Sunlight
Imagine it is a dark winter morning before the dawn. It has snowed during the night and your world is white. As you gaze out of the window you can see the sky in the east becoming brighter and brighter. Soon the Sun appears over the horizon. It lights the whole sky and makes the snow shine. You rush your breakfast, put on your outdoor clothes and run out to play in the snow.

All morning, as you make a snowman and snowball your friends, the Sun keeps rising in the sky. At midday the Sun is at its highest point in the sky. It makes snow and ice sparkle. While you have lunch, the Sun begins to move lower in the sky. It gradually sinks to the western horizon.

A large cloud moves slowly across the sky and covers the Sun but there is still plenty of daylight. You take your sledge and spend the afternoon speeding down a hill in the park. The cloud moves away and the Sun shines down on the snow once more. The Sun is near the horizon now and soon it sinks from view. You watch the orange glow of the sunset fade away then go home to get warm and have a meal. When you have finished eating, you feel so tired that you go to bed. As you close the bedroom curtains, you notice the night sky is black once more.

Long ago, people used to think that the Sun was a god that travelled across the sky each day bringing light and warmth. Today we know that the Sun is something different. It is a huge ball of gas a million times larger than the Earth. You may know the word 'gas' from seeing gas cookers and gas fires. They use a gas which comes out of the ground. This gas is called natural gas but there are many other kinds. The Sun is made from two gases. One is a gas called helium, which you may sometimes have brought home from a party. Helium is the gas that is used in party balloons. Helium is so light that it floats in the air and makes a balloon rise to the ceiling, if you do not hold onto it. The other gas in the Sun is called hydrogen. This gas is also lighter than air and was used to make balloons. However, hydrogen catches fire easily so it is not used for balloons today.

You may wonder how a ball of gas manages to hold itself together in space. Here is a clue to how it does it. Hold up a pen a few centimetres above the table and let it go. The pen falls and hits the tabletop. The pen moves because it is pulled by a force called gravity. This force pulls everything towards the centre of the Earth. When you jump up, the force of gravity pulls you down again. There is a force of gravity on the Sun. However, because the Sun is much larger than the Earth, the force of gravity is much larger too. It holds the gases together. In fact it does more than that. At the centre of the Sun the hydrogen gas is so squashed up due to the force of gravity, that it changes to helium. As this change happens, heat and light are made. They leave the Sun in all directions.

Every second four million tonnes of hydrogen are changed to helium. This means that the Sun is producing light and heat all the time ? even at night. The reason the Sun does not make the night sky light, is because of the way the Earth moves. The Earth spins round like a top but the Earth is so much larger than a top that it takes twenty-four hours to spin round once. The way the Earth spins explains why the Sun appears to move across the sky. If you could look down on the top of the spinning Earth you would see that it is turning anticlockwise. Back on the Earth's surface this means that objects that are in space appear above the eastern horizon, move across the sky and disappear below the western horizon. The Sun is no exception. As the Earth turns, the Sun moves across the sky and disappears. While it is out of the sky where you are, it is crossing the sky above another part of the Earth. Later, as the Earth keeps turning, the Sun rises again and another day begins. It has been producing sunlight all the time. You simply don't see the light at night because the place where you live is facing away from the Sun.

Look out of the window. Is the place where you are now facing the Sun or away from it?

How far is the Sun from the Earth?
It is 150 million kilometres. If you began walking now and did not stop until you reached the Sun it would take you over two and a half thousand years to reach it! If you walked for twelve hours every day and had twelve hours rest it would take you five thousand years!

How long does it take light from the Sun to reach the Earth?
Light travels much faster than you walk. In fact light travels at the fastest speed known in the universe. It travels at 300,000 kilometres in a second. This means that light takes just over eight minutes to travel from the Sun to the Earth.

How far are clouds above the Earth?
Clouds are much, much closer to the Earth. In fact
they are in the lower pat of the Earth's atmosphere. A large sheet of cloud that can cover the sky may be only about a kilometre above the ground. Some clouds can be so low that they form fog at ground level. The highest clouds, which appear as white wisps in the sky, are called cirrus clouds and may be up to twelve kilometres above the ground.

Why does it remain daylight when clouds cover the sky?
Sunlight is very powerful and the clouds are translucent. They let some of the sunlight pass through.

Why are clouds white?
Clouds are made from billions of tiny drops of water. When the light passes into the cloud it is reflected off the water drops in all directions. We call this the scattering of light. When light is scattered in this way, it makes the substance which does the scattering appear to be white. Milk is made from tiny fat drops in water. They scatter light just like the water drops in a cloud. This scattering of light makes the milk appear white.

Why does the Earth spin?
About five billion years ago there was a huge cloud of gas and dust in space. A star exploded close by. The force of the explosion pushed on the cloud and made it spin. At the centre of the cloud the Sun formed. Around the Sun formed the planets. The Earth is one of the planets. The Sun and the planets all spin owing to the way that they were made from the spinning cloud of gas.

If the Sun is using up its hydrogen, how long will it last?
It is thought that the Sun has about another five billion years to go before it runs out of hydrogen. It will then swell up into a large red ball and may spread out as far as the Earth. This would burn everything on the Earth to a cinder. The Sun will then shrink and become a small white star and eventually it will stop producing light. It will then become a black dwarf star. If there are any people alive when the Sun begins to change, it is hoped that they would be able to travel in spaceships to planets around other stars.