Professor Dave again, let’s rethink the
solar system. When we left off with Aristarchus, he had
correctly proposed that the sun is the center of the solar system, rather than the earth. But he was way ahead of his time, and the
geocentric model remained in place for centuries. This model was perfected by Ptolemy in second
century Egypt. He improved on pre-existing models by finding
a way to explain elusive phenomena like the retrograde motion of the planets. This was when a planet would seem to turn
around and go back the other way in the sky for a few days, before resuming normal motion. Now we understand very well that this happens
because the earth is passing the planet in its orbit, and will thus appear to move backwards. But back then, with the geocentric model,
it needed some serious explaining. He proposed that planets move on epicycles,
which are smaller circles that themselves move around the larger circle that is the
planet’s orbit, and this model was able to predict the motion of the planets with
reasonable accuracy. But it wasn’t perfect, so it was tinkered
with, making things more complicated, requiring a different formula for each planet. This went on for centuries until the 1500s,
when it just no longer made sense. It was at this point that a paradigm shift
in astronomy was necessary. The first to suggest a heliocentric model
since the time of Aristarchus was a Polish man named Copernicus. He demonstrated with great precision that
all of the problems with the geocentric model instantly vanished if the sun is placed at
the center of the solar system. Objects closer to the sun orbit faster, and
the retrograde motion of a planet like Mars is now simply the result of earth overtaking
Mars at a particular time in their respective orbits. As we pass by Mars with a greater angular
velocity, there is a moment where it appears to move backwards, just like when you pass
a car on the highway, and it appears to you to be moving backwards, given its slower velocity. Copernicus even estimated the distances from
the sun to each planet with great precision, using some ingenious geometric calculations. But there were still some issues to iron out. Critics of the heliocentric model had one
very valid point. If the earth goes around the sun, the apparent
position of the stars should shift. It would be several hundred years before we
had telescopes powerful enough to see it, but this shift does indeed occur. It is just incredibly small, because of the
staggering distances to the stars. We use this slight shift today to measure
the distance to certain objects in space. To understand why this works, hold a finger
in front of your eyes. Then alternately open and close one eye at a time. Your finger will shift left and right depending
on which eye is looking at it, because your line of sight is originating from a different location. Keep doing this as you move your finger away
from your face. The shift in location becomes less pronounced
with greater distance. If you try this with an object across the
room, it already starts to be relatively negligible. Now imagine that the earth being on one side
of the sun is your right eye, and the earth being on the other side of the sun is your left eye. A distant star will change its position in
the sky, just like your finger, but the star is so far away that we need mechanically precise
machines to measure the apparent shift. But we do have these machines in the form
of modern telescopes, and because we know the distance from the earth to the sun, which
is one astronomical unit, or about 150 million kilometers, if we then measure this apparent
shift, we can cut it in half to get what we call the star’s parallax. Then we make a right triangle with the earth,
the sun, and the star, and simply use trigonometry to find the distance to that star. If you need to, check out my trigonometry
tutorials for loads of information regarding triangles and angles. Parallax, among many other phenomena, strongly
corroborates the heliocentric model, such that there has been no doubt in its validity
in the past few hundred years. The Copernican revolution had a tremendous
impact on human culture. The notion that the earth goes around the
sun rather than the other way around is a fairly innocent claim to a scientist. One model fits the data much better than the
other, and that’s all there is to it. But the philosophical ramifications of this
shift cut very deep. If the earth goes around the sun, the earth
is not the center of the universe. Other astronomers like Giordano Bruno reasoned
that if the sun is the center of this system of planets, and all the countless stars in
the night sky are suns just like ours, possibly with their own planets, and perhaps even living
creatures, then mankind is no longer the pinnacle of creation. The Catholic Church, very much in political
power at the time, saw this way of thinking as a threat to its supremacy. As a result, Bruno was tried for heresy and
burned at the stake. It seems that mankind was not quite ready
to be dethroned from its status as the only reason the universe exists. Our quest for knowledge has had many detractors
along the way, which should serve as a reminder of how privileged most of us are in modern
times, to have free speech, to have access to all the information we could ever want. We must value these freedoms, and defend them
from tyranny at all costs. But we haven’t quite made it to modern day
yet, there is more of the story left to tell, so let’s see what astronomers did with the
Copernican model next.
