Lesson video

Hello and welcome to this lesson on the spinning Earth.

This is from our unit called "The Solar System and Beyond." My name's Mr. Norris.

It's really exciting to be teaching about space.

It really taps into that big question of, what is Earth and where is Earth's place in the universe? It's really exciting to zoom out from Earth and look at what's out there in space.

So, let's start.

Here's the outcome for today's lesson.

By the end of the lesson, hopefully you'll be able to describe the motions of Earth to explain a day, night, and a year.

And you'll be able to describe the motion of the Moon to explain why the Moon appears to change shape in the night sky.

Here are some key words that we'll be talking about this lesson.

Sphere, spinning, day, orbit, and year.

We'll explain each word as it comes up in the lesson.

This lesson is split into three parts.

The first part focuses on Earth being a planet that spins, or rotates, on its axis.

The second section, we'll focus on the Earth's orbit of the Sun.

And in the third section, we'll look at the Moon's orbit of Earth.

Let's get going with the first section.

So, Earth is shaped as a sphere.

There's a picture of a sphere and there's planet Earth.

They're about the same shape.

Earth is roughly spherical.

How do we know this? Well, people first realised that the surface of Earth must have curves to it because when boats sail out to sea, the bottom of a boat disappears from view before the top because the bottom of the boat has gone over the curve of the Earth so you can no longer see it.

And that made people think, "Well, does that mean that all of Earth is round like a sphere?" People began to test this idea out at different points in history.

They tried to travel as far as they could in one direction.

And we now know that travelling in one direction across Earth brings a person back to where they started because they've gone all the way round.

So if they started at that point on the Earth and travelled east across the Pacific Ocean, they'd eventually end up in Mexico or the West Coast of the United States, and they would continue travelling in that direction and they'd eventually end up where they started.

So there is no edge of the Earth that you can fall off by walking in a certain direction.

If you walk in any direction continuously, you'll eventually have walked all the way around the Earth and end up back where you started.

So that's pretty strong evidence that Earth is a sphere or round.

Once we began to develop spacecraft and aircraft, we could start to see the curve of the Earth when we're flying very high above Earth's surface.

So you could see that there's a curve to the Earth there.

If you're not convinced, then get a ruler right now and put the edge of your ruler up to that picture on your screen, and see that the curve, that the Earth from that picture curves beneath the ruler.

The horizon there is not a straight line.

It's curved.

Another piece of evidence is that when Earth's shadow falls on the Moon, Earth's shadow on the Moon appears to be curved.

So curved shadows tend to be caused by a curved object.

So there's an example of Earth's shadow on the Moon appearing curved.

And finally, more recently in the past 50, 60, 70 years, where we've developed spacecraft that can make it very high up, so kind of into space and even to the Moon that we've taken photographs of Earth, which shows the Earth is round like a sphere.

So, quick check.

Which picture best represents the shape of Earth? Is it A, B, C, or D? Five seconds.

Off you go.

Well done if you said C.

Earth is shaped like a sphere.

Earth isn't still.

The first kind of motion that Earth has is, Earth spins once every 24 hours on its axis.

An axis is kind of the imaginary line that a spinning object is spinning around.

So you can see in the picture, Earth's axis goes through kind of the middle of the Earth, and Earth is spinning around that imaginary line called the axis.

And Earth's axis has a tilt to it, and that'll be important in future learning.

And Earth's spin explains why we have day and night, something I'm sure many of you will be familiar with.

Let's do a quick check about Earth's spin.

How long did we just say it takes for Earth to spin once on its axis? Is it one hour, one day, one month, or one year? A, B, C, or D? Choose now.

Five seconds.

The correct answer is B, one day.

Earth takes one day to complete one spin on its axis.

So, let's talk about what happens during one day and how the Earth's spin can help explain those observations.

So, during daylight hours, the Sun appears to move across the sky in a curve from east to west.

So, this picture is taken looking south.

Have a look at kind of the compass needle on the right of the screen and imagine standing where the arrows cross, so in the centre of that compass, and you're looking south.

You'd have east is on your left and west is on your right.

So we're looking south, and what you would see, every time there's a sunrise, the Sun rises in the east.

From wherever you're looking on Earth, the Sun rises in the east.

We're gonna be able to explain that in a minute.

Why is it always the east? Then at midday, the Sun is directly overhead.

the Sun is appearing to move in that curve in the sky from east to west.

And the Sun always sets in the west.

So if you're looking south, that'll be to the right, as in this picture.

And once a day after 24 hours, the Sun will appear in the same position in the sky.

So it takes 24 hours for this cycle to go back to where it starts.

So why is that? Let's have a look.

