The Science of Everything Podcast - Episode 88: Cartography and Earths Seasons

Episode Date: September 29, 2017

A discussion of the shape of the Earth and the difficulties and conventions involved in describing a three-dimensional surface on a two-dimensional map, including an overview of some of the major map projections and their various limitations. This leads in to an overview of Earth's axial tilt and variation in solar insolation by latitude as an explanation for the seasonal variation in weather across the planet. Recommended pre-listening is Episode 87: The Geography of Planet Earth.

You're listening to The Science of Everything Podcast, episode 88, Cartography and Earth's Seasons. I'm your host, James Fodor. So the two topics that I've chosen to name this episode after might initially seem a bit distinct. I mean, what do the maps have to do with the seasons of the Earth? But actually, if you think about it a little bit, they're very closely related,

because explaining Earth's seasons relates to the axial tilt of the Earth and the different latitudes, or basically distance from the equator. of different parts of the world and when they're pointing away and towards the sun. And understanding that has everything to do with understanding maps and latitude, longitude, the equator, and all of the details about the language of those. So that's why these two subjects are put together in one episode, because one directly relates to the other.

So don't mind if you don't understand those concepts that I've been talking about. That's what I'm going to be explaining in this episode, particularly we'll talk about the language used to describe the earth, so latitude, longitude, equator, poles and all that stuff. We'll talk about map projections and the trade-offs involved in those briefly. And then we'll apply that knowledge to talk about the seasons. So what a season is and a little bit on the background there. And then we'll focus a lot of the episode on talking about axial tilt and how that changes between equinoxes and solstices. And we'll also look at the differences in the seasons between Equatorial.

mid-latitude and polar regions and how that differs from place on the globe. And we'll talk also a little bit about solar insulation and how also that varies between across latitudes and how that relates to the seasons. Recommended pre-listing is episode 22, our place in the cosmos, and episode 87, the geography of planet Earth, might be somewhat helpful in setting the scene for this, although that's not super necessary. Okay, so let's start by outlining and discussing some.

of the concepts used to describe the planet Earth. So the study of the shape of the Earth, together with its gravitational field, is called geodicy. And the first step, really, in approaching geodesy is to be able to describe the shape of the Earth using mathematical means. And the way that's done, obviously, because the Earth is not a perfect sphere. It's got mountains and valleys and so on, and the height, the elevation varies across the planet, among other things. We have to make some simplifications to describe this mathematically. The way that's done is by modelling the surface of the Earth by what's called a geoid, which essentially is a three-dimensional geometric shape.

Particularly what's called an oblate spheroid is used to model the Earth. An obloat spheroid essentially, if you imagine a sphere, and think about a basketball, if that's helpful, and situated so that the top of the basketball is facing up and the bottom is facing down. So you've got the axis about which it spins pointing up and down. Now, if you were then to push on the top of the basketball and squash it just a little bit so that it bulges a little bit around the center, that's an oblate spheroid. It's essentially a bit of a fat sphere, one with a bit of a belly that bulges out in the center.

It's contrasted with a prolate spheroid, which is sort of a tall, skinny one, kind of like a football stand stood on end. So the earth is an oblate spheroid. That means that the distance from one pole to the other pole is not the same as the distance around the equator. The equator is an imaginary line on the surface of the earth that's equally distant from the North Pole and the South Pole, which divides the Earth into Northern and Southern Hemisphere, so the Northern Hemisphere being the half of the Earth between the equator and the North Pole, and the Southern Hemisphere being the half between the equator and the Southern Pole. The distance around the equator, so the circumference of the Earth about the equator,

is 40,075 kilometers compared to the distance from one pole to the other, which is 40,08 kilometers. So those might sound essentially the same, and they are essentially the same. So that's why we think of the Earth as a sphere, because it's pretty close to a sphere, but it's not quite a sphere.

In terms of the difference between polar and equatorial circumference is about 1% roughly. So the Earth bulges slightly around its middle, essentially, and that's what we mean when we call it an oblate spheroid, or an oblate ellipsoid, it's sometimes also called. Also, when we're describing the Earth mathematically using this geoid approximation, we talk about the shape of the Earth

as if the entire Earth was covered by ocean. So we forget about all of the land masses and mountains and so on, and just imagine that the Earth was covered completely by ocean. Average sea level, then, is what we model when we talk about the shape of the Earth, at least to this level of abstraction. So, as you would hopefully know, and if not go back to one of the previous episodes,

I'm recommended, 22 and 1 on gravity. The Earth rotates about its axis, so it spins around its axis once every day. That's why we have day and night. And the axis about which the Earth rotates gives us the location of the poles. So essentially the, you can imagine the Earth as essentially a sphere. I'll often call it a sphere, even though you know it's not, because it's just easier. Imagine the Earth is a sphere, and you stick a long straight, well, stick through it, a pole through the centre. and then spin the earth about this pole. Again, imagine a basketball on a stick, and you're spinning it around. You're spinning the stick around.

That stick that goes through the middle of the sphere is the axis of rotation of the earth. The earth rotates about that axis once every day, and it goes through the north pole and through the south pole. So the earth rotates about an axis that projects out the north and south poles. And as I said before, the equator is a line that intersects the surface of the earth exactly midway between each of the two poles. So you could imagine drawing lines that are, say, closer up towards the South Pole or closer towards the North Pole.

Those would also go around the Earth, but they wouldn't go around the equator because they wouldn't be equally distant between North and South Poles. Or to put it another way, if you imagine going around the Earth up near the North Pole, then it would be a much shorter distance than if I would walk all the way around the Earth at the equator. Because I've got further to go around the equator because the Earth is a sphere, right? It curves outward, and the point,

or the line around which the distance that I would walk is maximal is around the equator. Understanding the poles and the equator is essential for understanding the concept of latitude and longitude. Latitude and longitude are two geographic coordinates that describe uniquely your location on the planet Earth. Now, there's another coordinate you could use, which is elevation, right? Because if you want to describe your position in three-dimensional space, you need three coordinates. However, here we're just thinking about the earth in terms of the mean sea surface level approximation. So there's no elevation, according to this metric. Everything's just on the surface of the earth. So we're not concerned with elevation for this episode. We're just interested in latitude and longitude. And typically, these are the two that are used by geographies in mapmaking and cartography, cartography by the... way is just the science of mapmaking in case you didn't know what that word meant.

