Lesson video

Hello, welcome to this lesson called Changing Ideas about Earth.

My name is Mr. Norris.

This lesson is from the unit called: Our Solar System and Beyond.

So nowadays, we've got a pretty good idea about Earth's place in the Universe, but people have had very different ideas about Earth over history, and that's what this lesson is going to explore, and why we're now so convinced that we're on the right track about Earth's place in the Universe.

The outcome of this lesson is hopefully, by the end of the lesson, you'll be able to describe how and why our understanding of the position and motion of Earth has changed over time.

Here are some keywords that will come up this lesson, sphere, geocentric model, heliocentric model, and telescope.

Here's the definitions for each keyword.

You might like to pause the video now if you think it would be helpful to review these definitions now, but each word will be explained as it comes up in the lesson.

This lesson is divided into three sections.

In the first section, we'll look at how we know the Earth is not flat.

In the second section, we'll look at how we know the Earth is not the centre of the Universe, and in the third section, we will extend that to look at telescopes.

Let's get going with the first section.

So most ancient civilizations in history have thought that Earth is stationary, it doesn't move.

And what most ancient civilizations thought was that, actually it's the Sun, and the Moon, and stars that move across the sky, and Earth is stationary below the heavens.

And you can see why they think that, because if you track the path of the Sun across the sky during the day and the path of the Moon, the Moon appears to move across the sky during a night, during a clear night, and the stars appear to move across the sky over the course of one night, you can see why ancient civilizations would think that, and they tried to come up with stories, and myths, and legends to explain those observations.

For example, the Egyptians told the story that the Sun god, Ra, rode the Sun across the sky during the day, and then rode it back at night, but through the underworld.

So you couldn't see the Sun being taken back, to then rise again the next day.

And we now know, of course, that it's Earth, which is orbiting around the Sun, and the Moon is orbiting around Earth.

We also know that Earth is a sphere because it was thought for a long time that Earth might be flat or might be a different shape.

In fact, the idea of Earth being a sphere was, evidence for Earth being a sphere was discovered over 2,000 years ago by the ancient Greeks, and we'll talk more about that later in this learning cycle.

About a thousand years ago, Arabic astronomers carried out detailed studies of the stars.

They named an incredible number of stars, and recorded in detail how the positions of the stars in the sky changed through the year.

And that knowledge gradually spread through the world at that time.

Around the same time, across the other side of the world in Central America, the Mayan civilization were also observing the planets and using the patterns they saw to work out their own calendar to keep track of dates, and the time of year, and the seasons.

Before astronomers had made those kind of careful observations starting with the Mayans and the Arabic astronomers, many people thought the Earth was flat, as we previously mentioned, but we now have a lot of evidence that Earth is shaped as a sphere.

Let's do a quick check, which picture best represents the shape of Earth? This should be very easy, five seconds to decide.

Well done if you said picture C, Earth is shaped as a sphere.

It was about 2,400 years ago, a Greek called Aristotle described Earth as being a sphere.

In fact, he was very certain in what he was describing, but we'll see if we trust his certainty or not.

Aristotle said, "Earth must be a sphere because Earth must be perfect, and a sphere is the most perfect solid shape," in his opinion, I dunno what his criteria for perfection were.

So this is just his opinion kind of his ideas about perfection, which is quite subjective, kind of depends, and can vary from person to person.

And importantly, he doesn't really quote anything, any testable evidence, so it doesn't ever go beyond his opinions or his ideas to something which you can actually test in the real world, and that's why this idea of Aristotle's that Earth must be a sphere because it should be perfect, it's not a scientific idea, scientific ideas are underpinned by evidence or statements that you can test in the real world to prove and show using evidence.

Aristotle actually did also give a scientific reason why he thought Earth should be a sphere, as well as his non-scientific reason.

The reason he gave was this, "If Earth is flat, then a ship sailing away would be seen to get smaller and smaller, but you would still always be able to see the whole ship as it just gets smaller and smaller." However, that's not exactly what we see.

