Lesson video

In progress...

Hello and welcome to this lesson called Seasons on Earth.

This is from a unit called Our solar system and beyond.

My name's Mr. Norris.

Understanding the seasons is complicated.

You firstly got to understand heating by the Sun, so how the Sun heats different places on Earth differently depending on how far they are from the equator.

You've also got to understand about Earth's tilt and how Earth's tilt can mean that it's summer in the northern hemisphere and winter in the southern hemisphere at the same time, for example.

And what we're gonna focus on this lesson is putting all that knowledge together to look at how the seasons change over the course of a year.

And of course, why some areas of Earth's surface close to the equator don't experience those kind of seasons at all.

So let's get going with that.

The outcome of this lesson should be that by the end of the lesson, hopefully, you'll be able to explain how and why the seasons change and why some countries do not have summers and winters.

Here are some key words that we'll be talking about this lesson, summer, winter, hemisphere, tilt, and orbit.

Each word will be explained as it comes up in the lesson.

This lesson is divided into three sections.

In the first section, we will recap about summers and winters.

In the second section, we'll then look at the main parts of the lesson, which is how the seasons change over the course of a year.

And in the third part of the lesson, we'll look at areas of Earth near the equator which don't experience seasonal change in the same way.

Let's get started with the first section, recapping about summers and winters.

So we know that in the U.

, there's a yearly cycle of season.

Spring becomes summer, summer becomes autumn, autumn becomes winter, and then winter becomes spring, and the cycle continues every year.

It's important to say that the seasons change gradually, okay? The seasons kind of fade one into the other without a definite start or end point.

But it's very important to say that the same pattern repeats every year.

And we're gonna look at why that is this lesson.

Okay, quick check on what we've said so far.

You are starting off summers and winters so far.

Try and fill it in the blanks for this, please.

Pause the video now.

Have a go.

Okay, we're gonna look at some feedback now.

So the warmest season is when the Sun is higher in the sky and when there are more hours of daylight, that's called summer.

And I'm sure you can fill in the opposite for the coldest season.

Fill in the blank of this second section of writing, please.

Off you go.

So the coldest season when the Sun is lowest in the sky and when there are fewer hours of daylight is called winter.

Well done if you've got most of that right.

It's important to say right at the very start of this lesson that not all countries have the same kind of seasons as the U.

Particularly, countries that are close to the equator do not have summers and winters.

They don't have a warmer season and a colder season.

So if there's the equator, okay, that imaginary line that runs all the way around the middle of Earth, and countries close to the equator have warm temperatures and about the same number of daylight hours every day all year round.

So they don't experience the same kind of seasonal changes to warmer seasons and colder seasons as we do.

They just have similar temperatures and similar number of daylight hours all year.

Now, countries that are further from the equator do have summers and winters like the U.

They're repeating a yearly cycle, like we've said.

But again, it's important to say that the summers and winters in all of those different countries, north and south of the equator, they can all have different temperatures and different numbers of daylight hours at different times of their different summers and winters.

So summer and winter is not the same everywhere.

And some countries close to the equator don't even have summers and winters.

And the final thing we should say is when countries in the northern hemisphere have summer, countries in the southern hemisphere have winter, and vice versa.

Remember, the northern hemisphere is everything above the equator, and the southern hemisphere is everything below the equator 'cause the equator splits Earth into two halves, the northern hemisphere and the southern hemisphere.

Let's do a check on that.

Which options might be one, might be more than one.

You decide.

Which options for completing the sentence are correct? When it is the middle of summer in some countries, A, it is the middle of summer in all countries.

Is that correct? B, when it's the middle of summer in some countries, it is the middle of winter in some other countries.

Is that correct? C, when it is the middle of summer in some countries, it is the middle of autumn in some other countries.

Is that correct? Five seconds to decide which ones, which one or ones of those are correct.

Okay, I'll give you some feedback on this now.

The only correct answer was B, when is the middle of summer in some countries, such as countries of the northern hemisphere, they all have summer at the same time, it's the middle of winter in some other countries, the countries of the southern hemisphere, which would all have winter at the same time.

And the countries around the equator, they don't experience the same season.

So they're not part of this.

And that works the other way around as well.

When it's the middle of summer in the southern hemisphere, it's the middle of winter in the northern hemisphere.

So it's never summer in one of the hemispheres and autumn somewhere else.

When one hemisphere has summer, the other has winter.