Now, latitude and longitude, what's the difference between them? Latitude is a coordinate that specifies the north-south position, whereas longitude describes east-west position. That's the simple, easiest way of understanding them. More specifically, latitude is an angle which ranges from zero at the equator up to 90 degrees at the north or south poles. So latitude is defined as some number of degrees north or south, meaning towards the north or towards the south pole. So you can think of it as you start at the equator and then you rotate your angle up some direction either to the north pole or to the south pole. And however many degrees you rotate, that's your, that's your latitude. So if you're at the equator, you don't

have to rotate at all because, well, you're already there. So that's zero degrees. If you're at the North Pole, that's 90 degrees. You've rotated your angle all the way from pointing to the equator up to pointing to the North Pole. So that's a 90 degree angle, and therefore you're 90 degrees north Likewise, south pole, 90 degrees south. If you are halfway in between equator and the north pole, you'd be a latitude of 45 degrees north, likewise 45 degrees south if you're in the southern hemisphere. Lines of constant latitude are called parallels, and they run east-west as circles parallel to the equator. So this is what I was talking about before when I was talking about walking around the earth at different distances from the pole. This is equivalent to walking around the earth at different latitudes,

or flying around the earth, same thing. The idea is that parallels, that is lines of constant latitude, get longer as you move further away from the pole. The parallel right at the pole is, well, zero. Parallels get longer as you go further and further south until when you get to the equator, the parallel at the equator, which is just the equator, is a maximal distance, that 40,00075 kilometres. So that's latitude. What about longitude?

Longitude is a geographic coordinate that specifies your east-west position, as I said. It's also an angular measurement, so similar to latitude and it specifies an angle. But in this case, it doesn't specify an angle rotating up or down, north or south, from the equator, but instead it specifies an angle rotating either east or west of some point on the earth's surface. Now, in the case of latitude, we have an obvious parallel, which can serve as the zero point, namely the equator. It's the longest parallel, and it's the one equal distance between each of the poles. But what about in the case of longitude? Well, lines of equal longitude, that is, if you imagine drawing a line from the North Pole right to the South Pole, that's a line of equal longitude.

It's the same east-to-west position, just at different latitudes. These meridians, there's no obvious one which can serve as the zero point because any meridian is equally long as every other. There's none that are equal distant from the poles or from the equator or anything like that, just because of the fact that meridians run, in a sense, parallel to the axis of rotation rather than perpendicular to it, that they don't exactly run parallel. because they circle around the earth. But hopefully you get what I'm saying. Parallels form... You can draw a plane through a parallel which intersects the axis of rotation of the earth.

It goes through that plane, whereas you can't do that for a meridian. So for that reason, there's no obvious meridian to serve as the zero point. So one had to be chosen arbitrarily. And after some debate, mostly I understand between the French and the British, the Royal Observatory in Greenwich,

was chosen as the arbitrary prime meridian. That's where we say that the angle that we're measuring east and west from is at zero degrees. So it's sort of the analog of the equator, but for longitude in this case. Zero degrees runs through Greenwich, England. If you move east of that, you increase your angle. So that's said to be a plus or an eastward longitude. Or if you move westward, then your longitude angle decreases, or it says to be a westward longitude. So because there are 360 degrees in a circle, that means that

longitude ranges from 180 degrees eastward to minus 180 degrees westward. So those add up to a total of 360 degrees. So you can circle right around from zero degrees at the prime meridian through to plus 180 degrees and then plus 180 degrees maps straight into minus 180 degrees, obviously because the world is circular and so then you come back through the minuses and back through up to zero degrees. Now, You may be wondering at this point, why is it that longitude goes from essentially plus or minus 90 degrees at the poles, whereas longitude goes to plus or minus 180 degrees around eastern west? Now, the reason for that is because latitude only describes a angle or a position, north to north and south,

between the equator and up to the pole. If you think about that, that maps out sort of one quarter of a circle, and there's one quarter going from North Pole down to the equator and another quarter going from the equator down to the south pole, so that's one half of the Earth's surface, whereas longitude wraps around the entire world, so that's 360 degrees. So to put it another way, it would be redundant for latitude to go out to 180 degrees, because in fact if you go at 91 degrees latitude is actually the same as 89 degrees latitude,

if you just sort of try and visualize going up to one degree just before the pole and then one degree across down from the pole, those are actually the same thing. So it would be redundant to have any more than 90 degrees plus or minus 90 degree latitude. So I think that's enough about latitude, longitude, and those other terms. I emphasize that because it is important to understand these, because I'll be talking a lot about them in other parts of the episode. Now I just want to briefly talk about map projections just to include this important content. So a map is a two-dimensional representation of the surface.

of the Earth, particularly I'm talking about global maps at this point. And it's mathematically impossible to accurately represent distance, shape, and size all at the same time on two dimensions when you're projecting from a spherical or close to spherical three-dimensional shape. So that's mathematically impossible to do, which means that every world map is inaccurate in some respect. Either it distorts shape, or it distorts distance between points, or it distorts area, or it distorts some combination of the three. Note that this is true for all maps, however, maps of smaller areas don't have as much distortion because a small section of a sphere is always roughly flat, so the curvature is not noticeable unless you get to a sort of large

portion of the globe. But when you're trying to depict the entirety of the Earth, the problem becomes very severe. So every map projection has to make compromises and trade-offs about what it's trying to represent and how it represents it. Some of the most common methods of representing the earth on a two-dimensional surface are called projections, because the original method of doing this was to literally project an image of the earth, that is a spherical globe of the earth, onto some sort of flat surface. And the different types of flat surfaces defined the different types of projections. So for example, you can have a conic projection, which is essentially imagine that basketball again, which represents the earth, and you put like a really big ice cream

cone sitting on top of it, or really you could put it anywhere on it. The fact that it's an ice cream cone is not important, just a conical shape that sort of sits on top of it. That is a conical projection. Essentially what it does is you project the spherical surface of the earth onto that cone. So obviously when the cone is directly lying against the curvature of the earth, that's going to be most accurate there because essentially the projection distortions are going to be smallest. However, as the straight surface of the cone moves further and further away from the curved surface of the basketball, you'll have more and more distortion. That is more and more difference between what the two-dimensional map shows and what the three-dimensional surface of the

earth actually is. So these conical projections can be useful for projecting mid-latitudes because the place of the least distortion tends to be neither on the equator nor at the pole, but somewhere in between just because of the way a cone sits on a sphere. Just try and visualize that. You'll see what I mean. Another type of projection, which is probably the most commonly seen type of projection, still to this day, unfortunately, although it's not as bad as it used to be, is a cylindrical projection, which is essentially just placing a big cylinder around a sphere, usually around the equator. And so this projects, again, you visualize where does a cylinder touch the sphere, it touches it on the equator, and it gets

further and further away from it as you move towards the poles. So this means that the equator, or the equatorial regions will be the most accurately represented on a cylindrical projection, and the poles will be the least accurately represented. So these cylindrical projections are useful for depicting the equatorial regions, but pretty much useless for depicting the polar regions. The most famous cylindrical projection is called the Mercator projection, and it's probably the single map projection that you're most familiar with. because they have been, as I said, a little bit, not as much now as maybe a few decades ago, but still to a large extent used for wall maps and in atlases and so on,

particularly in schools and other places like that. The Mercator projection and similar sorts of cylindrical projections are very useful, and one of the reasons that they're so widely seen is because they were used for centuries by navigators, because on a Mercator projection, lines of constant latitude and lines of constant longitude, that is meridians and parallels appear as straight lines, whereas on most other map projections, that's not the case. And having meridians and parallels appear as straight lines makes it really convenient for navigation, obviously, because you just decide on a bearing and follow that, and you can mark it as a straight line along a map. So very useful for ship-based navigation.