What we also see is the bottom of a ship sailing away, disappears before the top of the ship does, and that's because Earth is a curve 'cause Earth is a sphere, and that is what we see, so that is a statement that you can test.

If Earth is a sphere, then you would see the bottom of ships disappearing before the tops.

If Earth is not a sphere, if Earth was flat, then you wouldn't see that, but we do see that, so that is a test of Aristotle's idea that Earth is a sphere, and Aristotle's idea passes the test.

So that makes us more likely to believe it, and based on evidence, things we can see and things we can test.

Let's do a check on what we've just been saying.

People can believe different ideas for different reasons.

Which is a scientific reason for believing an idea? A, the idea feels right, it's intuitive, is that scientific? B, evidence from real-life observations fit with the idea.

C, lots of people say the idea is true, which is a scientific reason for believing an idea? Five seconds to decide.

The correct answer is B.

If evidence from real-life observations fits with the idea, then that is a scientific reason for believing the idea.

Scientific reasons for believing the idea are reasons based on evidence.

So another ancient Greek called Eratosthenes went a step further.

Working in Egypt, he measured the length of shadows at the same time each day in different places.

And what he realised was his measurements were consistent with the idea of Earth being a sphere, but they were inconsistent, they couldn't work with the idea of Earth being flat.

So that's really powerful scientific data to back up the idea that Earth is a sphere.

And what Eratosthenes could actually do, was he could actually calculate the diameter of Earth from his measurements, and from the distances between each place.

So that gives a value for the diameter of the Earth, which can then be further tested in other experiments by other people later down the line.

And it turns out that Eratosthenes' value for the diameter of the Earth was pretty accurate compared to what we now think the diameter of Earth is.

So in the present day, here's some further evidence that absolutely convinces us in the modern day that Earth is a sphere.

For example, photographs of the Earth from space show it's a sphere.

And we can sometimes see the curve of Earth from an aeroplane, and during a lunar eclipse, so that's when the Earth passes between the Sun and the Moon, so the Earth's shadow gets cast on the Moon.

The shape of Earth's shadow is always a curve, and the only shape that will always cast a curved shadow no matter what angle it's illuminated from is a sphere.

So that's pretty good evidence that Earth is in fact a sphere.

Let's do a check on what we've just said.

Which of the following is evidence that Earth is a sphere? Pause the video now, read through each option, and choose which you think are evidence that Earth is a sphere.

Right, I'll go through this one.

Earth's shadow on the Moon is a circle or curved.

That is evidence that Earth is a sphere.

The bottom of boats that are sailing away disappear first, that's certainly evidence that Earth has a curved surface or is a sphere 'cause it happens at every ocean on Earth, so it must be curved all the way around like a sphere.

And then statement C, we can fly all the way around Earth without changing direction.

So you can go all the way around Earth and come back to where you started without changing direction, and you can do that on a sphere as well, so all of those are evidence that Earth is a sphere.

Well done if you got those right.

Time for a task now.

So Laura once went on holiday to Chicago in the United States, and while she was there, she took a photograph of Chicago from the opposite side of Lake Michigan, which is 60 miles across, and you can really do this.

Explain why Laura's photograph shows the tops of the skyscrapers in Chicago, but none of the shorter buildings.

Now you only need to write three or four sentences for this, but think carefully about what exactly you're gonna say, and perhaps start with what is the most important idea that explains why you can only see the tops of the skyscrapers in Chicago, and why you can't see the shorter buildings.

Pause the video now and have a good go at that task, three or four sentences to answer the question.

I'll give you some feedback now, here's a model answer.

Hopefully, you mentioned it in your answer that the main reason why Laura's photograph only shows the tops of the skyscrapers and not the shorter buildings is because Earth is shaped as a sphere.

And then from 60 miles away, so over a distance of 60 miles, the curve of the Earth between Laura and Chicago, would be noticeable, which is a bit like a hill between Laura and Chicago, and that's why she can only see what's sticking up above the curve of the Earth, and not what is hidden below the curve of the Earth.

So well done if your answer was along those lines and conveyed to those ideas, pause the video now and make any improvements to your answer that you think would help.