When one hemisphere has spring, the other would have autumn.

So the seasons in the northern hemisphere and southern hemisphere are always outta sync by six months, half a year.

So you can never have autumn in somewhere or in one hemisphere when another hemisphere has summer.

And we'll look at exactly why that is later in the lesson.

Let's go over what causes summers and winters now.

So summers and winters are caused by Earth's tilted axis.

Remember, the axis of rotation is the imaginary line around which an object spins, and Earth's axis of rotation is tilted.

It's not vertical, it's tilted.

It's an angle, like in the diagram.

In June, Earth's northern hemisphere is tilted towards the Sun.

The yellow arrows represent the Sun's rays.

So you can see at this point in Earth's orbit of the Sun, the northern hemisphere is pointing more towards the Sun's rays than the southern hemisphere is because of Earth's tilt at that time.

And this means that at this time in June, Northern hemisphere countries are in Sunlight for more than half a day.

Look at the pink arrow, which shows the path of the U.

as Earth spins.

Much more than half of that circle is in Sunlight.

So much more than half of a day is spent in Sunlight.

Countries in the northern hemisphere in June when Earth is tilted like this, also see the Sun higher in the sky.

So the Sun's rays are less spread out over the ground, and the Sun's rays are gonna have more of a heating effect at this time 'cause the northern hemisphere countries are effectively being heated from closer to directly overhead by the Sun at this time with the Earth tilt in this way.

So at this time, June, northern hemisphere countries have a summer for both, or for those reasons I've just gone through.

More daylight hours and more of a heating effect from the Sun's rays.

So in June, it's summer in the U.

because Earth's northern hemisphere is tilted towards the Sun, and the U.

is in that northern hemisphere.

So is it winter in any of these places in June, A, B, C, D, E, and F? Choose the ones you think would have winter at this time, June, with the Earth tilted as shown in the diagram.

Pause the video now to decide.

Well done if you realised that the locations in the southern hemisphere would have winter in June when the U.

and the Northern hemisphere has summer.

So locations D and F, because they're in the southern hemisphere, far from the equator, and the southern hemisphere is pointed away from the Sun at this time.

So southern hemisphere countries would have winter.

Not E, of course, 'cause E is too close to the equator, and countries, but close to the equator have different kinds of seasons, not the same seasons that we have, in terms of summers and winters.

So well done if you've got that.

Okay, time for a task based on what we've just done explaining about summers and winters and what causes them.

So fill the gaps to explain about the southern hemisphere in June.

We explained just now about the northern hemisphere in June.

Now, just do the same for the southern hemisphere at the same time of year.

Pause the video now to complete those gaps.

Well done for your effort on that task.

Here's some feedback.

So in June, Earth's southern hemisphere is tilted away from the Sun.

You can see that in the diagram.

So the southern hemisphere countries away from the equator have the season of winter.

Those southern hemisphere countries have fewer daylight hours.

Look at the amount of time spent in the shadow side of Earth compared to in the lit side of the Earth that's lit by the Sun.

And those southern hemisphere countries also see the Sun lower in the sky.

So the Sun's rays are more spread out over the ground, so have less of a heating effect.

Very well done if you've got most of those right.

So now we've recapped about summers and winters and what causes them, let's now look at how the seasons change, which is the main focus of this lesson.

Now the seasons change over a year, and a year is Earth orbiting the Sun once.

So the seasons changing over a year is gonna be something to do with Earth's orbit of the Sun because that's what takes a year.

That's what a year is.

So the two must be linked in some way, but let's look at exactly what that link is.

So Earth orbits the Sun in an almost perfect circle, with the Sun of the centre, and the time taken to complete one orbit is one year.

And what this means is Earth does not move further from or closer towards the Sun during a year 'cause it's orbiting in a perfect circle with the Sun at the centre so Earth doesn't get any closer or further from the Sun at any point during its orbit.

If the Earth did move further or closer to the Sun during a year, for example, if the orbit had the Sun to one side, or if the orbit was a kind of more oval-type shape, then everywhere on Earth would have a warmer season, so a summer, at the same time, but this does not happen.

Countries in the southern hemisphere have winter when countries in the northern hemisphere have summer, and vice versa, which means the opposite way around two.

And countries near the equator do not have warmer or colder seasons.

So not everywhere on Earth has a warmer season, a summer at the same time.