So that's why it was typically used. But it's not accurate in terms of representing regions as they're close and closer to the pole. And this is a particular concern for the northern hemisphere, because there's much more land. and in the northern hemisphere near the polar region. So the southern hemisphere is pretty accurately represented, except for Antarctica. But the northern hemisphere, particularly the more northerly regions, are hopelessly inaccurately represented.

Actually, it does shape correctly. Moseol projection preserves shape quite well, but area is dramatically inflated near the polar regions. So particularly Greenland is the most obvious example of this on a Mercator projection. If you just look at a Mercator projection, Greenland looks bigger than Africa, whereas, in fact, Greenland is, like, less, than one-tenth the size of Africa, I think. It's way, way smaller than Africa. That's because

the polar regions are grossly distorted in terms of the area. They look much larger than they should be. And if you actually look at most Mercator projections or similar ones are clipped at near the poles because of this extreme distortion when you get near the poles, they actually chop them off near the top and the bottom. And actually, I think it's a bit misleading the way this is done because typically the latitudes where they chop them off is different in the the north and in the southern hemisphere. In the southern hemisphere, they typically chop them off just somewhere around 65 degrees south, which is around where you just see Antarctica poking out, 65 to 70 degrees south. So you'll see just a little bit of the coast

of Antarctica down the south of many of these Mercator projections, but most of the rest of Antarctica is just cut off the map. If you actually look at a full Mercator projection, or closer to full one, you'll see that the total size of the area of Antarctica on a Mercator projection, if you're allowed to see the whole of it and they don't chop it off, is actually about the same size as the total land mass area of the entire rest of the world. That is, the area of Antarctica on a Mercator projection, when they don't cut it off, is so grossly distorted that it actually swamps the rest of the, like half of the map is Antarctica, basically, or it looks like it's half of the map. And that's why it's typically cut off, just because it's so distorting, it looks ridiculous.

and Antarctica is not nearly that large in reality, but because it essentially sits right over the South Pole and the polar regions are the most distorted, its area is just dramatically distorted by a full Mercator projection. But the point I was making before is that they typically cut off the Mercator projection around latitude 70-ish degrees south. But up in the north, if you were to cut off at the same latitude, you'd be cutting just north of Iceland,

and yet you'd actually cut off the very northern parts of Canada, Half of Greenland would be missing. You'd cut off the northern parts of Siberia, and so that would remove areas of the earth that people are typically interested in, more so at least than Antarctica. So they typically cut Mercantor projections much further north, somewhere around 85 degrees north, which is much, as I said, much closer to the pole than in the south. So they're actually lopsided, in a sense, many of the Mercator projections that you see. Those that don't show a grossly distorted Antarctica are generally going to be lopsided in that. They've actually cut off more of the southern hemisphere than the northern hemisphere. And I think that while this might

be very useful for navigation, especially in the past, it's a very misleading way of portraying the Earth. And that's actually why the National Geographic Society and other geography societies like that strongly recommend against wall maps and geographic atlases displaying the world on a rectangular map, because the world isn't rectangular, it's spherical, and so a problem map should show at least some curvature there. So the Mercator projection for that reason is quite inaccurate because of this need to cut off the north and south poles and the tendency to do that differentially in the north and south hemisphere. It leads to a lot of misperceptions about the relative sizes of parts of the world. So that's some knowledge about the Mercator projection that might

be useful to you. Next time you go around looking at maps, the Mercosur projection, as I said, is a type of cylindrical projection, so it distorts the poles most severely. In some sense, the opposite of the cylindrical projection is an azimuth. or a plane projection, which is, instead of placing a cylinder around the equator, it's kind of like the plane or azimuthal projection is like putting a dish, or essentially a two-dimensional circle, at some point on the surface of the earth, often at the pole, although it doesn't have to be at the pole, but if you imagine putting it at the pole, then this little circle that you've put on the top of the earth meets the sphere at the pole and then increasingly diverges from the curve, as the sphere curves away,

as you move further away from the pole. So these types of planar projections represent the pole most accurately, and as you move further and further away from the pole towards the equator, the distortions increase there. So these sort of azimuthal projections are particularly useful for depicting the poles. Better world maps, if they use a cylindrical projection

like the Mercator, or even something somewhat like that, which tends to represent mid-latitudes and equatorial regions more accurately but distort the poles, Good maps will also include planar projections of the north and south polar regions so that you can get a more accurate view as to what those look like. So those are three of the major projections that are used. There are many others as well, and these days, cartographers aren't limited to actually physically projecting the light from spheres to two-dimensional shapes because they use computers to do this, and so they've come up with all of these very complex mappings and ways of representing the earth. Another type of projection that might be useful to know about is an interrupted projection. This is essentially where you try to represent the Earth by literally unfolding the sphere and then flattening it out. And if you do that, what you'll see is that there are blank spaces.

So it won't be a nice circle or a rectangle. It will be sort of blobby and patchy. And generally the way this is done is that they sort of cut it so that these missing patches are over the oceans, not over the continents. This helps preserve the shape and size more accurately, but it does lead to a sort of a funny-looking map, and I don't think they look as aesthetically pleasing because you've got all these big white areas sort of in the middle of the earth like that.

But that's the price you pay for trying to represent three dimensions into two. I can't help mentioning my personal favorite projection, which is the Robinson projection, which I think is, if you're going to use one of these, for projecting, representing the whole earth, is one of the best to go. with. The National Geographic Society used to use this, but they've since switched to a slightly

different one. The Robinson projections unique because most of these map projections, they start off with sort of a mathematical formalism, or that is a mathematical transformation to represent three dimensions to two, and then they carry that out and see what it looks like. Whereas what Robinson did is he essentially said, well, I'm going to figure out what looks good, and then reverse engineer and figure out what transformation will produce that. That is, he fiddled around with the parameters of his projection so that he got what he thought was the right balance between distortion of shape at the poles and distortion of area and accuracy of relative distance and things like that. So he sort of balanced all those out so that it achieved what he

thought was an aesthetically pleasing balance. And I think actually it's a really good, a really good balance. So it's particularly instructive to compare, say, a Mercator projection to a Robinson projection and see how they differ. So anyway, that's enough about cartography and map projections. Now I need to get on to talk about the seasons, which is sort of the reason we set up all of this discussion about maps and longitude and latitude and stuff like that, so I can talk about the seasons.

