The Earth, the world we live on, is a great ball of rock. It measures over 12,750 km across at its widest point, the equator. But rocky material, or land, covers less than a third of the Earth's surface. More than two-thirds of the surface is covered by the water of the oceans.
Water is one of the two vital substances that makes possible life on Earth. The other vital substance is the layer of air that surrounds the Earth. This layer, the atmosphere, is made up of a mixture of gases.
All living things, plants and animals alike, need to breathe one of these gases, oxygen, in order to stay alive. Oxygen makes up about 21 per cent of the air by volume. The rest is mostly nitrogen gas.
Another vital gas in the air is carbon dioxide. Plants need to take in this gas to make their food. They combine it with water they take in through their roots to make sugar. But they can carry out this process only in the light. They need the energy in sunlight to get it going. That is why the food-making process is called photosynthesis (which means 'making with light').
All life on Earth in fact hinges on photosynthesis. Animals can't make food themselves, and must eat plants for food, or eat other animals that feed on plants.
The energy we receive from the Sun is not just important for making food for living things. It also keeps our world at a suitable temperature in which living things in enormous variety can thrive and multiply. Without the heat from the Sun, the Earth would be a deep-frozen, dead world.
The atmosphere also plays a vital role in keeping our world at a reasonable temperature. It acts as an insulating blanket at night to stop the Sun-warmed Earth cooling down too much.
Compared with the size of the Earth itself, the layer of atmosphere is extremely thin. It is thinner, for example, than the peel of an orange is relative to the size of the orange.
The atmosphere extends upwards over our heads, but it is not the same all the way up. Because of its weight, it is densest, or thickest, at the bottom, at sea level. It gets gradually less dense, or thinner, the higher up we go.
At sea level, the weight of the air pressing down exerts on each square centimetre of surface a pressure of about one kilogram. This is termed the atmospheric pressure. Meteorologists - the scientists who study the weather - measure the pressure in units of millibars. Sea level pressure averages about 1,000 millibars.
The atmospheric pressure falls - the air thins - progressively as you climb higher and higher above sea level. At the height of
Everest (nearly 9 km), the air is so thin you can hardly breathe. Here you are approaching the top of the layer of atmosphere known as the troposphere.
At twice this height, where the supersonic airliner Concorde flies, you are in the next layer of atmosphere, the stratosphere. And the pressure is so low that if you were exposed to it, your blood would boil.
Within the stratosphere is a layer of ozone, a different form of oxygen, which has three instead of the usual two atoms in its molecules. The ozone layer also plays a vital role in helping life on Earth. It does so by filtering out from sunlight most of the harmful ultraviolet rays. These are the rays that cause sunburn.
Without the ozone layer, the intensity of ultraviolet radiation would have a devastating effect on both plant and animal life. That is why the observed thinning of the ozone layer because of atmospheric pollution is so worrying.
The air gets thinner and thinner as you climb higher and higher through the atmosphere. The stratosphere ends at a height of about 50 km. Thereafter, the air that is still present exists not as molecules, but as charged particles, or ions. This part of the atmosphere is therefore called the ionosphere.
In this region there are layers that reflect radio waves, a property used for long-distance, round-the-world broadcasting.
It is also in the ionosphere that displays of the Northern and Southern Lights take place. These colourful light displays occur in polar regions when streams of charged particles from the Sun run into the ions in the atmosphere. The streaks we call meteors also originate in the ionosphere, as rocky specks are attracted by gravity into the upper air and burn up.
The air thins rapidly going up into the ionosphere. And by a height of about 250 km, there is scarcely any air left at all. To all intents and purposes we are in space.
At this height satellites can orbit the Earth without experiencing any appreciable air resistance. They can therefore maintain their speed and their ability to stay aloft. The space shuttle sometimes orbits as low as this on its relatively short journeys into space.
But a satellite orbiting for a long time at this height will be gradually slowed down by the faint traces of air remaining. When its speed falls below orbital velocity, 28,000 km and hour, it will drop back to Earth.
When the space shuttle orbiter wishes to return to Earth from space, the shuttle pilot fires rockets to slow it down. It drops from orbit down towards the ground. As it drops lower, the air outside begins to thicken, but at first there is little effect on the orbiter. Not until it has dropped to a height of about 120 km
does the air begin to offer noticeable resistance, or drag. This marks the re-entry interface for the orbiter, which is now travelling at a speed of about 26,000 km an hour.
As the orbiter drops lower and lower, the drag on it increases dramatically, rapidly slowing it down. Friction between the orbiter and the air generates tremendous heat. This causes the tiled underside of the craft to glow red-hot. In less than 20 minutes the orbiter's speed has been halved, and it is beginning to fly like a plane rather than a spacecraft. The air continues to brake the orbiter, and in another ten minutes or so it will land on the runway at a speed of only about 350 km an hour.
