Video Transcript: Models of the Solar System

These mental models that we have we take for granted but someone in some groups of people had to figure these things out first but he's purely on observations of the night sky.  

There's a great story throughout the history of astronomy of trying to figure out which of two models are the correct model for our solar system.  On the one hand we have the geocentric model, that is the model which places the Earth at the very center of our solar system and, in fact, at the time all that was known about the sky was our solar system so this was really putting the Earth at the center of the whole universe.  But this idea can be traced way back to Aristotle and even before Aristotle, I'm sure.  That is illustrated in this first picture in the gallery.  

The notion is that our entire sky is really a series of concentric shells or glass spheres, and that on each of these shells are the things progressively further away from the Earth, so the closest is the Moon, and then you have Venus, and the Sun, and Mars, and in all planets considering the Sun, Moon, Venus, Saturn, Mars, Jupiter, and Mercury, those things that the ancients knew about as Wanderers in the sky. 

Now there's a lot of good reasons to think that this is a correct model.  This certainly matches our own experience.  We see things rising and setting from day to day.  Sitting here on the Earth, if I sit here as still as I can it doesn't feel like I'm moving.  It certainly doesn't feel like I'm rotating around the center.  In fact, this was one of the strongest proofs that the geocentric model must be the correct model.  If I were to take an object and drop it, it falls straight down; it doesn't go zooming across the room.  If the Earth were rotating or moving, surely a bird wouldn't be able to fly through the air and keep up with this enormously fast speed that the Earth must be rotating.  So to ancient people, including Aristotle and people all the way up until the Middle Ages, it was considered sort of obvious that the Earth must be stationary and be at the center of our solar system.  So this geocentric model had a lot of weight to it.  

But it wasn't the only idea.  In fact, early on Aristarchus suggested there's another possibility which is that the Earth is at the center and everything else is going around it.  He realized that all of the things we observe can be explained… I'm sorry I misspoke…  that if the SUN is at the center… the SUN is at the center and everything goes around it.  Now this idea while it's an ancient idea was really made more sophisticated and put to the forefront of thought in the Renaissance by Copernicus, and so this is a drawing that's accredited at the time of Copernicus sort of illustrating this heliocentric, the Sun-centered model, for our solar system where the Sun is at the center, and Mercury and Venus go around it, and then the Earth and the Moon, and then Jupiter and Saturn, and the stars, again, are really far away.

Now Copernicus’ model, this heliocentric model. was put forth at a good time.  The Renaissance was a good time for new ideas.  You got the printing press, you got sort of the birth of science happening, people are open to new ideas.  Copernicus didn't see that in his own life but he knew that this was a controversial idea.  This was also a time of religious persecution.  The Inquisition was going on, witch trials had been happening, and so really people's faith was always being called into question in particular if they put forward any ideas that could potentially be viewed as heretical or counter to the teachings of the Bible.  And so it begs the question how does this idea of a Sun-centered solar system, how does that conflict the teachings of the Bible?  

And one of the great examples that the religious leaders at the time of Copernicus suggested was looking at the story of Joshua and the battle, and I, of course, remember the vague idea of the story but the details I have to remind myself.  And so this is when the Israelites were fighting the Amorites and Joshua prays to God.  He says, “O Sun stand still over Gibeon; O Moon over the Valley of Aijalon.”  So the Sun stood still and the Moon stopped, till the nation avenged itself on its enemies, as it is written in the book of Jashar.  And then it says the Sun stopped in the middle of the sky and delayed going down about a full day.  So, what this suggested to the to the church leaders and potentially to us as well is that the Sun is what stopped.  It says the Sun stopped, and so the Earth must not be moving.  The Bible doesn't say that the Earth stopped rotating.  It says the Sun stopped.  And then it says, which is worth noting, it says there has never been a day like it before or since a day when the Lord listened to a man.  Surely the Lord was fighting for Israel.

Amazing story to be sure, but it begs the question of how we make sense of the Bible and of nature, and how we make sense of those things at the same time.  We know that God created both of those things. God has written his name both in His Holy Word and in the universe He created so they cannot possibly be in conflict with one another.  What God is telling us and what nature is telling us can't disagree.  Where the confusion comes is in our interpretation; science being our interpretation of nature, but also our interpretation of the Bible.  And so when it says the Sun stopped, does that mean that we should take that to mean that the Earth is at the center of the of the solar system?  I don't think any of us today would say that that's what that Bible verse is telling us.  It's telling us something much more meaningful about God's action on the part of the Israelites and the profound miracle that He did and that He's capable of.  