So before we get on to the reason for the season, we need to talk about what is a season. Now, most people have a fairly intuitive idea about what a season is. It's just the division of the year that's marked by changes in the weather and daylight, ecology and so on. You know, trees will lose their leaves or gain their leaves, or you'll have bushfires during parts of the year, or the temperatures will be hotter or birds will migrate, stuff like that. These define the seasons. Now, most places there aren't formally defined seasonal boundaries. In mid-laditude regions, typically there are four seasons a year,

summer, winter, autumn, or fall in the US, and spring. In the northern hemisphere, summer is generally defined to be the months that are the hottest, so those will be during June, July and August, whereas in the southern hemisphere, those are actually the coolest months, so those are winter. So the seasons are swapped between the hemispheres. Summer in the northern hemisphere is winter in the southern hemisphere, and likewise autumn in the northern hemisphere or fall is spring in the southern hemisphere. But that's only really relevant to the mid-latitude regions, which is where I'd say most of the world's population leaves. But at the equator, the four seasons isn't really a relevant model, because if you recall from

the previous episode on World Geography. In the equatorial regions, it's pretty much hot and wet all year long. So in the equatorial regions, really, there are only two seasons, basically the wet and the dry season, or in some places the wet and the somewhat less wet seasons. The reason for this essentially is because there's a rain belt, which can be thought of as existing roughly along the equator, but it doesn't sit exactly over the equator. It moves over the course of the year. to the north and then across the equator back to the south and then back again. So when the rains move up into the northern hemisphere, that occurs roughly over the period of northern summer,

but it also extends sort of a few months either way. So the tropical rain belt then moves back into the southern hemisphere where it sits for roughly around the southern summer, plus or minus couple of months, so October to March, and then back up to the north, rainwater to September, and then back over again. So if the rain belt's over where you are, that's going to be the wet season, and if it isn't, then that's going to be the dry season.

So this is a more relevant seasonal variation for tropical regions. So why do we have seasons? Why is it that during part of the year, we have relatively warm weather in summer, say, and in the other part we have relatively cool weather, or downright cold weather, depending on where you live, in winter? What's up with that? Now, naively, I think a lot of people reason that in summer we're closer, the earth is close, to the sun, and in winter, the Earth is further away from the sun. Now, this turns out to not be the case, or in other words, this isn't the reason for why the seasons exist.

But if you think about it, it actually can't be the reason. And that's because when it's summer in the northern hemisphere, it's winter in the southern hemisphere. So suppose during the summer in the northern hemisphere, the Earth was a lot closer to the sun, and that's not a case. but just suppose it were the case. If the northern hemisphere is hot, that is, it's summer there, because it's closer to the sun, then that must also be true for the southern hemisphere as well, because if the northern hemisphere is closer to the sun, the southern hemisphere is closer to the sun as well, that they kind of go together.

And that would seem to imply that when it's summer in the northern hemisphere, it should also be summer in the southern hemisphere. And then when the earth moves further away from the sun, then it would be winter in the northern and southern hemispheres. But that's not how it works. When it's summer in the northern hemisphere, it's winter in the southern hemisphere. So that doesn't make any sense at all. It can't be the case that seasons are primarily the results of distance of the Earth to the Sun

because then seasons would be the same in northern southern hemisphere, but they're opposite. So what's going on here? It clearly can't be a distance from the Earth, a distance from the Earth to the Sun. In fact, the main reason for seasons, globally speaking, is the axial tilt of the Earth. So let's explain this a little bit. So we know that the Earth rotates about the Sun once every year, and the Earth also rotates about its axis once every day. The plane of the Earth's orbit about the Sun is called the ecliptic,

and the plane perpendicular to the rotation of Earth's axis is the equatorial plane. The axial tilt is the angle between the two of them. So in other words, you may not have known this, but the plane about which the Earth rotates every day is not the same as the plane about which the Earth or Earth always. orbits the Sun, there's an angle between them. In fact, in general, there's no reason you would expect them to be the same. And in the case of some planets, they're quite similar. In the case of the Earth, they differ by about 23 degrees. In fact, you can't give a precise number because the axial tilt

actually fluctuates within a margin of about 2 degrees over a 40,000 year period, and that's due to tidal forces of the Moon. So, not only is the Earth's axis of rotation tilted relative to its orbit about the Sun, but that tilt sort of wobbles a little bit over a thousand, of years. But coming back to the sun and the earth, this roughly 23 degree angle between the ecliptic and the plane of equatorial plane is called the axial tilt of the earth, and it is the main reason why seasons exist on Earth. So to understand how this works, you need to visualize the Earth as it rotates about the Sun. So think about that basketball on a pole again, with the pole extending through the axis of rotation of the Earth. And now imagine that that pole is sticking

straight up and down and we then orbit that, we take hold of the pole and orbit about the center, the center of a big circle, which is the sun. So the pole is up and down perpendicular to the plane of orbit around the sun. This would be the case, or this would be a description of the Earth about the Sun, if there was no angle between the Ecliptic and the equatorial plane. That is, if the Earth had zero axial tilt. In fact, that's not the case, obviously. We know that there's 23 degrees of actual tilt. So what we need to do is take our pole that sticks through the Earth and rotate it a little bit,

about one quarter of the way towards full horizontal. So we tilt it a little bit and then orbit it, and then keep sort of moving it, circling around the centre of the, which is the sun. So we've got this slightly diagonal pole that's sort of sticking through the Earth and now rotating around the sun. The Earth, meanwhile, is rotating about this pole.

So the axis about it which is rotating is not the same as the axis about which it is orbiting the sun. Now, the single most important thing to realize is that because of this axial tilt, effectively, at any given time, one of the hemispheres is pointing relatively more towards the sun, while the other hemisphere is pointing more away from the sun. So think about it this way. If the axial tilt was zero, then that pole about which the Earth roared, rotates is sticking right up and down. The North Pole, polar region, so the Arctic and the Arctic and the South polar region, so around Antarctica, are both pointing, sort of facing perpendicularly

away from our plane of rotation about the sun. Now that means when the sunlight comes in, obviously the sunlight is coming in straight, so the sunlight comes in parallel to the plane of our orbit about the sun. The sunlight is not going to hit either the north or the south pole, because for it to hit the north or the south pole, it would have to come vertically downwards, right? But the sun, the sun that doesn't come from there. It comes sideways. So if that were the case, if there was no axial tilt, effectively the north and south pole would basically never receive any light. They'd never be able to see the sun. That's not strictly true because there's also the Earth's atmosphere to take into account, and that scatters the light a bit. But we're imagining the Earth didn't have an

