Outdoor lighting
Imagine that you are out in the countryside and darkness is falling. You are on a hilltop and below you on every side are fields and woods. Far in the distance in one direction you can see the roofs of a town.

As the light of the setting Sun fades, you see points of light in the sky. Some twinkle. They are stars. Two points of light do not twinkle. They shine steadily. They are planets. One shines with a red light. It is the planet called Mars. The other planet, which shines with a white light, is called Jupiter.

As you look at the stars and planets, the Moon comes out from behind a cloud. It has a large disc tonight but sometimes you have seen it as a crescent shape or even looking as if half of it is missing. The moon does not twinkle. It shines with a steady light like the planets.

The reason the Moon and planets do not twinkle is that they receive a large amount of light from the Sun and reflect it across space and into our eyes. The stars twinkle because they are so far away and only a small amount of light reaches us. This light is so weak that it cannot shine strongly through the atmosphere like the light from the Moon and the planets. In the atmosphere, air is moving up and down and this makes the weak light keep changing direction as it comes towards us. This changing direction of the starlight makes the stars appear to twinkle. If you could travel in a space ship above the atmosphere you would see that the stars shone with a steady light like the planets.

As you look up, you see a small streak of light streak across the sky. It is a meteor. Some people call meteors shooting stars but this is wrong. A meteor is a small piece of rock, which leaves space at a high speed and enters the atmosphere. As it rushes through the atmosphere it rubs against the air and becomes so hot that it glows and sends its light to the ground. The heat causes most meteors to burn up before they reach the ground.

After the light from the meteor has died away, you see another object shining with a steady light, like a planet, but moving at a steady speed in an arc across the sky. This is a satellite. It is shining because it is reflecting light from the Sun as do the planets and the Moon.

Another light in the night sky catches your eye. This time the light is flashing and moving towards you. As it passes overhead you realise that it is on the underside of an aeroplane. You decide that you have probably seen everything there is to see in the night sky and turn to go down the hill to the road. As you walk down the path you see a small light in the tall grass close by. You look closer and find a small beetle with its tail lit up. This animal is a female glow-worm. She is using the light to attract a mate.

Just as you reach the road you see a pair of headlamps approaching. From nearer you can see that they belong to a bus. The bus stops and you sit at the front and enjoy a ride to town. The light shining out from the headlamps strikes the road many metres in front of you and illuminates the way for the driver. As the bus rounds a corner a pair of eyes shine at you from the middle of the road. The driver slows down the bus and you realise the eyes belong to a rabbit. They blink, then the rabbit hops away.

A few minutes later you come out of a wood and into the orange glow of streetlights. As you travel towards the centre of town you see lamps on the front of houses and security lights shining on the sides of factories. On entering the bus station you see the lights from shops and restaurants. You are glad to see that someone is there to meet you and take you home.

If you have a chance to look up into the night sky see if you can tell a planet from a star, spot a meteor or track a satellite.

Can you see only two planets in the night sky?
No. The most easily seen planets are Venus, Mars, Jupiter and Saturn. They may not all be in the sky at once. It just depends where they are in their orbits as they move around the Sun. Mercury is too close to the Sun to be seen easily. (Remember that you must not look at the Sun because it can damage your eyes and cause blindness.) The other planets are too far away to be seen without a telescope.

Why does the Moon seem to change its shape?
The Moon moves in an orbit round the Earth. While it is moving round the Earth the Sun is always shining on it but the part that is lit up is not always fully seen from Earth. For example, when only a small part of the illuminated surface can be seen we see the Moon as a crescent. Another time half the surface may be seen and the Moon appears as a half Moon. At a full Moon we can see the entire surface that is being lit up, by the Sun.

Is the Moon as far away from the Earth as the Sun?
No. The Sun is 150 million kilometres away. The Moon is about 384,000 kilometres away. If you could walk to the Moon and began walking now, and did not stop until you reached it, you would arrive in six and a half years. If you carried on walking to the Sun you would not arrive there for about another two and a half thousand years.

Is the Moon near the clouds?
No. The highest clouds are only about twelve kilometres above the ground. The Moon is thirty two thousand times further away.

How far away are stars?
They are huge distances away. The distances are measured in light years. That is the distance that light travels in a year. As light travels at 300,000 kilometres in a second you must multiply this number by 315,360,000 which is the number of seconds in a year. Another way of thinking about these huge distances is to remember that the Sun is a star and sunlight only takes about eight minutes to reach us. Many stars are over a hundred light years away. The light from them takes over a hundred years to reach us.

Do any meteors ever reach the ground?
Yes, they do. If the meteor is large it will not burn up completely as it passes through the atmosphere and may reach the ground. It is then called a meteorite. Some meteorites are so large that they make a crater in the ground.

What is a satellite?
It is a spacecraft, which goes around the Earth. It may be used to send telephone messages or television programmes around the world. It may also be taking pictures of the world so that scientists can study the weather or the way crops and forests are growing.

Does a glow-worm burn itself when it makes a light?
No. The substances which make the light in a glow-worm do not make any heat so the glow-worm is not harmed.
There are insects called fireflies which make light. Many deep-sea animals have spots which light up. The light is made from substances which do not produce heat.

Does light shine out of eyes?
Some people used to think that we see because our eyes make light and it shines out onto the object so that we can see it. This is wrong. You have only to go into a dark room to realise that your eyes do not work in this way. Sometimes the light shining into eyes can be reflected out again. This makes the eyes appear to shine but they are not producing light.