So this takes us to the second part of the lesson.

Earth is not the centre of the Universe, but how do we know this? Let's find out.

So most ancient civilizations thought that the Earth was stationary, and that the Sun, Moon, and stars moved across the sky, and they thought this because you can see the Sun, Moon, and stars moving across the sky over the course of a day, or over the course of a night.

So because it felt that everything is moving around the Earth or past the Earth, then it felt natural to assume that Earth was at the centre, and everything moved around the Earth, and that's called the geocentric model of the Universe.

And, of course, it turns out, that's not correct.

The word geocentric, geo means Earth, and centric means centre.

So a geocentric model of the Universe is a picture of the Universe that has Earth at the very centre of everything, like in this picture here, with everything orbiting around the Earth.

So the Sun, and Moon, and stars do all appear to move across the sky each day, and Earth does seem to be stationary because we can't sense Earth's movement through space.

We now know, of course, that Earth is rotating, and at the same time, Earth is orbiting the Sun.

And at the same time, the solar system is kind of orbiting within the Milky Way Galaxy, but we can't sense any of that movement, and that's partly because we're moving with the Earth, and it's also partly because the Earth's atmosphere is moving with us.

So careful observations of space have proved the geocentric model of the Universe to be wrong.

We know that Earth is not the centre of the Universe.

So what were some of these observations that helped lead the way to that realisation that the geocentric model is wrong? Well, the first observation was known since ancient times.

Planets were sometimes seen to change direction in which they moved across patterns of stars in the sky.

So have a look at the picture, and that shows the path that a planet can take over the course of a number of nights across the night sky, and that movement's unexplained.

And actually, for ancient people, this is what made the difference between a planet and a star because, of course, we didn't know the difference, people didn't know the difference in ancient times.

The word planet comes from the Greek word planetes, which means a wanderer.

So planets were points of light that kind of wandered on these strange paths through the stars, sometimes, whereas stars had more fixed positions that travelled in a smooth curve across the sky over the course of one night, whereas planets were wanderers, and people couldn't explain this, people couldn't explain why, what would cause planets to wander in this way.

And it was a particular problem for the geocentric model because the geocentric model of the Universe has the Earth at the centre with planets moving in, what were assumed to be perfect circles around Earth because Earth was assumed to be the fixed centre of the Universe, But if planets are moving around Earth in fixed perfect circles, then what causes their path across the night sky to kind of wobble and zigzag in this way? And it was completely unexplained.

So the geocentric model of the Universe couldn't explain what we actually see, it didn't fit with what we actually see.

And people worked really hard for hundreds of years to try and come up with ways to make what we see fit with the geocentric model of the Universe, but it couldn't ever really be explained in a satisfactory way.

In the 16th century, that means the 1500s, a Polish astronomer called Nicolaus Copernicus, came up with an alternative model that could explain the odd movements of planets across the sky.

And this is called the heliocentric model.

The Earth and the other planets orbit the Sun, and that it's the Sun that's the centre of the Universe.

Helio is Greek for Sun, and centric means centre, so the heliocentric model of the Universe is a model of the Universe that has the Sun at the centre.

Now this is closer to being correct because the planets do orbit the Sun, but that's just within the solar system, actually, the Sun is not the centre of the Universe, the Sun is just one star amongst billions in the Universe, and it's not even thought necessarily that the Universe has a centre.

So Copernicus's idea isn't correct, but it was certainly closer to being along the right lines than the geocentric model, for the solar system, anyway.

It's important to add that Copernicus wasn't the first person to come up with the idea of a heliocentric Universe, and the ideas have been debated by Greek, Arab, Indian astronomers, and others too in history.

But what Copernicus did do is he set out in detail how a heliocentric Universe could work, and could explain the motions of the planets in a different way to the assumed geocentric model of the time.

Copernicus published his ideas in a book, but he couldn't convince the world he was right.

And the main reason why the world wasn't convinced, well, two main reasons actually, well, the first one, was actually that the model that Copernicus published, actually had just as many complexities as the geocentric models of the time to explain those wandering planetary motions in the sky.