Well, we've already said it's all to do with Earth's spin.

Earth's spin is what explains day and night.

So, looking at the animation on the screen, we can see that half of the Earth facing the Sun has daylight because it's facing light from the Sun.

But the other half of the Earth facing away from the Sun has got nighttime because it's facing away from the Sun.

So the shadowy side of the Earth is not facing the Sun's light.

And as Earth spins, some of the points on Earth's surface move into the light, and other points on Earth's surface that are on the other side of the Earth are moving into the shadow as Earth spins.

And every day-night cycle takes 24 hours because that's how long it takes for one spin of the Earth.

So it takes 24 hours to move into the Sun's light and then out of the Sun's light and then back to where you started.

Let's do a quick check of that.

Which diagram explains why we have night and day on Earth? Is it A, at night, the Moon covers the Sun? Is it B, at night, the Sun's moved behind the Earth? That's why it's dark.

Is it C, Earth is spinning? That's what explains day and night.

Is it D, Earth orbits the Sun? Which one of those explains why we have day and night? Well done if you said the correct answer is C.

The fact that Earth is spinning on its axis is what explains why we have day and nights because Earth's spin takes 24 hours, so it takes 24 hours for a point on Earth surface to turn into the sunlight and then away from the sunlight and then back into the sunlight again.

For a full rotation it takes 24 hours.

That's why a day or a day-night cycle takes 24 hours.

Right, time for a task now.

You are going to model day and night.

You will need a lamp for this.

You'll need to stand a few metres from the lamp, and you should start facing the lamp and then slowly spin round on the spot, turning anticlockwise, just like Earth does.

Earth spins anticlockwise.

So just look carefully at that drawing to check what direction anticlockwise is and that's what direction you should spin.

So the lamp is gonna represent the Sun, and the pupil represents the spinning Earth.

And the first thing I would like you to do is, in the boxes or in your own boxes or on paper, draw a picture to show where the Sun appears in your vision as you turn when you are that spinning pupil representing the Earth.

So, you're effectively drawing what someone on the Earth sees as the Earth spins because you're spinning, representing being the Earth.

So have a go at doing that task now.

Wherever you are, you should be able to have a go at that task.

'cause all you need is a lamp and you just need to spin and then record what you see in your vision.

In a moment I'll show you an example video of what you should have seen and you can check that that matches with what you did see when you did it.

So, pause the video now and have a go at the task.

Okay, well done for your effort by getting up to do that model, be part of that model yourself by getting up out of your seat and rotating.

And well done for your effort in recording what you saw in your vision as you rotated.

I'm going to show you an example video now, and you could check that what you saw matched with this.

In this video I'm representing the Earth, and you can see the lamp represents the Sun.

I'm going to start rotating just like the Earth rotates or spins.

I've just added on the points of the compass on Earth on me.

So north would be working its way up towards the top of Earth, and on the Earth as we're looking at them, west is to the left and east is to the right.

That will be helpful in just a moment.

I'm now going to show you what that looked like from my point of view, from the Earth's point of view.

What does it look like from the Earth as the Earth rotates in front of the Sun? So here's what Earth sees at midday.

It's midday because the Sun is directly in front of us, kind of directly overhead, we would say, looking from the Earth.

Here are the directions of east and west as seen from this point of view.

They're the opposite direction to normal because we're looking out from the Earth, and normally we're looking in towards the Earth.

So from this point of view, this shows the direction of east and west.

Now, let's see what it looks like when I start rotating anticlockwise, when the Earth starts rotating anticlockwise from midday.

We can see from the Earth's point of view, it appears that the Sun has moved to the west.

In fact, it was the Earth that was rotating anticlockwise, but it appears to us that the Sun has moved to the west.

This is sunset.

That's why the Sun always sets in the west.

So in a moment the Sun will disappear off the edge of the screen, or disappear off the western horizon, and the Earth will continue to spin.

From this point of view, it seems that the Sun has just appeared in the East.

This is sunrise.

Really, the Earth has just continued spinning anticlockwise, but from this point of view, that makes it looks like the Sun then appears in the east.

That's why the Sun always rises in the east.

As the morning continues, the Sun will move across the sky westwards and we'll return back to midday.

So there we are.

It's now midday again.

The Earth has spun once on its axis.

That took 24 hours, and it's a new day.

Here's some feedback for the part of the task where I asked you to record what you saw when you were the Earth rotating in front of the Sun.

So you should have seen something like this with the Sun moving to the right of your vision and then disappearing from the right of your vision.

That would be the western horizon.

Then no Sun while you were facing away from the Sun.