But more than that, I think what we can see - well actually not more than that - I think that's the big picture… like that's what that verse is about… but, as a caveat I would say how much in our language today do we still talk about Sunrise, Sunset, oh the Sun is so high in the sky, it's low in the sky.  In reality, the Sun is sitting there.  It's all our perspective that's changing.  Sunrise, Sunset; we don't say Earth rotate, Earth rotates some more.  The language we use to describe the night sky is based very much on our experience of being what we perceive as stationary on the surface of the planet.  So there's a danger I think that can come from reading into those details in the Bible as though God is trying to tell us something in those words that are used.  And here's the beauty; and I'm not saying we shouldn't look carefully at that, but here's the beauty is God is also teaching us.  He's telling us about his creation and about himself in the world that he created.  And so we're going to see in not only in this video but the next video is that God was revealing to us something else, too, in the world He created that we could observe; details that if we carefully studied them we could see the message from God quite literally, what He was telling us, this is how I made the solar system, this is where the Earth is located in the solar system.  You know the book, in a sense, the book about how God made the universe and all the subtle details it's everywhere around us.  Now of course the Bible tells us too, but it doesn't tell us all the details, all the details about how God did it, but we look around us and we can see it.  He's written it on the fabric of the universe and that is tremendously exciting to me.

Okay, so let's look at this heliocentric and the geocentric model.  Let's just come here them to one another a moment.  In the geocentric model, now again I mentioned this in a previous video but these ovals are meant to be more or less circles; we're just looking at them at an angle, so imagine that most of these orbits are basically circles.  In a geocentric model you'll know is that Mercury, Venus, Mars, Sun, they're all orbiting around the Earth but you'll notice that many of the planets, in particular Venus, Mars, Jupiter, Saturn, Mercury all have a little loop on top of the loop.  You know, there's a big loop, so this is the… okay, there's a big loop, the whole orbit, and then there's a smaller loop.  That smaller loop is called an epicycle.  An epicycle.  And the reason it's there is because we need to try to make sense out of the retrograde motion that planets exhibit in the night sky.  Now you may recall, we talked about how do planets appear to move in the night sky, and one of the things they do is they do these loops which we call retrograde motion.  

Now to explain that is actually kind of tricky in a geocentric model.  If the planet is just orbiting the Earth it should just keep going the same direction all throughout the year.  Why would it turn around and go backwards?  So to make sense out of those observations in the geocentric model, you have to add an epicycle.  And this is partly why some of those early astronomers, and Copernicus, suggested a heliocentric, a Sun-centered model is more elegant, it's more simple.  It doesn't require you to add these ugly epicycles.  You know what I mean?  Like in science, we often have an assumption which is that the universe is relatively simple; it can be explained by simple rules and simple laws.  It would be called, you know, it's an elegance, an elegance, as Christians, we see as evidence for our designer that God has carefully crafted this universe and it is an elegant universe.  And so scientists have recognized that as well, and they say that these little extra epicycles on here, they aren't elegant.  And so, check it out.  A heliocentric model can explain retrograde motion without epicycles.  So let's take a look at epicycles a second first.  

In a geocentric model, where you have the Earth at the center, this is the fourth picture in the gallery, the Earth at the center, a planet going around it, but that planet is riding on an epicycle as it orbits, then every once in a while it's going to do a little loop-de- loop in the sky.  And that is the retrograde motion.  But there's no mechanism to explain what causes this epicycle.  It's simply added just so that we can get this this retrograde motion.  Now by contrast, if you were to look at the fifth picture in the gallery and look at a heliocentric model, then as the Earth orbits the Sun, and Mars orbits the Sun, then there is a special time a little over once a year as the Earth passes Mars when - it's kind of like when you're if you're driving in a car down the highway and you're going faster than someone else, from your perspective as you pass the person on the highway, it looks like they're going backwards.  Of course, you're both going forward very quickly on a highway but, because you're going faster, from your perspective, they look like they're going backwards just as you're passing them.  In the same way, as the Earth passes Mars in its orbit, Mars will temporarily look like it loops and goes backwards in the sky and then goes forward again.  And so this heliocentric model can explain retrograde motion without any need for these circles on circles, these epicycles, and so this is viewed as a more elegant solution to explaining retrograde motion.

Now here's the thing we may want elegant solutions or we may believe that the elegant answer is always the correct answer, but ultimately we need to make observations of the universe and say does this match with what we can see and observe?  That's what the nature of science is all about; does this idea match with what I can measure and observe.  And so this is a great idea but is this really what's happening in our solar system?  And in our next lesson we're going to look at key observations that were finally able to answer this question and say definitively whether or not the Sun was at the center of our solar system.  

All right.  We'll see you next time.