atmosphere, and we're imagining it's perfectly sphere and other simplifications to get the main point across. but if that were the case and if there was no axial tilt, in fact, there would be a perfect relationship between how far away from the equator you were and the amount of sunlight that you got. Obviously during the daytime, we don't get sunlight during the nighttime. So at the equator, you'll get the maximal amount of sunlight, which is called solar insulation, because the sunlight's coming right directly and hitting you face on. Whereas up at the poles, the sunlight's not hitting you at all. It's sort of skimming right across you, right past you, and you're missing it. Halfway sort of between the poles and the equator,

you'll get some sunlight, but you won't get as much because the earth curves away from those rays of sun as they're coming in, and so the sunlight's going to be spread over a wider area, and that will increase, that is the area that the incoming sunlight is spread over, increases as you move towards the pole, until at the pole itself, the sunlight's coming in right parallel to the surface of the earth there, and you get no sunlight whatever. And so you never, see this sun. This, I emphasize, would be the situation if there was no axial tilt. You'd get maximum solar insulation at the equator and progressively less as you move towards the poles, at which point you would get exactly none. But what happens when the Earth does have an axial tilt?

Well, in that case, one of the hemispheres, let's say it's the southern hemisphere at this point, will point somewhat towards the sun, not completely towards the sun. That would only happen if the axial tilt was 90 degrees, then the pole would point directly toward the sun, and it'll be the equator that wouldn't get any sunlight. But that's an extreme case. If there's only a little bit of axial till, you know, 23 degrees, then the south pole would point somewhat towards the sun, while the North Pole would point somewhat away from the sun. Now, what does this mean? Well, it means that the southern pole would now get more light or more solar insulation than it did before, because it's relatively towards facing the sun. So it's getting more of those, well, before it didn't get any,

and regions right near the pole barely got any, and now the pole's getting some, and regions that are just near the pole are getting more than they did before. To get the maximum amount, it'd have to rotate all the way up to 90 degrees, so it was directly facing the incoming sun rays. Of course, it doesn't go that far, but it goes somewhat of the way there.

What happens to the North Pole? Well, the North Pole before was directly perpendicular to those incoming sun rays, and so exactly at the North Pole, you got no sunlight at all, but just a little bit away from it, you got barely any. but a tiny amount of sunlight because it was spread over such a wide area.

Now, however, the North Pole as a whole is pointing away from the sun. So not only does the North Pole exactly at the North Pole get no sunlight, but even some distance around the North Pole doesn't get any sunlight either. And exactly what that distance is, we'll talk about in a moment, but it depends on the axial tilt.

Another thing that's happened as a result of this axial tilt is that the place of the Earth's surface that gets maximal solar insulation that gets the most sunlight per unit area is no longer exactly on the equator because there's been a tilt, there's been a relative rotation. So in fact, it's some place, if we keep with our analogy

or our case of the southern hemisphere pointing towards the sun, it will be some region just a little bit somewhere below the equator in the southern hemisphere, which will actually be directly facing the incoming solar rays. And so that region, not the equator itself, will receive maximal solar insulation.

Now, so far I've been imagining as if the Earth was just sitting still and the sun was shining on it according to its axial tilt. But of course, the Earth is rotating about its axis. So how does that affect the situation that I'm describing? Well, that rotation about its axis is going to bring one half of the Earth facing the sun, one half the day, and then the other half facing the other half of the day. So you're going to have a continual rotation around there. And so the what's called the Terminator, so the region, the demarcation between day and night, in our hypothetical example is a clearly defined line

because we're imagining there's no atmosphere and other complicating factors which blur it. But the Terminator is going to continually move across the Earth as the Earth rotates. However, what it won't do is in this situation where the South Pole is facing towards the Sun, the Terminator will never actually cross over the South Pole. It might take a moment to realize why this is the case,

But again, imagine our sphere with the pole pointing straight up and down. That's the zero-axial tilt case. Now imagine tilting the axis so that the southern pole points somewhat towards the sun. And now imagine the sun is shining its light rays on the earth. What part of the earth is illuminated by those light rays? It's going to be exactly the half of the earth, as seen from the sun, that the sun can see. And then now imagine rotating that earth about the pole that's sticking through our basketball. If you do that, hopefully you can visualize what I'm saying here, you'll notice that although for most of the Earth's regions, they'll pass into the illuminated region, then out of it as they rotate around and then back again, some region around the South Pole will always be facing towards the sun, and so will always be in daylight.

And conversely, some region around the North Pole will always be facing away from the sun, and thus will always be experiencing nighttime. and how big that region is, again, depends on the axial tilt. If the axial tilt was maximal so that it was 90 degrees, and this pole now faces directly towards and away from the sun, then what you would have is the entire southern hemisphere would constantly point towards the sun and therefore would always be in daylight, whereas the entire northern hemisphere would constantly point away from the sun

and therefore would always be in nighttime, and the earth would rotate around, but it wouldn't make any difference as to which part was illuminated, because the rotation would not bring any new, part of the Earth to facing the sun. But that's the extreme case. In the moderate case where there's only 23 degrees of actually... There's only a relatively small region around the southern pole and the northern pole that experiences these periods of all night and all daytime. Okay, but what does all this all this have to do with the seasons you might be wondering? Well, this has all been

necessary set up to explain why we have seasons, because so far I've just been imagining a case where the southern hemisphere is always pointing towards the sun. But that, that's a very, and That is not what happens. What actually happens is that the Earth is constantly rotating about the Sun. The axis of Earth's rotation, however, is always pointing in the same direction. It doesn't rotate around the Sun with the Earth. So if it did, the Earth would be what's called tidily locked to the Sun. That's the way the Moon is to the Earth. The same side of the Moon always faces the Earth. So its axis of rotation effectively is locked with respect to how it faces the Earth. But that's not the case with the Earth and the Sun. That pole always points in the same direction, the pole about which the Earth rotates,

whereas its orbit about the Sun continually changes the Earth's relative position to the Sun. So if at one time of the year, say when, imagine again our situation where the Sun's at the center and the Earth is on the left, we've got that pole that's sticking relatively towards the Sun in the Southern Hemisphere, now imagine rotating that Earth, pulling it around the circle, so that it's now sitting on the other side, the right-hand side of the sun. Now, which direction is that pole going to be facing? The pole's going to be facing towards the sun in the northern hemisphere, because it's sloping diagonally down from the top left to the bottom right,

and it was when it started, and it still is. It doesn't change direction. But now, because it's on the other side of the sun, the northern hemisphere is going to be, or the northern polar region specifically, is going to be facing relatively towards the sun, whereas the southern region is going to be facing away. So you've had an inversion, whereas previously the southern pole was always in the sunlight.