So this fact that Earth's average temperature is actually constant over a year because when the northern hemisphere has summer and is warmer, the southern hemisphere has winter and is colder.

So those temperature differences on different parts of the Earth at the same time average out, meaning that Earth's average temperature is constant actually over a year.

That strong evidence for Earth having a circular orbit around the Sun.

Before we go further, let's do a check of what we've just said.

Which statements are evidence for Earth's orbit of the Sun being circular? A, the seasons repeat with a cycle of 365 1/4 days.

B, countries near the equator do not have summers and winters.

C, countries on Earth do not all get warmer at the same time.

D, the average temperature across the whole of Earth's surface does not vary much over a year.

Which of these are evidence for Earth's orbit of the Sun being circular? Choose the ones you think.

Five seconds, off you go.

Okay, the answer was that B, C, and D are all evidence for Earth's orbit of the Sun being circular.

A is true but is not evidence for Earth's orbit being circular.

A could be true with different shapes of orbit, but B, C, and D all suggests that Earth stays the same distance from the Sun, therefore, the orbit should be circular.

Well done if you've got all three of those.

Okay, so the seasons change with a yearly cycle because Earth's tilted axis does not change direction during its circular orbit of the Sun.

So we've drawn Earth's circular orbit here from an angle.

So it might not look circular on the screen, but it is a representation of a circle viewed at an angle.

And remember this diagram's not to scale.

But the key thing here is that over a year, Earth's tilted axis doesn't change direction.

So we've got the Earth pictured there in June, and three months later, that's a course of a year, Earth would've completed a quarter of an orbit.

So June, July, August, September.

Three months later, we're in September.

And look at Earth's tilted axis.

It's in the same direction, even though Earth has moved around a bit on its orbit.

Three months later, October, November, December, Earth is now gone a total of six months round its orbit, and that's half a year.

So it's gone from June to December.

And look at its tilted axis, which has not changed direction as Earth has orbited the Sun.

And then three months later, January, February, March, that's what Earth kind of looks like on its orbit at that time, with its tilted axis in the same direction as it's always been.

So how does this cause the seasons? Let's see.

Well, let's start by looking at June because we've looked at what happens in June quite a lot in lessons so far.

So we know that in June, the northern hemisphere is tilted towards the Sun, so the northern hemisphere has summer, and the southern hemisphere is tilted away from the Sun, so has winter.

And if we kind of showed a different diagram of that, like this, that kind of recaps all of that.

So you can see the U.

, the U.

takes the top pink path as the Earth spins.

So it spends more time in daylight, longer daylight hours, more daylight hours, and the Suns rays are closer together when they hit Earth, whereas a country like the southern part of Chile takes the lower pink arrow as Earth spins.

And you can see that it spends most of its 24-hour day in the dark nighttime side of the Earth.

So it has fewer daylight hours.

And also the Sun's rays are more spread out because of the angle Earth makes to the Sun at this time of year in June.

So now let's compare that to December, where Earth has orbited six months around the Sun, but Earth's axis has not changed direction.

So his December.

We can see now that the northern hemisphere is tilted away from the Sun.

So it's gonna have winter.

Six months later from summer, you get winter.

And the southern hemisphere is tilted towards the Sun.

So now how summer, just as you'd expect six months later.

So here's the other picture of that.

And again, we can see now how that area of the south of Chile in South America is tilted more towards the Sun, and the U.

's path is tilted away from the Sun in the northern hemisphere there.

And here is June and December's pictures next to each other just so you can pause the video if you need to just to compare the two.

Let's just mention about March and September, because in those months, the Earth's tilt is neither towards or away from the Sun for the northern hemisphere or for the southern hemisphere.

So that means March and September are not the hottest or coldest months for any country.

So no country has summer in March or September.

And likewise, no country has winter in March or September.

Let's do a check now on the really key points that we've covered so far in this learning cycle.

So in countries that have summers and winters, how and why do the seasons change? So there's a sentence there to fill in for your answer.

So try and fill in those gaps to answer that question.

Pause the video now and have a go.

Did you get it? Let's see.

So in countries that have summers and winters, the seasons change because Earths tilt, I suppose you could say Earth axis there as an alternative, does not change direction during Earth's orbit of the Sun.

Very well done if you've got all of those 'cause that is the key idea of the lesson, which explains why the seasons change, and all of the other detail about how the seasons change is built on top of that understanding.

I'll just mention that the seasons could be modelled using a globe and a lamp in a dark room.