And then as you continue to rotate, the Sun appearing in the left of your vision, which represents the east of the Earth because the Sun always rises in the east, and then the Sun moves from left to right back to that midday position and the cycle would continue if you continue to rotate.

So well done if yours looked very similar to that.

So, there is a second part of this task.

So, we've got two descriptions here for the same thing.

The first description is a person rotating in front of a lamp or Earth rotating in front of the Sun.

That's what the rest of the class saw when you were rotating.

But there's your description as well when you were the Earth, which was that you saw the lamp moving, the Sun moving from left to right in your vision, disappearing and then appearing again.

And the question I want you to answer is this.

How can those two descriptions describe the same thing when they are so different? You'll have to get your thinking caps on a bit for this one.

So discuss this question with a partner first before writing a short answer which explains how those two descriptions can describe the same thing when they're so different.

It might actually be a bit simpler than you initially think.

So try and give a really simple, like one-sentence answer to that question.

How can the two descriptions describe the same thing when they're so different? Pause the video now and complete this part of the task.

Off you go.

So, I'll give you some feedback now.

I think there's actually quite a simple answer to the question of how the two descriptions could describe the same thing when they're so different, and I think it's something along the lines of this.

It's because the same motion is being viewed from a different perspective.

That just means a different viewpoint.

And this is kind of an important idea in physics, actually.

The same motion can look very different to different observers depending on how they're moving and where they're viewing from.

So well done if you've got an answer that's along those lines.

So, that's the end of the section of the lesson focusing on Earth's spin.

And now let's look at Earth's orbit of the Sun.

So, as it spins, Earth also moves around the Sun in an almost perfect circle, which has the Sun at the centre.

And the path that Earth takes is called an orbit.

Earth actually takes 365 and a quarter, so 365.

25 days, to complete one orbit of the Sun, and we call that one year.

And as the Earth moves around the Sun over a year, we get different seasons in the UK, and they happen because of Earth's tilt, Earth's tilted axis as Earth orbits the Sun.

There will be future learning on that.

So, let's look at one year.

So, when the Earth has gone round a quarter of its orbit, that represents a quarter of a year, which is three months.

So let's say that's January, February, March takes you a quarter of a year, three months.

And three more months takes you to half a year, six months.

So perhaps April, May, June takes you half a year so halfway around Earth's orbit.

And then the next three months take you to the next quarter of a year.

So that might be July, August, September takes you to here.

And that leaves the final three months of the year for the final quarter of Earth's orbit of the Sun, so October, November, December.

However, on our calendars, we only give a year 365 days, whereas it actually takes 365 and a quarter days to complete one orbit of the Sun.

So after 365 days of one year according to a calendar, Earth actually won't have quite completed one full orbit.

Earth would need another quarter of a day to get back to where it started to complete a full orbit.

So after one year, Earth falls about a quarter of a day behind where it needs to be to complete the orbit.

So after two years it would fall another quarter of a day behind, so that's half a day behind.

After three years, it would fall another quarter of a day behind, making it three quarters of a day behind where it needs to be to complete its orbit.

And after a fourth year, it would fall another quarter of a day behind, making it a full day behind where it actually needs to be to complete a full orbit of the Sun and return to where it started from.

So what we do about this is, on our calendars, we give every fourth year an extra day, so 366 days instead of 365, and we call these years leap years.

And that gives Earth the extra day it needs to catch up to where it should be to complete its orbit in the same place around the Sun.

So if we didn't do this, after four years, all the seasons would effectively start one day later.

Now, we might not notice that effect, but after 40 years, we probably would notice a bit of an effect that the seasons would all be starting about 10 days later.

And if we waited 400 years, then all the seasons would start about 100 days later.

That's 1/3 of a year later than when they do now.

And we don't want that to happen.

We want the seasons to match up with the months of the year every year, and we don't want the changing weather patterns of the seasons to kind of move around the calendar over a period of hundreds of years, so that is why we have leap years.

There's one last thing we should say about Earth's orbit of the Sun, and that is that it's an almost perfect circle.

So Earth actually stays the same distance from the Sun all year.

And Earth's circular orbit, you need to be aware that it's sometimes drawn viewed from an angle, which can make the circular orbital path look like this.

Now, that shape is supposed to represent a circle, but it's drawn from the side, a bit like how a circular plate could look like a more oval shape if it's viewed from the sides.

They're the same circular plate drawn to look like an oval.

So just bear in mind that in diagrams where Earth's orbit looks like an oval path, it's really a circular path just drawn from the side.

Let's do a quick check now about what we've just been talking about, like the key idea, how long does it take Earth to complete one orbit of the Sun? One hour, one day, one month, or one year? A, B, C or D? Five seconds.