Now, the northern pole is always in the sunlight, and the southern pole doesn't experience any sunlight during that time of year. So those are the two extremes when the North Pole is facing away and when the North Pole is facing towards the sun. What about halfway in between? So in that case we can imagine, remember, in the first case, the Earth was on the left-hand side of the sun. In the second case, it was on the right-hand side. But what if it was halfway in between so that if you imagine the sun is in the middle, the earth is between the sun and us,

and then we're looking at it from face on? What about in that situation? Well, in that situation, remember, we've still got the pole that sticks through the earth that is the axis of rotation still goes from the top left to the bottom right. It still points in the same direction. It always points in the same direction,

conservation of angular momentum. So in that case, the earth is rotating about that axis, but the axis itself doesn't rotate. So in that circumstance, what we actually have is neither the North Pole nor the South Pole is pointing towards the Sun. In fact, both are sort of pointing sideways relative to the Sun. Hopefully you're following me and my visualization here. The axis of the Earth's rotation, instead of pointing more or less in the same direction as the Sun's rays, as it did in the left hand and the right-hand case, it's now completely perpendicular to the direction of the Sun's rays,

which are coming into and out of the picture, that is, towards us, whereas the axis of Earth's rotation is, diagonal from top left to bottom right. So in this case, neither the North Pole nor the South Pole is pointing towards the sun. In fact, no part of the Earth is pointing relatively towards the sun. In this situation, all of the Earth experiences exactly 12 hours of daylight. So there's no region of the Earth that's always in sunlight and no region of the Earth that never has sunlight. So this situation corresponds to the equinoxes.

During the equinoxes, neither the North Pole nor the South Pole is pointing towards the sun, all regions of the earth on the exact day of the equinox experience exactly 12 hours of day and 12 hours of night. Because essentially, in that situation, the terminus, the demarcation between night and day, runs directly along from the north pole to the south pole. And so therefore, there's no tilting of the terminus one way or the other. Or in other words, no biasing of the north or the southern hemisphere. So each gets exactly 12 hours of day and 12 hours of night, regardless of what your latitude is. The two can. cases that I started off with, remember, when the Earth is on the left and the southern hemisphere points towards the sun, and when the Earth was on the right and the northern hemisphere points towards the suns, those cases correspond to the solstices. That is, days of the year when you either have the longest day, that is the most sunlight or the shortest day, the shorter sunlight. When the southern hemisphere points towards the sun, that's the southern summer solstice, because their days are longer, they're getting more sunlight, because their hemispiece is pointed relatively towards the sun. The further towards the South Pole you are, the longer your days become.

until you reach that region where the sun never sets, and therefore you have maximal day length at that time. Whereas at that time in the northern hemisphere, the North Pole is pointing away from the sun, therefore the days are shorter, the further north you go until you reach that region around the North Pole, where, in fact, you have maximally short days, that is you don't have any sunlight,

because that region is never illuminated, and the exact reverse happens when six months later the Earth is on the other side of its orbit. Now, the northern hemisphere is experiencing its summer solstice because it's pointing relatively towards the sun, and its days are longer, and vice versa for the southern hemisphere pointing away. Now, these regions that I talked about that experience periods of complete darkness, that is no light during winter and 24 hours of complete sunlight during summer, these correspond to the Arctic and the Antarctic circles, obviously the Arctic up north and the Antarctic

down south. and they located around 66 degrees latitude. So that's about two-thirds of the way, roughly, from the equator up to the North Pole in terms of angular distance. The Arctic Circle is located just sort of, it cuts across the northern region of Canada, across through Greenland,

just north of Iceland, and just at the very northern tip of Scandinavia and across northern Siberia. The Antarctic Circle basically just circles right in Antarctica. So Antarctica, it sort of marks out the boundaries of Antarctica, loosely speaking. So these regions mark out the areas of Earth where at least some of the time during the year, there are days either of no sun or of no setting sun. That is, when the sun never sets during summer or when the sun never rises during winter. If you'll write on the, let's say,

Antarctic Circle, right down south, right on there, then the only day when you'll experience the when you'll experience the full extent of that, that is, say, the 24 hours of sunlight, is on the solstice itself, right on the summer solstice. On the summer solstice, right on the Antarctic Circle, at that latitude, regardless of your longitude, at that latitude, you'll experience 24 hours of sunlight. Vice versa during winter, and on the Antarctic Circle, the winter solstice, you'll experience no sunlight. The sun won't rise on that day, because it won't ever clear the horizon, because you're located on a part of the Earth that's pointed away from the sun.

So right on the circle, it's only one day of the year. the solstice. As you move further towards the pole, more and more days of the year will be like that until you get to the pole itself, which experiences six months of sun and then six months of night, essentially. So its days right at the pole are six months long. Again, this is an ideal case where there's no atmosphere. In the rural world, it's going to be a little bit more complicated than that, because you can see the sun, even if it's not quite clear the horizon because of the bending effects of the atmosphere and so on. I'm not going to go into all those things. details here. I'm just trying to emphasize how axial tilt affects the seasons. So in an ideal

case, the pole is either facing towards the sun, in which case it always receives sunlight, regardless of the rotation of the earth's axis, or it's always facing away from the sun, in which case it never receives the sunlight. And if you think about it this way, the earth is rotating about its axis, which extends from the north pole to the south pole. That means the further away from the pole you are, the greater your velocity is, as you, as that part of the Earth's spins around. At the pole itself, there's actually no velocity. The pole isn't orbiting about itself, because the pole is at the place where the orbit is centered. So, unlike other places on the earth, where the rotation of the earth causes you to move into and out of the region of

sunlight, that doesn't happen at the poles. You're either in the region of sunlight or you're out of it. The rotation of the earth isn't going to help you there, because you're right where it is. So at the poles, winter is the same thing as night, and summer is the same thing as day. That is, the summer is six months long, as is the day, the winter is six months long, as is the night. As you move further away from there, you get more and more balanced days and nights until when you're at the equator, you've got the most balance. So that's the Arctic and Antarctic circles. That's why those are important. Those are the places where, right on the solstices, you get either no sun or a 24-hour sun. depending on whether it's winter or summer.

What about the equator? Well, the equator is the place where you get maximal sunlight. Actually, not at the solstices, remember, because at the solstices you've got a relative tilt. Where do you get maximal sunlight at the solstices? I said it was either somewhere to the north or to the south of the equator, again, depending on the season, whether it's winter or summer.

But in the southern hemisphere, there's a latitude called the Tropic of Capricorn, which is about 23 degrees south of the equator, so sort of a third of the way towards the pole, very roughly speaking. In the northern hemisphere, there's something called the Tropic of Cancer, which is, again, about a third of the way

up towards the North Pole. The Tropic of Cancer marks out the latitude where the sun is directly overhead at the summer solstice, that is the summer solstice in the southern hemisphere, because it's at this region, this latitude around the earth, that the sun is directly overhead.

