The Sun glows so fiercely that it is not safe to look straight at it, even though it is so far away. The Sun is a ball of gases.

The temperature at the centre of the Sun is about 15 000 000 degrees Celsius! The surface is about 5 500 degrees Celsius. That is extremely hot! Can you see the explosion from the surface of the Sun in the picture?!

The Sun is a star, because it produces its own energy. The Sun appears bigger and brighter because it is much nearer to Earth than the other stars.

The Sun is about 420 times bigger than the Earth and about 1700 times bigger than the Moon! The Sun is much further away than the Moon from the Earth. The Sun produces light and heat. The heat warms the surface of the Earth.

Study the image of the Sun, and then answer the questions that follow in the activity below.

Long, long ago, people watched the stars at night. Shepherds looking after sheep and cattle would lie down and look at the night sky. People in hot regions like Mesopotamia (now Iraq) slept on the roofs of their houses. They had plenty of time to look at the stars. They knew the patterns of stars in the sky and how the stars moved across the sky during the night. The patterns were fixed (they did not change). For example, you can find a pattern like the Southern Cross if you look up towards the South. You can see it in the first photo below. The second photo with white lines helps you to see the Cross. It always looks the same because the stars are always the same distance apart.

Patterns of stars are called constellations. The next image shows some other famous constellations of stars which you can see in the night sky.

People noticed that some bright objects did not behave like the others. These objects are close to a star one night and then the next night, at the same time, the object has moved away from the star. Night after night, these objects appear in new positions among the stars.

The Greeks of those times called these objects "the wanderers" because they were in a slightly different position each night. The Greek word for wanderer is "planetes" and so we get the English word "planet". A person who wanders is someone who walks around wherever he feels like going.

The planets were a science puzzle

People who study the stars are called astronomers. The planets were a puzzle for ancient astronomers. Why did they move differently to the stars? Were they just as far away as the stars? Why were they brighter at some times of the year than at other times?

The ancient peoples gave names to the planets. For example, Mars was named after the god of war. One planet was so beautiful that they called it Venus after the goddess of love and beauty.

You can see Mars at some times of the year. Mars is orange-red, and at most times it looks smaller than Venus. It is not as easy to find as Venus is; sometimes you have to look late at night to see Mars rising in the east.

How astronomers solved the puzzle

Over hundreds of years, astronomers set up observatories in places like India, Egypt, Iraq, England and countries in Europe. An observatory is a building that has permanent measuring marks. These marks are always in the same position. The astronomers make notes of where stars and planets are compared to these fixed positions, and they note the dates and times.

Over many years of careful observing, the astronomers kept records of where the planets moved. They used maths to predict where a planet would be on a future date. Then, on that date, they went to check if their prediction was correct. They became very good at measuring, doing maths and doing calculations with big numbers.

So they worked out that the planets are closer to us than the stars, and that the planets are moving around the Sun. Then they realised that Earth is a planet too, and it is moving around the Sun!

This idea upset some people who believed that the Earth was the centre of the universe and that everything moved around the Earth. Nowadays we know exactly where each planet in our solar system is at any time, and we can actually send spacecraft to the planets.

The Sun together with the planets moving around it is called the solar system. You will learn about that next.

The solar system consists of the Sun and all the planets that orbit around it. It also includes asteroids and the planets' moons.

In the first image, you can see a diagram of the solar system. In the second image in the next activity, you can see another diagram of the solar system. Both these diagrams try to show you what the solar system is like.

In the solar system, each object's force of gravity pulls on all the other objects. Gravity is a force of attraction between objects. The Sun is the biggest and heaviest object in our solar system and so it exerts the greatest force of gravity on all the planets. This force of gravity makes all the planets move in circles around the Sun.

The names of the planets in our solar system are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Mercury is the planet closest to the Sun and Neptune is the planet furthest from the Sun.

Let's now use our bodies to create a model of how the planets move around the Sun!

The planet Earth orbits around the Sun in 365.25 days and we call that one year. As Earth moves to new positions around the Sun, we have four seasons: summer, autumn, winter and spring, and then summer comes again.

Now in Space, the Earth keeps on going around the Sun at more than 100 000 kilometres every hour. However, there is no string pulling on the Earth, so what keeps it moving?

The force of gravity pulls the Sun and the Earth towards each other.

There is no string in Space between Earth and the Sun! The Earth would move away, but the Sun "traps" the Earth with its much greater force of gravity. The Sun pulls on the Earth and the Earth pulls on the Sun with the force of gravity. The pull is so strong that it works at a distance of 150 million kilometres! Like the string, force of gravity keeps the Earth moving in its orbit around the Sun, year after year.

It does the same for the other planets too. The force of gravity pulls the planet Neptune into its orbit even though Neptune is 30 times further away from the Sun than Earth is.

The four inner planets are rocky.

Look at the picture below of the solar system again. The 4 planets closest to the Sun are called the inner planets of the solar system. They all are made of rock; some of them have a thin layer of gas on the outside. Earth has a very thin layer of water and soil too.

The next image shows us what the core of each of the rocky planets looks like. The core is the inner part of the planet and it is made up of different layers.

The four outer planets are gas giants.

These planets are very far from the Sun. They don't have a hard surface that a spacecraft can land on. Instead, they are giant balls of very cold gases. Astronomers think that these planets have hot, solid cores, deep down inside them.

This next image shows us what the inside of the gas giants is like. There are also different layers of gases.

Let's now take a closer look at each of the planets.

Mercury is closest to the Sun, and the smallest planet. It has no atmosphere and its grey surface is marked with thousands of craters. A crater is the mark that we see where a rock has crashed onto a planet or a moon.

