Video Transcript: The Seasons

Let's start by considering the dramatic variation that we see on our own planet and how that's different depending on where you are.  So here's the first picture in the gallery which is again, an animated GIF, so you could see it cycling in the gallery, but I'll just show you one time through.  I want you to notice here we are starting and it's summertime in the northern hemisphere and you can tell because it's all green in Canada, and Alaska, and the United States.  And so wherever you see greenish, that means there's plant life growing and then of course, you see snow in certain places like in and Antarctica and maybe in the mountains here of Chile a little bit.  As we go forward in time, you'll see that first, all the green disappears here in the northern hemisphere, and I would be pointing to some of these places like South Africa or Australia, but they don't have as much green, as there’s a lot of desert. 

But let's watch what happens as the snow starts to appear here in the northern hemisphere. What do we start to see happening in the southern hemisphere?  It starts to look a little more green down here in South America and even in Africa, south of the equator, we see it getting greener and greener and then of course, the snow melts as our season changes once again.  And so what we see is that near the North Pole or the northerly latitude, we see this huge variation where the snow comes down and recedes again, whereas kind of near the equator it stays relatively green throughout the year. 

This seasonal change impacts life for so many different creatures, us included.  And a lack of seasonal change, if you live near the equator also has a tremendous impact.  Just consider, for example, the jungles of weather itself, let's say South America.  These rainforests which stay relatively warm throughout the year because they're near the equator have a whole host of just amazing creatures, you think of these giant insects who live there, and, this enormous diversity of life that lives there throughout the year in the rain forest.  And part of that is because there isn't this huge freeze that comes through and forces these creatures to either die or hibernate, or somehow burrow underground. 

You know, for a time I lived in Florida and one of the things I didn't like as much about Florida was that the bugs grew so big there; the spiders, oh my goodness, the spiders were so huge.  And here in Michigan, we get some big spiders, but nothing like what you saw, what I saw, in Florida. And the reason for that is because it's warmer climate and the seasons don't vary quite as much.  Now, by contrast, think about way up here in the North, there are very few animals who can survive in these Arctic environments because of this huge variation in the seasons.  So this has a tremendous impact, and that's just thinking about animals.  Think about all the plants and the variation that happens throughout a season.  The plants all around us the trees, at least where I live, their leaves fall off every single year, and they depend on that variation in the seasons for their seeds to spread.  So there's this beautiful, marriage between the seasons that are hard-wired into our planet and the living creatures and plants that makes this planet their home.

Now, I see that as this beautiful design that God has made where the living world and the nonliving world have been woven together in an amazing way.  And we see many, many examples of this.  But I think this is one of the most striking, because it involves our entire planet, all at once.  So we're going to try to understand these seasons a little better. 

The ultimate cause of these seasons can be illustrated.  We kind of touched on this before with motions in the sky that we see and that is the changing position, or the seeming, the apparent change in position of the sun over the course of the year.   Now, this can be illustrated another way as another animated GIF, the second picture in our gallery.  This other animation, which shows, if we were to, you know when you look at a map of the Earth or a globe, you tend to kind of keep the Earth fixed, you know?  It's sitting right here, the north poles here, the south poles there, and it's fixed on that axis.  That's how we perceive our planet because we live on it.  From that perspective, well, there's a dramatic change that happens over the course of the year.  From month to month, what happens is the illumination from the Sun changes.  

And you're seeing what side of the Earth is being illuminated here in this animation.  And you can see now it looks like the sunlight is coming from further south.  And as you go further along, the sun's coming from further south, so the southern hemisphere is getting more light and the northern hemisphere is getting less.  As you continue on in the year, it appears from our perspective that the sun is getting higher in the sky, more northerly; it's moving to the north.  And so the northern hemisphere is getting more light while the southern hemisphere is getting less.   So this is the apparent change in where the sunlight is coming from over the course of the year. And our goal, of course, is to try to understand, well, if this is what it seems like to us, what's really going on?  How can we make sense of this apparent change?

Well, here's how we can make some sense out of that.  It all comes back to the tilt of the Earth's axis.  So while we perceive our experience here on the planet as our axis is north and south, compared to our orbit around the sun, the Earth's axis is tilted by 23 ½˚.  So as we orbit around, that axis is tilted.  What that means is sometimes we're tilted towards the sun – you can see that in this example - the Earth's axis is tilted towards the sun, whereas on the other side of our orbit, we're still tilted in the same direction in space but now, since we're on the opposite side of the sun, we're tilted away from the sun.  So we're tilted towards sometimes; we're tilted away in other times.

Now, as a result of that - now, when I say toward and away, that's a very ‘northern hemisphere’ way of thinking about it, because here the northern hemisphere is on top and it's tilted toward the sun, while the southern hemisphere is tilted away.  But on the opposite time of the year, in December in January, the northern hemisphere is tilted away from the sun, but look, the southern hemisphere, it's tilted toward the sun.  And that's why we see this opposite between the northern and the southern hemisphere. 