And the second reason, he was going against the widely held opinions of the time, which were held by powerful religious organisations like the Catholic Church, so he was going up against the church too.

So the world wasn't convinced by Copernicus in the 1500s.

And the heliocentric model of the Universe wasn't really adopted until well into the 1600s, the 1700s, that's when an astronomer called Tycho Brahe collected huge amounts of extremely accurate astronomical measurements, which another astronomer, a mathematician called Johannes Kepler, used to produce a much simpler heliocentric model, of planetary motion around the Sun, that fit almost perfectly with what is seen.

And then, also in the 1600s, Galileo Galilei used one of the first telescopes to find evidence that not everything orbits Earth, more on that later, and he caused a big controversy with the Catholic Church, which ensured his ideas were widely-read 'cause people were interested in them because of the controversy.

And finally, in the 1600s, Isaac Newton came up with a theory of gravity that explained how it all worked, with essentially just one simple elegant equation, which just worked and fit with the observations.

So Copernicus didn't cause an overnight scientific revolution, it took between 100, and 200, maybe even 300 years for his ideas to be widely accepted about the Sun being the centre of the Universe.

And as we've said, of course, the Sun is actually not the centre of the Universe, the Sun is the centre of the solar system, the planets orbit the Sun within the solar system, and all of the other stars are just part of a much wider Universe that may not even have a centre.

Let's do a quick check on the most important idea we've just talked about.

What is the name of the model in which the Sun is at the centre of the Universe? Choose which option, five seconds.

I'm sure you've got this right, well done, heliocentric model is the name of the model of the Universe that has the Sun at the centre.

The heliocentric model also gives a simple explanation for how patterns of stars change through a year.

And it's because as the Earth is orbiting the Sun, then looking out from the side of the Earth that has night, you will see different constellations in the background at different times of year as the Earth orbits the Sun in one year.

So when Earth is on one side of the Sun, we can see star patterns in the night sky looking away from the Sun, and we can't see star patterns that are on the other side of the Sun.

So in the position shown, we can see the constellation Orion, but we can't see the constellation Cygnus.

Six months later, the opposite is true.

We can see the constellation Cygnus, but we can't see the constellation Orion 'cause they're on the other side of the Sun.

Let's do a check on what we just said.

Which of the following is evidence that Earth orbits the Sun? Pause the video now, which of those are evidence that Earth orbits the Sun? Okay, I'll give you the right answer.

It's only B, okay? A could be explained if the Sun and stars orbited Earth, and C could also be explained if the Sun orbited Earth, but B can only really be explained if Earth orbits the Sun.

Well done if you got right.

Time for you to do a task on this learning cycle.

There are pictures of constellations on classroom walls, have a look at the picture.

And Jun is modelling the heliocentric model of the Universe.

Imagine his head is the Planet Earth, and the beach ball is the Sun.

And imagine the beach ball is too bright to look at as the Sun would be.

And then there's three questions to write an answer to, quite short answers will be fine.

So Question 1, which constellations can Jun see in his current position in the picture? Question 2, as Jun walks around the Sun, will he always be able to see Orion? And he should explain that answer as well.

Question 3, why will Jun always see the same constellations at the same time each year? Pause the video now, and have a good go at that task with your best effort.

Right, I'm gonna give you some feedback now, well done with your effort on that task.

Question 1, which constellations can Jun see in his current position? Well, he can only see Orion and the Plough, he won't be able to see Cassiopeia in his current position because that's behind the Sun.

As Jun walks around the Sun, will he always be able to see Orion? No, not when he's on the other side of the Sun.

And Question 3, why will Jun always see the same constellations at the same time each year? It is basically because Earth orbits the Sun once per year.

So at a certain time of year, Jun could look away from the Sun and see a certain constellation, and then if Jun waits one year, he will have completed one orbit of the Sun, and then he'll be back at the same position, so if he looks in the same direction out from Earth, he'll see the same constellation in the same direction at the same time of year because Earth has returned to the same point on its orbit around the Sun.