Choose now.

I'm sure you will have said option D, one year.

That is the time taken for Earth to complete one orbit of the Sun, a year.

That's what a year is.

Earth's orbit of the Sun over one year explains why the same star patterns are seen in the same positions in the sky in the same seasons every year.

So for example, in the summer in the UK, we can see the constellation Cygnus.

That's a star pattern that you can only see in summer in the UK.

and then in the autumn from the UK, you can see the constellation Pegasus.

In the winter, you can see Orion in the UK.

And in the spring in the UK, you could see the Great Bear, or to give it its kind of Latin name, Ursa Major.

So how does the Sun's orbit of Earth explain that? Well, it's because at each time of year, if you're looking out in the night sky, you're looking in the opposite direction from Earth to the Sun.

And that's a different direction in summer compared to say, three months later in autumn.

You're looking in a different direction so you see different star patterns.

And then in winter, you're looking in a different direction again, opposite to the Sun, so you see different star patterns.

And the same in spring.

But then three months later is summer again and you see the same star patterns you see in the sky every summer because you've returned to the same point along Earth's orbit of the Sun and you're looking in the same direction again 12 months later.

We should also say that the seasons repeat in a yearly cycle, too, and that's caused by Earth's tilt or Earth's tilted axis.

As Earth orbits the Sun over one year, we get the same cycle of seasons that lasts a year, but that's mainly caused by Earth's tilt as Earth orbits the Sun.

So, quick check on that.

What causes the seasons? A, Earth's tilt as it orbits the Sun in one year, B, Earth gets closer and further from the Sun explaining summer and winter during one orbit, or C, the Sun heats up and cools every year causing summer and winter.

Which is the correct answer? Five seconds to decide A, B, or C.

Let's see how you got on.

Well done if you chose A.

I did just explain that, but hopefully you've not been distracted by answers B or C, which students sometimes think but are not correct.

Let's go into more detail about why option B was not correct.

If Earth did get closer and further from the Sun in its orbit, which it doesn't because Earth's orbit is an almost perfect circle, so Earth doesn't really get any significantly closer or further to the Sun during its orbit over a year.

But if that did happen, then everyone on Earth would have warmer and colder seasons at the same time because everywhere on Earth would have the warmer season when Earth was closer to the Sun and everywhere on Earth would have a colder season at the same time when Earth was further from the Sun, but that doesn't happen.

For example, countries near the equator don't really have hot seasons and colder, hotter seasons and colder seasons.

They have a steady average temperature all year, countries close to the equator.

And when it's winter in the northern hemisphere, it's summer, a warmer season, in the southern hemisphere and vice versa.

So when it's summer in the northern hemisphere, like in the UK it's a warmer season, at the same time it's winter in the southern hemisphere.

So the whole of Earth does not warm up or cool down at the same time.

Earth can't be moving closer or further to the Sun during its orbit, otherwise the whole of Earth would warm up or cool down at the same time, and every country would have hotter and colder seasons at the same time, but they don't.

So quick check, what is the shape of Earth's orbit around the Sun? A, an almost perfect circle, B, an oval with the Sun at the centre, or C, an oval with the Sun to one side.

Choose your answer now.

And on this question, I'm not gonna give feedback just yet because there's a secondary question.

So now justify whatever answer you chose for A, B, and C by choosing either D or E as well.

Did you choose your answer because D, when Earth gets closer to the Sun, that's what causes summer or did you choose your answer because of E, warmer and colder seasons do not occur everywhere on Earth at the same time? So make a decision between D and E to justify your answer now.

Okay, so you should have a final answer from A, B and C and a final answer also from between D or E.

So the correct answer is A, the shape of Earth's orbit around the Sun is an almost perfect circle.

And the reason why you should have chosen that one is because warmer and colder seasons do not occur on Earth everywhere at the same time.

It's time for a quick task now on everything we've done in the lesson so far.

In this task, you just need to use the words spin and orbit to fill in the gaps.

So every gap is either the word spin or it's the word orbit.

So, fairly straightforward.

You should pause the video now and give this a go.

Well done for your effort on that task.

Here are the correct answers.

You should pause the video now to check your work and make any corrections.

So, well done for your effort with this lesson.

We're now going to look at the last part of the lesson, which is about the Moon's orbit of the Earth.

So the first thing to say is, like Earth, the Moon is shaped like a sphere.

However, the shape of the Moon appears from Earth to change in a cycle that repeats about once per month.

So here's the Moon's cycle going from a new moon to a crescent moon and a quarter moon, and then the Moon appears to increase in size to a full moon.

Of course, the Moon doesn't really increase in size.

