Sundials
About five billion years ago there was a huge cloud of gas and dust in space. If you could have entered the cloud in a space ship you would have had to travel one and a half million, million kilometres before you came out of it. The distance was the same no matter where you entered the cloud ??from the top, the bottom or the side. Not far away a star exploded. It sent out all kinds of material in every direction at great speed. Some of this material caught the edge of the cloud and pushed it. This made the cloud start to turn. The cloud gathered speed and began to spin. At the centre of the cloud some gases formed into a huge ball. In time this ball began to glow and became a star ? the Sun.
Around the Sun flew pieces of dust. They crashed into each other and stuck together to form small rocks. These small rocks crashed together and formed larger rocks. The larger rocks crashed together and formed even larger rocks. Eventually when some huge rocks crashed together they spun round and formed balls of rock called planets. The Earth formed in this way. Today the Sun and the planets are still spinning. For much of the time that people have been here, they have used the spinning Earth and the shining Sun to help them tell the time.

The Earth is like a giant spinning top. It turns round once every twenty four hours. If you could travel in a space ship above the North Pole and look down on the Earth for a while, you would see that the planet turns anticlockwise. This movement means that to everyone on the Earth the objects in the sky move in a special way. They appear over the eastern horizon, move across the sky and disappear below the western horizon. The most obvious object to make this journey is the Sun. Let's think about how the Sun moves in the sky over your home.

Early in the morning the sky on the eastern horizon begins to get brighter. This is because the place where you live is turning towards the Sun. As the Earth keeps moving the sky gets brighter and brighter and the Sun appears over the horizon. The time when this happens is called sunrise. After sunrise, the Earth keeps turning and Sun moves across the sky and upwards. All morning it appears to travel in this way. At midday it is at its greatest height. In the afternoon, as the Earth keeps turning, the Sun travels further across the sky but also sinks lower. Finally it reaches the western horizon and sinks below it. When this happens we say that the Sun has set. Even after sunset there is light in the western sky but eventually, as the Earth turns your home further away from the Sun, the whole of the sky becomes dark.

If you had watched the shadows that formed as the Sun travelled across the sky you would have seen them change in an orderly way. At sunrise the shadows were very long and pointing towards the west. As the Sun rose in the sky the shadows became shorter and pointed to the north if you live in a country north of the equator, or south if you live in a country south of the equator. At midday the shadows were at their shortest and pointed directly north or south. In the afternoon, as the Sun sank in the sky, the shadows became longer again but this time pointed towards the east. At sunset the shadows were at their very longest and pointed eastwards.

The way in which the shadows change is the same every day and in the past people made devices to measure time by looking how the shadows fell. The best known device for measuring time by shadows is the sundial. A sundial has a metal rod in the centre of a flat surface. On the surface are marked lines to indicate the time of day. When the Sun shines, the rod casts a shadow across the surface and strikes one of the lines. If you read the time that is stated by the line you can tell the hour of the day.

You can make your own simple sundial. Just stick a pencil straight up in the centre of a piece of white card. Put the card and the pencil outside early in the day. Every hour look at the shadow of the pencil and mark the position of the shadow on the card. At the end of the day you should have a number of lines and next to each one a time of day. On the next sunny day set up your sundial exactly in the same place and use the shadows instead of a clock to tell you the time.

Does the Sun spin like the Earth?
Yes, it does. The Sun however is a million times larger than the Earth and so takes longer to spin round once. It takes the Sun nearly twenty five Earth days to spin round once.

Do other planets spin round like the Earth?
Yes, they do. But some planets spin round more slowly while others spin round more quickly. For example Mercury takes over fifty eight Earth days to spin round once while Jupiter takes less than ten hours.

Are all the planets made of rock?
No. The rocky planets are Mercury, Venus, Earth, Mars and Pluto. The other four have rocky centres but are mostly made from gases and liquids. These planets are called Jupiter, Saturn, Uranus and Neptune.

What is the equator?
It is an imaginary line, which runs around the middle of the Earth. This line divides the Earth into the Northern and Southern hemispheres. Examples of countries in the Northern Hemisphere are England, The United States of America and Japan. Examples of countries in the Southern Hemisphere are Australia, New Zealand and South Africa.

Does the Sun seem to move across the sky in exactly the same path every day?
It will do this if you live on the equator but if you live in the Northern or Southern Hemisphere the path will change during the year. For example, in England the Sun does not rise as high in the sky in winter as it does in summer.

How are the shadows different in winter and summer?
As the Sun does not rise very high in the sky in winter the shadows are longer. In winter the shadows are longer at midday than they are at midday in summer.

Why does the path of the Sun change?
This is because of the way the Earth moves in space. The Earth moves round the Sun in a path called the Earth's orbit. It takes a year to move round the Sun once. During part of the is time each hemisphere is tipped towards the Sun and then away from the Sun. When a hemisphere is tipped towards the Sun, the path of the Sun rises higher in the sky and we call that time summer. When a hemisphere is tipped away from the Sun, the path of the Sun does not rise high in the sky and we call that time winter. Remember that the Sun does not actually move across the sky. It is the turning Earth that makes it look as if the Sun is moving. The Sun just stays in one place in space spinning round approximately once a month.