It's not directly overhead of the equator because the equator is actually tilted a little bit away from the sun or a little bit relative to the sun at this time of year. Vice versa, if you move six months ahead, the Tropic of Cancer will be, in the Northern Hemisphere, will be the place where the sun is directly overhead when there's a summer solstice there or when there's a winter solstice in the southern hemisphere.

Of course, when I say the sun is directly overhead, the sun obviously moves across the sky over the course of the day as the Earth rotates. So I'm talking about at noon, where is the sun? you may have been told that the sun is always directly overhead at noon, but that's not correct. That's only true at one particular latitude of the earth on any particular day. So that is, on the summer solstice, the sun is only ever directly overhead at noon along the tropic of cancer in the summer solstice in the northern hemisphere.

And the summer solstice in the southern hemisphere, the sun is only ever directly overhead, meaning 90 degrees from all of the horizons, smack bang right over the top of you, if you look sort of straight upwards, that's what I mean by directly overhead. The sun is only directly overhead in the southern hemisphere during the summer solstice along the tropic of Capricorn on the day of the summer solstice. What about at the equator? When is the sun overhead at the equator? The sun is overhead at the equator at the equinoxes.

So that's remember the time when the earth, when neither the south pole nor the north pole is facing the earth, but the earth is halfway in between those extremes, so that our axis of rotation is from the top left to the bottom right is perpendicular to the direction that the sun is coming, which is towards us, we're imagining. On that day, the equator is neither tilted away nor towards the sun, and so the equator itself is the region or the area of latitude, which will experience the sun directly overhead at noon on that day. So these regions, the tropic of cancer up to the north, down to the equator, and then down to the tropic of Capricorn down in the south, these are called the tropics. And it's these regions I talked about in the previous episode where the rainforests are found

and also the savannah regions are mostly found. These are hotter regions because they receive more solar insulation, because of the fact that they're close to the equator. They also have less seasonal variation compared to the north and the South Pole's because there's not much difference between the winter and summer near the equator, because winter and summer, the difference between them is defined by whether you're tilted away or towards the sun. The closer you are to the poles, the more extreme the difference will be until when you're at the pole, you get maximal extreme difference of seasons when either you have six months of sunlight or six months of nighttime. That's an extreme difference in seasons. That's about as much as you can get.

At the equator itself, there's very little difference between the seasons, because all you have essentially is a very slight fluctuation of the the total solar insulation as you move from one seasons to the next. But there's not much of a change. Pretty much all of the time you're getting close to 12 hours of daylight and pretty much all the time you're getting nearly the maximal amount of solar insulation. Because even in winter, you're still pointed pretty close to the sun rays still come nearly directly face onto you. Or in other words, at noon the sun is nearly overhead. So there's not much variation in the seasons around the equator, around the equator,

around the equatorial regions, except as I mentioned, the difference between the wet season and the dry season, as the rain band moves up and down during the seasons. As you move further away from the equator, the seasonal variations become more extreme. So a very crude way of thinking about this is to imagine that the seasons are caused by the difference between how much sunlight or solar insulation, your latitude is. expecting to get versus how much it actually does get, and the difference of course caused by the axial tilt. If there was no axial tilt, then the polar regions would receive essentially no sunlight at all, as they rotated about the sun. It wouldn't make any difference,

that the sunlight always sort of just passes them by, whereas the equatorial regions would receive maximum sunlight, and the mid-latitude regions would receive somewhere in between. Just the further away from the equator you were the less sunlight you would receive. Again, I should note that I'm ignoring the effects of cloud cover here, which complicate the picture. the picture. We'll get to that later, but here we're just talking about purely in terms of the curvature of the earth away from those incoming solar rays. And if that was the case, you would also have no seasonal variation. There was no axial tilt. However, because of the axial tilt, during the southern summer, the southern polar regions and areas surrounding them, in fact,

the entire southern hemisphere, but especially the polar regions, get sort of more than expected amount of sun, because of that tilt towards the sun. Whereas the northern hemisphere gets less than expected and progressively more extreme as you get towards the pole. Vice versa, obviously, during the northern summer where it gets more sunlight than expected, and the southern hemisphere gets less. So because of this seasonal variation across different parts of the year, as the Earth moves about around the sun in its orbit, there are differential amounts of sunlight that regions of the Earth are receiving, and therefore different temperatures.

There's actually a lag between, what's called seasonal lag, between when an area receives the most amount of sunlight and when that area actually reaches maximum temperatures. So, for example, if you live in the southern hemisphere, you receive the most amount of solar radiation on the summer solstice when the southern hemisphere is most pointed towards the sun. And that happens around near the end of December. However, the hottest months in the southern hemisphere, at least in mid-latitude regions, are generally in January or sometimes even February. there's a lag of around six or so weeks between when you get the most solar radiation

and when you actually reach the highest temperatures. And that's essentially because the Earth takes a while to heat up and cool down, especially because of the oceans. The oceans suck up a lot of heat during summer and then slowly release that heat during winter. So that delays the period as well. I'll talk more about that in future episodes about the mediating and moderating effects of water. But it's important to understand that seasonal lag, and that's sometimes where you get variations in the definition of summer and winter.

Do you define them by when you get maximum solar insulation, or do you define them when you get highest temperatures? Because there'll basically be a one-month shift in the later direction towards the end of the year if you define it by temperatures because of that seasonal lag effect. One final point that I want to discuss is sun paths. That is, what paths does the sun progress across as it passes overhead? Now that varies depending on two factors. It varies depending on your latitude, and it varies depending on the time of year. Now, the simplest case, which is perhaps what you'd been taught or imagined, is when you live on the equator.

When you live on the equator, on the days of the equinox, the sun rises directly in the east, and passes right overhead at noon, and then sets exactly in the west. Very simple case. But that only happens at the equator, and it only happens on the equinoxes. So for the rest of the world and for the rest of the world, and for the rest of the, the time of year, it's a bit more complicated than that. So what happens at the equator during other times of year? Well, at the equator away from equinoxes, say at the solstices, as you moved away from the equinoxes towards the solstices, the sun gradually moves either towards the south

or towards the north in terms of where it rises and sets. And it does so as sort of a fractional degree every day, so that during the southern summer solstice, the sun actually rises at the equator, 23 degrees south of east and passes in a straight line over, never goes directly overhead, and then sets also 23 degrees south of west. So that 23 degrees roughly is the axial tilt. So as you get closer to the equinoxes, the sun gets closer towards rising in the east and setting in the west,

whereas as you get closer to the solstices, it moves either towards the south or towards the north. Now that's at the equator. The picture is similar at mid-latitudes, but it's a bit more complicated. So we'll jump straight from the equator up to the poles, because the equator is in the poles are the easiest to visualize. Whereas at the equator, the sun rises in the east, goes directly overhead and sets in the west, at the equinoxes. At the other times of year, it's a little bit shifted to the south or north of that, but the path is essentially the same. It passes overhead and sets on the other side of the sky.