So this represents June because that globe is being lit from the right-hand side.

So the Sun would be on the right of that globe, lighting one side of the globe, one side of the Earth.

And how can we tell that that's June? Well, it's because we can see that the northern hemisphere is getting more heating and lighting by the Sun, more lighting and heating.

And we can particularly see that if we look at the Sun's view at this time.

You can see much more of the northern hemisphere from the Sun's point of view than the southern hemisphere.

That's why the northern hemisphere has summer and it's June.

So how could we model December in this way? Well, all we'd need to do is we would need to carry that globe round to the other side of the lamp.

So it's on the right of the lamp and the globe is lit from the left, and that represents Earth on the other side of its orbit of the Sun in December.

So we can see now that it's the southern hemisphere which is receiving more lighting and heating from the Sun because of Earth's tilt.

And if we look at the view from the Sun, we can see that we can see much more of the southern hemisphere, which is lit and heated by the lamp than at the northern hemisphere.

Okay, I've got some new pictures of a globe here using that model, and here's a quick question for you.

Should each label be added to picture A or picture B? So we've got six labels there.

Should each one of them be added to picture A or picture B? Pause video now and make your mind up for each of those six labels.

Off you go.

Now if you found that tricky so far, I'm gonna now also give you the view from the Sun in picture A and picture B.

So that's the view from the Sun in picture A and the view from the Sun in picture B.

So that might help you add the right labels to the right picture.

Five more seconds, then we'll go through some feedback.

Pause the video if you need a little more time, but I'm gonna go through the answers now.

So label number one, winter in North America, that must be photo B because North America is receiving less lighting and heating because of Earth's tilt in photo B.

Winter in South America must be therefore in photo A because South America receives less heating and lighting in photo A.

Summer in North America must happen at the same time as winter in South America.

So that's photo A again.

And again, if you look at the pictures, you can see that North America receives more heating and lighting in photo A.

So that's summer.

And summer in South America, therefore, must be photo B, when the southern hemisphere gets summer.

So June is when the northern hemisphere gets summer.

So that's photo A.

And December is when the sled hemisphere gets summer.

So that's photo B.

So very well done if you've got most of those right.

Here's a task for this section of the lesson then.

It's a very simple task.

There are 10 labels in the boxes and you just need to add each of those labels to the correct places on the diagram.

So there are only four places to add labels to, A, B, C, and D.

So you're gonna need to add more than one label to some of those places.

And in fact, two of those places have got lots of labels and the other two of those places don't have very many labels at all.

So with all of that information, I think it should be fairly straightforward to have a go at this task.

So pause the video now and do that.

Off you go.

Let's see how you got on.

So position A, that is where the northern hemisphere is tilted towards the Sun.

So the northern hemisphere has summer and the southern hemisphere has winter.

Well done if you've got all of those three for position A, and that is gonna be June when the northern hemisphere has summer, like the U.

C will be the opposite to all of those.

So that's the southern hemisphere tilted towards the Sun.

So the southern hemisphere has summer and the northern hemisphere has winter, so that's December.

And that only leaves March and September to fill in in the correct places.

Make sure you've got those two the right way round, because after December, you've got January, February, March, then April, may, then June, then July, August, September.

So you've got the months labelled in the correct order for the correct direction of Earth's orbit around the Sun, which is one year.

Well done for your effort on that task.

Well done for your effort so far on this lesson.

We're gonna finish this lesson by recapping about the equator and applying our knowledge to to there.

So close to the equator, Earth's tilt doesn't make much difference as Earth orbits the Sun.

So this is the same diagram we've seen before, but this time, we wanna focus on a place close to Earth's equator.

So here's the diagram for June, and we've got the equator labelled this time in pink or a point on the equator.

We'll follow that pink path as Earth spins.

So that is a representation of a point on the equator in June.

But what about six months later in December when Earth's has gone to the other side of its orbit, but its tilt has remained in the same direction? Well, you can see that a point on the equator is pretty much still receiving the same kind of intensity of lighting and heating.

The Sun's rays are just as spread out in December as they were back in June in the first picture, okay? So Earth tilt doesn't make much difference if you're close to the equator.

It's if you're further from the equator that it does make a significant difference to the lighting and heating you receive from the Sun's rays.

So another thing to mention here is that near the equator, there's about 12 hours of daylight every day of the year.

So there are no seasonal changes in the number of daylight hours.