Now I have to explain this picture, this is the third picture in the gallery, that this orbit that you see here, the white circular orbit is not actually meant to be like oblong or egg shape.  It's really a very circular orbit.  The only reason it's shown here to be kind of like an oval is to illustrate kind of the three dimensional tilt of the Earth's axis.  So when you picture the Earth's orbit around the sun, you should really picture a circle.  And I say that because this gives the wrong impression.  This implies that sometimes we're really close to the sun and other times we're really far away, but that's really not true. The Earth's orbit is almost a perfect circle and so we're almost always the same distance from the sun. Our distance to the sun has a very small, actually negligible, impact on the seasons.  The tilt is really this primary factor in determining our seasons.  

Okay, let's take a look now and consider from our perspective looking at the sky.  So here's the fourth picture, and this is a familiar picture we've already seen. This is how this looks in terms of the motion in the sky. The sun rises in the east and sets in the west, and when we're tilted toward the sun, the sun appears higher and higher in our sky.  The highest point it reaches in our sky we call the solstice.  It's not just the highest, it's the most northerly, or the most southerly depending on which hemisphere you're in, is the solstice.  That's when we reach the peak of summer or the peak of winter.  That’s that summer solstice or that winter solstice.  And the equinoxes then, is when the sun is rising due east and setting due west and that is the halfway point in between the fall and spring; we're halfway between those two extremes of the seasons.  So this is connecting.  That tilt really impacts us.  Now, why does this matter?  

It matters because this is what causes our ground, the ground to get warmer.  The reason we have these warm and cold seasons is because the ground itself, and the water all around us gets warmer and warmer and warmer.  Why is it warmer?  Because the sun is higher.  We're getting more direct sunlight.  That's one of the key reasons.  The sun is beating down on us.  By contrast, in the wintertime, the sun is kind of coming in, it's low in the sky, that sunlight is coming in at an angle and it doesn't, you're not getting as much energy on the ground, because the sunlight is coming in from low in the sky.  So one of those one of those ways our ground is heating up is because of the height of the sun in the sky.  The other ways can be illustrated in the last picture in our gallery, which is a complicated graph. I mean, it's kind of cool.  If you want to impress your friends, say, “Hey, I'm learning astronomy, check out this cool graph.” 

This is one of the more impressive graphs that we're going to see throughout this course. What this graph is showing is the number of hours of daylight that you get at any given point in the year.  So across the bottom, you'll see all the months of the year.  That's the horizontal axis.  And the vertical axis is how many hours of sunlight you're getting.  Now, as you can imagine, when the sun is higher in the sky, in the summertime, and you know this from your own personal experience, in the summertime, the sun generally is up for more hours.  It rises earlier, and it sets later.  And in the winter time, you get less sunlight; it rises late and sets early.  That's what this graph is showing.  But there's something subtle, I want to illustrate. 

So for where I live, I live around 45˚ north latitude.  So that's this kind of greenish line.  So for me, where I live in Michigan, which is relatively far north, in the wintertime, we only get about six hours of sunlight.  That’s not very much.  And in the summertime, at the very most, we get 18 hours of sunlight.  Think about that dramatic change between the hours of sunlight.  I like I like summer more than winter, a lot more sunlight. If you were to live even further north, maybe getting close to Alaska, or way up in Sweden and Norway, now you're approaching 70˚ north latitude, or maybe your way in the southern hemisphere in South Africa or southern Chile, you're at 70˚ south latitude.  In those cases, in the winter, you actually reach zero hours of sunlight.  You get to a point where the sun never rises for weeks on end.  The sun just stays just below the horizon. It's kind of a perpetual Twilight. And likewise, in the summertime, you get to a point where the sun is up for 24 hours in a day.  For weeks the sun stays up in the sky, it never sets.  What a crazy extreme.  And not many people live at the extremes of the earth and for good reason.  Who wants to live somewhere where the sun disappears for a month and then is up all the time for a month.

If you happen to live closer to the equator, here's 30˚ north, even 20˚ north, you see much less variation.  Much less variation.  In fact, if you live on the equator, you always get every single day, 12 hours of sunlight and 12 hours of darkness, all year round.  So you get this kind of perpetual nice weather.  You don't get the cold winters and the hot summers.  It's kind of always warm and potentially hot because the Sun is always there for 12 hours a day.

Okay, so we're seeing all these changes that have happened.  It's all because of that tilt of the Earth's axis.  And depending on where you're located on the earth, you can have a dramatic difference in whether there are seasons, and if there are, how much variation there is in those seasons from summer to winter.

All right, we'll see you next time.