At the poles, the sun actually doesn't pass overhead at all. It circles around the sky, and basically there are only two cases. Either you can see the sun, in which case it's circling around the sky, so it will pass from west to south to east to north and then back again, and it never crosses the horizon. When you can see the sun, that's obviously during summertime, when you're pointed towards the sun. The sun over the course of the year gets higher and higher in the sky

as you approach the solstices until it's highest in the sky on the solstices, but it's never directly overhead. It's only some degree over the horizon, the angle above the horizon that it reaches his highest on the solstices. Then as you move away from the solstices back towards the equinoxes, the sun progressively gets closer and closer to the horizon, still circling around, never going overhead and never setting as it does at the equator, but its circle constantly gets closer and closer to the horizon

until right on the equinoxes it touches the horizon. And again, in a hypothetical case with no atmosphere and everything else, exactly on the equinox the sun actually passes directly along the horizon. And then after the equinox, as you move towards the summer solstice in the other hemisphere, or in other words, your winter solstice, whether you're in north-less south pole, the sun is always below the horizon. It's still circling around, but it's just always below the horizon and you never see it. And in fact, it circles around further and further away from the horizon

until the solstice and then it comes back towards the horizon until finally six months later, at the next equinox, you finally, or just after the next equinox, it finally rises above the surface of the horizon again, and you can see it, and then it will circle around you once again. So that's at the pole. That's the extreme at the pole. At the pole, it's the sun circles around you at a height above the horizon dependent upon the time of year.

At the equator, the sun circles right above you from east to west, and only its relative position changes. It sort of shifts sideways towards the south-west of the north, depending on the time of year. What about at mid-latitude? So, say, halfway between the equator and the pole, because not many people live exactly at the equator and even few people live exactly at the pole.

Well, that's when the situation is most complicated, because if you imagine these sun paths, the paths that the sun is following as it traces across the sky. Of course, this is caused by the rotation of the Earth, but we imagine the sun going across the sky. Now, these paths form circles, which are parallel to the horizon at the poles, because it's circling around the horizon,

whereas if you go to the equator, these circles are perpendicular to the horizon. That's why the sun spends half of its time above the horizon, half of its time below the horizon. Well, at mid-latitudes, the lines that the paths that the sun follows are neither going to be perpendicular nor parallel, but halfway in between. They're sort of tilted relative to the horizon.

So what actually happens at mid-latitudes, let's start with the equinoxes, because that's the simplest case. At the equinox, the sun rises in the east. That's the same as at the equator. But instead of passing directly overhead, it sort of moves in the direction of the south, and it will be facing relatively southward at noon of the day, and then it will move towards the west and set in the west. So the sun never passes directly overhead at these mid-latitudes on the equinoxes.

It rises in the east and reaches its highest point above the horizon, at noon, but never gets fully directly overhead. The difference between winter and summer in these latitudes is that essentially these tilted curves that the sun path is following shift towards either the south or the north in the same way as they do with the equator. The difference though being now they're tilted. So in the winter time the sun rises to the south of the east, again that's the same as the equator, but now it rises somewhat above the horizon towards the south but it never gets very far above the horizon and then it sort of quickly sets again somewhere south of west, so it never gets fully over to the west. And so because of this, the sun doesn't spend very

long above the horizon and the days are short. Conversely, in summertime at mid-latitudes, the sun rises somewhere north of east, so on the other side of east now, and it passes all the way around, raising higher and higher above the horizon, but still never quite reaching directly overhead, and then curving back around and setting somewhere north of west. So in this case, it actually spends more than 12 hours above the horizon and you get very long days. The further north you are, or further south rather, further towards the pole, the longer the days will be. Of course, until you reach the extreme case where the sun actually never sets below the horizon and you get a day that's 24 hours long.

So this is much easier visualised than explain, but hopefully you've been able to mentally see what I'm getting at. And if you want to listen back over this when looking at a diagram, which I'll post up, or ideally even an interactive simulation, which you can find some really good ones, and I'll post a link to one of those on the Facebook page of the podcast, particularly because those sun paths are hard to visualize. But the basic point is that the path that the sun follows in the sky depends on your latitude. It circles around you when you're at the poles.

It goes directly from east over to the west above you when you're at the equator, and it follows a sort of complicated, curved path, sort of diagonally across the sky in some sense, relative to the horizon, when you're at mid-latitudes. and the amount of time that it spends above the horizon increases during summer and decreases during winter. And that's one of the reasons why summers are warmer than winter, because the sun spends more time above the horizon. The other reason is because you're tilted relatively towards the sun, and therefore the sun feels hotter. And you will actually notice the sun feels hotter in summer than it does in winter,

and especially if you live relatively close to the poles, that is at a more extreme latitude. And the reason for that is not because we're closer to the sun, it's because we're tilted relatively more towards the sun during the summertime. And so its heat or its energy is spread over less of an area, whereas during the winter we're tilted away, and so its energy is spread over a wider region, and so it's more diffuse. So that's why the sun feels so different in winter and summer. And it's a substantial difference as well. It's at least a factor of two, and it might even be a factor of three. It's hard to get the exact numbers because, of course, the atmosphere interferes and complicates the picture. So insulation at the top of the atmosphere is not the same as insulation at the Earth's surface.

But the point is, it is a substantial difference between the amount of solar radiation per unit area in winter and summer. So that's why the sun is hot, feels hotter in summer than in winter. But how hot the sun feels is not the same, say if you're standing out in the sun and just, well, how hot does it feel on my skin. That's not the same as the ambient temperature. The ambient temperature is much more dependent upon wind patterns and cloud cover and things like that, whereas how hot the sun feels is mostly dependent on just how close you are to your summer or winter solstices. So the other effects of climate and weather, including global circulations of air and water, cloud cover and these other factors, we'll talk about in some future episodes. Here I just focused on the earth seasons and the axial tilt and how they're related to each other.

So hopefully that was relatively clear. I know it's difficult without the diagrams, but if parts of that were unclear, Hopefully you'll be able to follow up with some diagrams and see what I'm talking about in some of the cases. If you enjoyed the show, please feel free to send me an email, letting me know. Fods12 at gmail.com, FODDS12 at gmail.com is my address, and give me some feedback, or even if you didn't enjoy the show, I'm always open to suggestions. Another way that you can support the show is to go into our Facebook page and like that page. That's a way of getting updates about new episodes, visual aids, and also sharing the show. with friends. You can also go onto one of the podcast aggregator websites like iTunes and leave

a favorable review. I always appreciate those. So thank you very much for listening and I'll talk to you next time.