Venus is second closest to the Sun. It is almost the same size as Earth. To us, it looks white and it shines brightly in the evening or morning.

Venus shines brightly like that because it is covered in a thick cloud of gas. It would be horrible to breathe the atmosphere on Venus, because the gas is mostly carbon dioxide and sulphuric acid! The atmosphere absorbs lots of heat from the Sun and does not cool down at night. Venus is the hottest planet in our solar system.

Then we reach Earth - the blue planet, our home in Space. You know a lot about Earth already. Earth has its Moon that we know well. The Moon moves in its orbit around the Earth.

Next furthest out is Mars, the red planet. We know quite a lot about Mars. Many spacecraft have been sent to Mars to take photographs and some spacecraft even landed to take samples of the soil.

Mars needs almost two Earth years to complete one orbit around the Sun, so if you lived on Mars you would have to wait a very long time for your birthday. Mars spins like the Earth, and a day on Mars is almost the same length as a day on Earth. A day on Mars is called a sol.

This planet is further from the Sun than Earth, and for this reason it is very cold. It has a thin atmosphere but this atmosphere is mostly carbon dioxide gas. Humans cannot breathe that atmosphere.

The surface is mostly sand and rocks. The sand is full of iron oxide, which is the same substance as red-orange rust. From Earth we see Mars as a small red-orange dot in the sky, because of the colour of the soil.

There are some very big valleys on Mars. Valleys are caused by erosion when water flows downhill, so we can make an inference that there was a lot of water on the planet long ago. If there was water, perhaps there were living things on Mars too. But we cannot be sure. Scientists have sent another spacecraft to look carefully at the rocks and sand. The spacecraft is called Curiosity and it will try to find signs of living things in the ground.

Now we have to go very far from the Sun, 5 times further than Earth is from the Sun. We begin to see the gas giants, which are the four outer planets. These gas giants have no solid surface we could land on. They are huge balls of gas and if we came close we would fly through clouds of cold gases.

Jupiter is the first gas giant we come to, and it is the biggest of all the planets. From Earth we see it shining white but close up, its colour is light pink-brown. It is bigger than all the other planets put together.

Jupiter is a huge ball of gases such as hydrogen, with clouds of ammonia. Winds blow from east to west on Jupiter's surface, and they blow at the speed of jet planes. The surface is very cold, and some of the gases are so cold that they have become liquid or solid.

Deep inside, Jupiter may have a very hot core of rock. Jupiter has four big moons, and sixty smaller ones.

If we now go even further into Space, to double the distance we went to reach Jupiter, we reach the planet Saturn. Saturn is almost as big as Jupiter but it has a light yellow colour. It is mostly gas and it has rings of rock spread out and spinning around it. Saturn has about 62 moons.

Then we get Uranus, a smooth blue-green ball of gas, with almost no marks and shapes on it that we can see. It has more than 25 moons.

Now we are more than 30 times further from the Sun than Earth is, and we see the last planet, Neptune. Neptune is also a ball of gas and looks like Uranus. It has about 12 moons, and possibly more. Neptune has a very 'stormy' surface. Images of the planet often show huge storms and winds.

What a strange word - asteroid! Have you heard this word before? Maybe when we mentioned the asteroid belt?

The asteroids are lumps of rock from planets that broke up long ago. Some of the lumps are bigger than a school building and some are only as big as small stones. They orbit around the Sun and so they are travelling very fast. Some of them are travelling as fast as 25 kilometres every second. That is much faster than a bullet.

The asteroids have gathered together in a ring in the solar system. This ring is called the asteroid belt.

In 1973, Pioneer 10 was the first spacecraft to travel to Jupiter. To get there, Pioneer 10 had to go through the asteroid belt. Do you remember how fast asteroids are moving? If an asteroid had hit Pioneer 10, the spacecraft would have been destroyed and smashed to pieces. However, Space is a very big place and the asteroids are usually far apart. Pioneer 10 got through safely and went on to take the first close-up photos of Jupiter.

Let's begin with Earth's Moon. You learnt about Earth's Moon in Gr. 4. So what do we already know about Earth's Moon?

Many people say that the full Moon looks bigger when it rises just after sunset, and then looks smaller late at night. Is this true? Let's do an investigation to find out.

The Moon has a pale grey surface with dark grey marks on it. Nobody knew what the surface was like until spacecraft landed on the Moon. The first astronauts to walk on the Moon stepped into fine, powdery dust. They collected rock samples (small pieces) to bring back to Earth.

The surface of the Moon is covered with holes called craters. These craters are made by space rocks that hit the Moon. These rocks may be as small as grains of sand or as big as a house. They travel so fast that they explode when they hit the Moon, and they make a round hole.

The light-coloured areas on the Moon are mountains, and the darker areas are plains. Some of these plains were made by huge space rocks that made craters 300 km wide. Other plains were made by volcanoes on the Moon a long time ago. Lava* flowed out from those volcanoes. Nowadays the Moon has no volcanoes.

The Moon has no air so there is no wind to blow the dust. The Moon has no water, so there is no rain to wash away sand and cause erosion. That is the reason why we see the craters on the Moon so clearly.

Other planets have moons, too. Below is an image showing some of the moons in our solar system. Not all of them are shown here. They have been scaled to be the correct size compared to Earth and our Moon.

Mars has two moons, and astronomers called them Deimos and Phobos. Phobos has deep craters showing that it has also been hit by fast-moving rocks. Can you see how small these moons are compared to our Moon?

On the other hand, Jupiter has 66 moons which we have identified, and each time humans send another space probe to Jupiter, more moons are being discovered! Only Jupiter's 4 biggest moons are shown below.