So well done if your answer was along those lines.

Pause the video now to make any improvements to your answers.

So that takes us to the third section of this lesson, which is about telescopes.

In the 17th century, and that means the 1600s, European scientists were able to use telescopes to observe planets and stars more closely.

Telescopes were invented in 1608.

And what do telescopes do? Well, they magnify distant objects so that more detail can be seen.

The first telescopes use the idea of refraction, which is when glass lenses bend rays of light.

So the first telescopes use that idea with a lens at each end of the telescope.

And if you change the length between the two ends of the telescope to make a shorter telescope or a longer telescope, then that would adjust the focus of the telescope, and the magnification it provides to get a clear image, depending on how far away the object is.

We've already talked about Galileo Galilei in the 1600s using a telescope to make observations, which cast serious doubt on the geocentric model of the Universe, which we now know is wrong.

Galileo actually managed to produce and invent his own version of the telescope just from hearing about the idea of the invention, and he worked out how it must work.

And because he lived in Venice, he had access to expert glass-making that was happening in Venice at the time.

So his telescope was potentially clearer than any other telescope and allowed greater magnification than any other telescope that had been invented so far.

One of the first things that Galileo looked at using his telescope was the Moon.

And when you look at the Moon in the telescope, you can see that it's absolutely not a perfect sphere, it has craters, and different patches on it.

So that completely cast doubt on the idea that everything in the heavens was perfect, or perfect spheres, or perfect circles.

And actually, you can see that the Moon isn't a perfect sphere from Earth, but looking through a telescope, really makes that absolutely clear.

And then one of the second and most important things that Galileo then looked at with his telescope was Jupiter, the Planet Jupiter, and he could see specks of light, which appeared to go around Jupiter, and this was evidence that not everything orbits the Earth.

Not everything has to orbit the Earth.

The Earth might not be the centre of everything, the centre of the Universe.

So Galileo had provided direct evidence that not everything orbits Earth, and Kepler had provided a simple, elegant, heliocentric mole that was backed up completely by detailed and accurate astronomical data collected by Tycho Brahe, but what was missing was a theory that tied it all together.

But they didn't have to wait long because in the late 1600s, Isaac Newton working in England worked out laws of gravity, and that gave an overall explanation, which tied together Kepler's model of a heliocentric, universal, or heliocentric solar system, and Galileo's observations, and Tycho Brahe's astronomical data could all be underpinned by Isaac Newton's elegant model of gravity, which explained the orbits of all the planets, and the observations that we then see.

And, in fact, Newton's laws of gravity were then used to predict the existence of planets that had not yet been seen, and that's a really good test of a scientific theory.

If a theory makes a prediction and that then later turns out to be true, then that is really strong evidence for believing in that scientific theory.

So in 1781, William and Caroline Herschel discovered the planet Uranus.

They used a telescope that made themselves, and they knew Uranus was a planet because they observed its circular orbit around the Sun, they could make observations, which showed that it was orbiting the Sun.

And, in fact, Uranus was originally named Georgium Sidus, George's Star after King George III.

Let's do a check on some of the ideas we just talked about.

Which of the following is the best reason for believing Earth orbits the Sun instead of the other way around? Is it A, we are more intelligent than people who thought the Sun goes around the Earth? B, there's a lot of evidence for many careful observations, or C, this is what most people think is correct, so that's why we should believe it too, which is the best reason for believing Earth orbits the Sun, instead it the other way around? Pause the video now and decide.

I'll tell you the correct answer, it's B, the best reason for believing Earth orbits the Sun is because there's a lot of evidence for that idea, there are many careful and well-checked observations.

So we can now collect evidence about space using a wide range of powerful and accurate telescopes.

We've now got reflecting telescopes, which take Galileo's design that used lenses, refracting telescopes kind of one step further, and use mirrors to reflect the light, to focus it, instead of using lenses, we can use radio telescopes to look, instead of looking at the visible light with our eyes, we look at the radio waves given off by different objects in space using radio telescopes, and that can tell us a lot about objects in space, and we can use telescopes, which are actually in orbit around Earth called space telescopes.