It's just the portion of the Moon we can see from Earth gets bigger.

The Moon stays the same size.

Then the full moon slowly starts to disappear.

Of course the Moon doesn't actually disappear.

It's just we can see less and less of the brighter side of the Moon that's reflecting the Sun's light.

And then we're back at the new moon again and that whole cycle takes about a month.

So, let's zoom out and look at from the perspective of space.

The Moon orbits Earth, and each orbit takes just over 27 days for one loop round the Earth.

And like Earth, the Moon doesn't give out its own light.

Like Earth, the side of the Moon facing the Sun is in light and the side facing away from the Sun is in darkness, and you can see that in the pictures on this slide.

And it's the Moon's orbit of Earth that explains why the Moon appears to change shape.

So let's look at what the Moon looks like at different points around its orbit.

So here's Alex at a certain point on Earth, and the blue box is what Alex sees on different nights during the month or so that it takes the Moon to orbit the Earth.

So when the Moon is in this position, Alex will see a full moon because he's fully looking at the side of the Moon which is lit by the Sun.

Once the Moon has moved on here, Alex will only see this because he can only see part of the side of the Moon that's lit by the Sun and so on.

At this point, Earth will see a half moon because he can only see half of the half of the Moon which is lit by the Sun.

And the right hand side of the Moon, as Alex is looking at it, is in darkness and so on as the Moon orbits the Earth.

At this point, Alex would see a new moon if the Moon is even visible from his part of the Earth at all.

Now, the next few cycles are a bit more complicated because we need to think about the perspective Alex is looking from.

So when the Moon is in this position, as Alex looks at it, the right side of the Moon is in light.

So Alex will see the right side of the Moon being lit up and the left side of the Moon as he looks at it, so on his left, that's the side of the Moon that is in shadow.

So at this point, Alex will see the right side of the Moon being lit up and the left side of the Moon in shadow, which is what's shown in the blue box.

And then Alex sees the other half moon, but this time the right side of the Moon, as he's looking at it, is lit up, and the left side, as he's looking at it, is in darkness.

So look in the blue box.

That's what he'll see from that position.

And finally, we've got the right side lit up and the left side is just a sliver that's in shadow, and then he's back at the full moon again about a month later.

So, you can see how the mood's orbit of the Earth explains the appearance, changing appearance of the Moon.

So the final thing we should say is that there's just slightly more than 27 days between each full moon.

Remember, the Moon takes 27 days, 27.

3 days to complete one orbit of the Earth.

But there's actually slightly more than that number of days between each full moon.

Let's look at why.

It's because during the time of one moon orbit, so 27.

3 days, Earth is at the same time moving around the Sun a bit, and the Moon will be moving with Earth around Earth's orbit.

So Moon will actually have to travel a bit further before the next full moon happens.

So this is what the situation would look like after one full moon orbit.

The Earth has moved along its orbit, the Moon has completed one full orbit, but it's not quite the next full moon yet because the full moon is when the Moon is on the completely opposite side of the Earth to the Sun so the Earth can see the full lit-up face of the Moon and you get a full moon.

So, we have to wait just a couple more days to get the next full moon compared to a moon's orbit of 27 days.

Let's do a little check of why the Moon appears to change shape.

Is it because of A, the Moon's orbit of the Earth, B, the Moon orbits the Sun, C, the Sun orbits the Moon, or D, is it to do with the Moon spinning? I'm sure you will have chosen answer A, which is correct.

The Moon appears to change shape because of the Moon's orbit of Earth.

Let's do the final task of this lesson.

There's part one, which is just a multiple-choice question.

Which diagram best shows how the Sun, Moon, and Earth move through space? And then part two of the task is a gap fill, but every gap is either Earth, the Sun, or the Moon.

So, fairly simple.

Pause the video now and have a go at part one and part two of this task.

Off you go.

Well done for your effort on this task.

I'll go through some feedback now.

So, for question one, which diagram best shows how the Sun, Moon, and Earth move through space is diagram C.

We've got Earth orbiting the Sun and at the same time the Moon orbiting the Earth.

And for question two, here is the correctly completed gap fill.

So you should pause the video now and check your work for question two and make any corrections that you need to.

So, well done for completing this lesson on the spinning Earth.

Here's a summary.

Earth is a planet.

It's a sphere that spins once every 24 hours.

And Earth's spin explains day and night and why the Sun appears to move across the sky from east to west during each day.

Earth also orbits the Sun in an almost perfect circle.

Each orbit of the Sun takes 365 and a quarter days.

We call that one year.

And finally, the Moon orbits Earth every 27.

3 days.

The Moon's orbit explains why it appears to change shape with a cycle of around 29 days.