More daylight hours in summer, fewer daylight hours in winter.

That's for countries that have summers and winters.

Near the equator, they don't really have those kind of seasonal changes.

So to understand why this is, compare the path of the equator, which is the pink line on the globes now.

Compare that to the U.

's path as Earth spins in each month.

So here's the U.

's path just added now.

So you can see in June, the U.

has got significantly more daylight hours than nighttime hours, whereas for the equator, it's about half and half, 12 hours and 12 hours.

Whereas look at December, for the U.

, you've got significantly more nighttime hours than daytime hours, but the equator is still split about half and half, 12 hours and 12 hours.

So it's the same as what we said on the previous slide really.

Earth's tilt doesn't make much difference at the equator.

So they don't really have the same seasons that we have at the equator.

And another way of thinking about that is at the equator, the Sun is pretty much face-on all year round.

And it so it's high in the sky all year round, so it's rays provide good heating all year round.

Let's do a quick check of your understanding of this.

There is one noticeable change at the equator between June and December, and that's the direction of the Sun in the sky.

So study these diagrams and use your understanding of them from previously in the lesson to fill in the gaps.

So at the equator in June, the Sun is seen towards the, I'm looking for a direction here.

So look at Alex in the June picture.

If he was looking straight up from Earth's surface from the equator at that time, look what direction that would be looking.

So where would he need to look? Which direction to be looking directly at the Sun? Which, of course, he shouldn't do, but this is just a thought experiment.

And then compare that to the December picture.

Alex in December on the equator, if he was looking straight up from the ground into the sky, he'd be looking in the direction shown by the line.

But what direction does he need to tilt his head to see the Sun in December when he's on the equator in these positions? So there's just two directions to fill in.

Try and do that now.

Five seconds.

Let's see if you got this.

So at the equator in June, so it's the first picture on the screen, the Sun is seen towards the north.

'Cause you can see in that picture, Alex needs to tilt his head up to be able to see the Sun, come towards the north of Earth.

Whereas in December, because of Earth's tilt, Alex needs to tilt his head down to look directly at the Sun from looking straight up in the sky.

So the Sun is seen more towards the south in December.

So this is one of the only noticeable changes at the equator between June and December, the direction of the Sun in the sky actually changes between being slightly to the north to being slightly to the south.

Very well done if you've got that.

It's now time to do a final task which will complete this lesson.

All you have to do in this task is explain why the U.

has summer in June and winter in December, but countries at the equator do not.

You can use the diagrams to help, but I'd like you to produce a piece of writing, or it could be bullet points that provides that explanation.

So pause the video now, have a think first, and then write your answer.

I'll see you in a few moments for some feedback.

Off you go.

Let's see how you got on.

So I'm gonna give you some example points to include.

So you are gonna have to read mine carefully and work out if yours cover the same kind of ideas.

And after I've gone through mine, you can always add to your answer to improve it.

So why does the U.

have summer in June and winter in December, but countries the equator do not? Here's what I put first.

The U.

experiences summers and winters because of Earth's tilt.

So well done if you mentioned Earth's tilt, because that is the key idea.

And what's the key thing that Earth's tilt does, or actually, we should say, doesn't do? Earth's tilt does not change direction as Earth orbits the Sun.

That is the key explanation of why places on Earth have seasons.

So because of Earth's tilt in June, the northern hemisphere or the U.

is tilted towards the Sun, but in December is tilted away from the Sun.

However, countries at the equator stay close to being face-on to the Sun all year.

And this means in the U.

, the Earth's tilt causes a big difference in the number of daylight hours and the heating effect of the Sun's rays between June and December, and that's what causes summers and winters in the U.

But at the equator, the Earth tilt does not cause much difference in the number of daylight hours and the heating effect of the Sun's rays.

So there is no summer and winter there at the equator.

So very well done if your answer was along those lines.

Remember to pause the video now and improve your answer to make progress.

Here's a summary of today's lesson.

The seasons on Earth change because of Earth's tilted axis, which does not change direction as Earth orbits the Sun.

Look at the diagram to remind yourself what that means.

In June, the northern hemisphere is tilted towards the Sun so has summer, and the southern hemisphere has winter.

In December, the northern hemisphere is tilted away from the Sun so has winter, and the southern hemisphere has summer.

And countries close to the equator do not experience summers and winters as they remain close to being face-on to the Sun all year.

I've finished the video