model to include something
called an epicycle.
An epicycle can be simply thought as a circle
within a circle.
Instead of having Mars orbit Earth in a perfect
circle, it orbited Earth on its own little
circle within its bigger circle.
Ptolemy used that to try to explain Mars’s
motion in the sky.
It worked, but to some degree.
Epicycles did improve the model, and more
accurately represented the retrograde motions,
but they were still not perfect.
The more precise you wanted to be, the more
epicycles you had to add.
And eventually, the whole system became way
too complicated.
As broken as this system was, it 
somehow survived well into the Middle Ages
and even all the way up to the Renaissance.
Which is kind of strange, because this model was broken.
Like if you tried to calculate the position of Mars, you would be
a few degrees off.
But I mean there were also some other reasons for the model being adopted and kept for so long.
One notable reason is the fact that the Church really liked to have Earth at the centre of the universe.
Because it fit perfectly with the idea that if God made the universe and the stars, he would
put Earth at the centre of it all, with everything
else revolving around it.
And when the Church adopts an idea like that,
it can be very hard to go against it.
After about 1,300 years of geocentricism,
which is a fancy word for Earth being at the centre,
we finally have somebody who challenges this
old, broken view of the universe: Copernicus.
Copernicus was a renaissance-era mathematician
and astronomer, who proposed that the Earth
was not at the centre of the universe, but
rather that it was orbiting the sun along
with all of the other planets.
This is what we call the heliocentric theory.
But Copernicus wasn’t the first one to introduce
such an idea.
This idea of the sun being at the centre had
been around since the Ancient Greeks.
But due to the strong movement of the Ptolemaic
model, the theory never gained much traction
until later on.
Copernicus realized that the retrograde motion
of the planets could be explained much better,
and without the need for epicycles, if the
Earth was orbiting the Sun rather than the
other way around.
It also provided with much better celestial
predictions, which convinced him that his
model was right and that Ptolemy was wrong.
Being a smart guy, he knew that if he went
ahead and published his findings, he’d most
likely face death by the Church.
I mean the Church would consider this heresy,
going against God himself.
So he didn’t say anything.
Not wanting to take his knowledge with him
to the grave, Copernicus decided to finally
publish his findings very close to his death.
In fact, the very day he died, was the day
that his book was first published.
He saw the first ever copy, and then died
that same day.
What a kind of perfect ending to his life.
Unfortunately, this book was largely ignored
for about 80 years, until a man by the name
of Galileo Galilei took up an interest in
planetary motion.
Galileo had done something no other human
had done before him.
He built for himself a telescope.
Now he didn’t invent the telescope, but
he greatly improved it and designed it to
allow him to observe the stars in a way that
had never been possible before.
He was able to magnify the image up to 30
times what the human eye could see.
With it he saw the different phases of Venus,
which served as proof that Copernicus was
right.
In the old model, Venus’ orbit is entirely
between the Sun and the Earth, making it impossible
to see Venus fully lit as it would have to
somehow be behind Earth.
On Copernicus’ model, however, this was
possible.
It was the first time somebody could actually
provide some tangible proof to the model.
Around the same time, a German mathematician
by the name of Johannes Kepler, was working
on another piece of the puzzle.
Copernicus' model was good. But it had some fundamental flaws.
For one, it was using circles, perfect circles, as orbits instead of ellipses.
Which of course, as you can imagine, gave rise to a lot of problems.
Kepler was very lucky that he had access to a ton of observations made by
the Danish astronomer Tycho Brahe, most notably
his observations of Mars’s motion through
the sky.
While Copernicus added more and more epicycles
to his model to explain irregularities, Kepler
realized that they could be explained if the
planets orbited in ellipses instead of circles.
And he could do it without the need for epicycles.
He used the data from Brahe about Mars, and
realized that Mars’ movements would only
make sense if it moved in an elliptical orbit.
This orbit, combined with the orbit of Earth,
would explain perfectly why we see Mars’s
retrograde motion in the sky.
Kepler was able to prove these orbits were
elliptical, but he wasn’t able to explain
why they were like that.
The final man who was able to find the missing
link, was none other than Isaac Newton himself.
Newton used Kepler’s observations and laws,
to deduct his own laws of the universe that
proved planets would move in elliptical orbits.
He was able to provide mathematical proof
and argued that if his equations are correct,
then the orbits had to be elliptical.
And of course, today we all know that
Newton’s laws are correct.
In fact they are so correct, that they have
become the foundation for modern physics.
These laws became universal, and are said
to affect the entirety of the universe.
The puzzle was finally complete, and the solar
system was finally understood.
Wow that was a ton of knowledge for one video!
If you’ve enjoyed this video, please leave
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And now it’s time for questions.
Do you like astronomy?
What is your favourite, cool fact about the universe that you would like to share?
Please do leave me a comment I would love
to read and answer them all, and bonus points
for you if you do leave a comment you might
get featured in next week’s video as a fan
of the week!
As you know and say at the end of every video,
I haven’t talked about everything regarding
Copernicus and the heliocentric theory, and the history of astronomy
That’s ok.
I'm actually doing this on purpose to get you guys to research more on your
own.
Because I believe in research and I believe
in getting you guys educated on a subject.
And if you want to learn more, as always there will be some links in the description where
you can start!
With that being said, my name has been Darius
Cosden, it’s been an absolute pleasure you
can follow me on social media the links will
also be in the description, and I’ll see you
all next Wednesday!