And it's because of observations from telescopes that we know all of these points, which are just gonna follow now.

So it's because of observations of telescopes that we know that the Sun is just one of many thousands of millions of stars in a structure called a galaxy that we call the Milky Way.

Observations from telescopes have told us that the Sun orbits the centre of the Milky Way galaxy about once every 230 million years.

Observations from telescopes have told us that there are many thousands of millions of galaxies in the Universe.

And observations from telescopes have told us that actually, most stars have planets.

And some people are searching for Earth-like planets called exoplanets, where life might also have evolved, and that's an interesting bit of work to potentially be involved with.

Let's do a check on what we've just said.

True or false, there are more stars in the Universe than people on Earth, what do you think, is that true or false? And once you've decided, justify your answer, did you choose true or false because of A, there are about 100 billion stars in each of over 100 billion galaxies in the Universe, and about 8 billion people on Earth? Or B, there are only a few thousand stars that can be seen in the night sky, which one justifies your choice of true or false? Pause the video now to decide between true and false, and A or B.

Okay, I'll give you the answer now.

The statement is in fact true.

And the reason it's true is because there's about 100 billion stars in each of over 100 billion galaxies, but there's only about 8 billion people on Earth.

Now, B is true, there are only a few thousand stars that are visible in the night sky, but there are far more stars than that, which are too far away to see.

Let's do a task on this now.

It's been estimated that there are about a thousand billion billion grains of sand on Earth.

Now I could read that number as a thousand billion billion because there's a thousand at the start, 1 with 3 zeroes, then a group of 9 zeroes means billion, and another group of 9 zeroes means another billion, so it's a thousand billion billion grains of sand on Earth is one estimate.

And we also think there are about a hundred billion stars in each of over a hundred billion galaxies in the Universe.

So all I want you to do is compare those two numbers, which is greater, is it the number of stars in the Universe or the number of grains of sand on Earth? And if one's bigger than the other, then roughly how much bigger is it based on those numbers? So pause a video, have a good think, you might need to do a calculation, you might need to think carefully about how you can multiply numbers with ones and lots of zeros, and give that task at your best go, and I'll see you in a few moments, off you go.

Okay, I'm gonna give you some feedback now.

So here is an example way of working out an answer.

So if there's about a hundred billion stars in each of over a hundred billion galaxies in the Universe, then to work out the total number of stars there might be in the Universe, you just multiply those numbers together.

So a hundred billion times a hundred billion, actually gives 10,000 billion billion.

So that's a 1 with 22 zeros after it, and you can work that out because each of the numbers, a hundred billion stars and a hundred billion galaxies had 11 zeros after it.

So when you multiply 1 with 11 zeros by 1 with 11 zeros, you get one with 22 zeros, which is 11 plus 11.

And you can read that as 10,000 billion billion stars.

Whereas the number of grains of sand was given as just a thousand billion billion.

So that is a 1 with 21 zeros after it.

So if the number of stars is a 1 with 22 zeros after it, that's 10,000 billion billion, and the number of grains of sand is a 1 with 21 zeros after it, that's only a thousand billion billion compared to 10,000 billion billion, then there's about 10 times more stars in the Universe than there are grains of sand on Earth, according to those numbers.

Here's a summary of the lesson.

Scientific ideas about the Universe help explain what we see on Earth.

Ancient civilizations often thought Earth was not moving at the centre of the Universe, and that the Sun and stars move across the sky.

But observations have provided evidence that this geocentric model is wrong, and careful observations prove that Earth and other planets orbit the Sun.

You need to be a bit careful about that idea because the Sun is not the centre of the Universe, so the idea of a heliocentric Universe is wrong, but the Earth and other planets do orbit around the Sun in the solar system, so the idea of a heliocentric solar system within a far, far bigger Universe is correct.

Telescopes magnify distant objects so they can be seen in more detail.

Evidence from telescopes shows that the Sun is just one of many stars in the Milky Way galaxy, and there are many more galaxies in the Universe than we can imagine.