Copernicus's theory that the Earth orbits the Sun, marks the beginning of the
Scientific Revolution but his theory was widely rejected for many decades,
not just by religious leaders but by other astronomers who raised troubling
questions that Copernicus couldn't answer.
The word "planet" comes from a Greek word meaning "wandering star".
The Greeks didn't know the earth was moving
and assumed that the planets orbited the Earth, but they moved strangely.
They would sometimes change directions.
The planets couldn't simply be circling the Earth because how could they change directions?
Ptolemy realized that he could explain this behavior by adding a second circle called an epicycle.
Each planet orbits around an epicycle and the epicycle orbits around the earth.
Copernicus had a different explanation which had the earth orbiting the Sun.
It was a more elegant solution, but it had several problems.
Copernicus soon realized that his simple model just didn't work.
It didn't match the observed positions of the planets so he added the epicycles back in.
He moved the Sun away from the center of the solar system.
But even with these changes, his model wasn't really any better than Ptolemy's.
Problem #2: What could be moving the earth? The earth is so big and so heavy,
What could be moving it around so fast? The fact that planets and stars move
wasn't a big problem because Aristotle explained that they are made of a
lighter substance that likes to go around in circles.
3. Parallax. If we're moving so fast,
then why don't we see the stars change positions as we move?
This is called parallax. You might think that's just because the stars are really far away,
but then we'd expect the stars to be pinpoints of light.
In telescopes, you see that they have a thickness to them
which means they'd have to be incredibly far away and gigantic.
4. The problem of projectiles.
If you fire cannon ball towards the South it will land due South as you'd expect,
but only if the earth is not moving. Now let's put the earth in motion.
Italy is rotating at 750 miles per hour. The cannon ball was moving at the same speed
when it was launched so we add this speed to the motion of the ball.
But the equator is rotating faster so the ball doesn't hit its target
because its target is moving faster. But this is not what we see.
We don't see the earth moving when we're on the ground. We only see the cannon ball
which should veer to the right if the Earth were rotating.
These problems with Copernicus would stump scientists for centuries.
Galileo made two key discoveries that supported Copernicus.
He discovered four moons orbiting Jupiter proving that there were objects that did not orbit the Earth.
Ptolemy and Copernicus both give similar predictions about where Venus should be.
Remember, they can only measure the angle to Venus, not the distance.
But Ptolemy predicts that Venus is in a crescent phase here,
while Copernicus predicts that it's nearly full, which is exactly what Galileo saw.
So now did everyone believe Copernicus?
Nope! Because there was still a good alternative.
Tycho suggested that the planets orbit the Sun but the Sun orbits the earth.
This avoids all the problems of a moving earth.
The man who would fix the Copernican model was a German astronomer named Johannes Kepler.
Initially, Kepler believed that the orbits of the six known planets were
closely linked to the five Platonic solids. Kepler was mistaken.
There was no real connection. But he sought out the man with the best data: Tycho Brahe.
Tycho was rich. His stepfather had rescued King Frederick II from drowning
and as a result died of pneumonia. The very grateful King supported Tycho's huge research program.
When Kepler finally got his hands on the Mars data, he predicted that he would solve the problem in eight days.
Eight years later, he was still working on it.
It was hard because he only had one data point to work with:
the angle between Earth, Mars, and the Sun. He didn't know the distance to Mars
or how much of the motion was caused by Mars moving and how much was caused by the earth.
But he found a way to isolate the motion of the earth. One year on Mars last 687 days.
If we use measurements that are spaced 687 days apart
Mars is always in the same location. Only the earth is moving.
And now Kepler estimated the orbit of the earth, then used that orbit
to estimate the orbit of Mars. Then he discovered the mistake
that Ptolemy, Galileo, Copernicus, and every other astronomer had made.
The planets don't orbit in circles they orbit in ellipses.
Isaac Newton explained how the earth moves through his laws of motion and gravitation.
We see so little parallax in the stars because they're so far away
but they're not absurdly large. The thickness of the stars is an illusion.
George Airy explained how telescopes distort the stars making them appear larger than they actually are.
In 1838, Bessel finally showed the stars do move just a tiny bit as the Earth orbits the Sun.
The motion of projectiles on a rotating body was not completely understood until
Coriolis derived his equations describing the motion.
However, this effect is only noticeable across very large distances.
When a batter hits a home run, this effect is smaller than the width of the ball.
The Copernican model worked great once Kepler realized that the planets move in ellipses.
Ellipses fit perfectly into Newton's laws of motion.
And the Copernican theory was further confirmed by observing parallax and the Coriolis effect.
For more astronomical videos, please click to subscribe.