A For most of history, sailors feared the coast at night more than the open sea. Hidden rocks and shallow waters could tear open a ship's hull in seconds, and without any warning lights, a captain had little idea how close he was to disaster. The lighthouse was invented to solve this problem: a tall tower with a bright light at its top, built to warn ships of danger and guide them safely into harbour.

B The most famous of the ancient lighthouses was the Pharos of Alexandria, in Egypt, built more than two thousand years ago. Standing perhaps a hundred metres high, it was one of the tallest structures of the ancient world. A fire burned at its top, and its light was said to be visible many miles out to sea. The Pharos stood for centuries before earthquakes finally destroyed it.

C For most of history, the light itself came from open fires or candles, which were dim and easily blown out. The great advance came in the early nineteenth century, when a French engineer, Augustin Fresnel, designed a special lens built from rings of glass. This lens gathered scattered light and concentrated it into a single powerful beam, greatly increasing the distance from which a lighthouse could be seen.

D Lighthouses were once tended by keepers who lived inside them and kept the light burning through every night — a lonely and demanding job, often on a rock far from land. Today almost all lighthouses are automatic, controlled by machines, and satellite navigation has reduced how much ships rely on them. Even so, many are still kept working as a backup, and are carefully preserved for the part they played in history.

A We tend to think of colour as belonging to objects — the red of an apple, the blue of the sky. In fact, colour is a property not of things themselves but of light and the eye. Sunlight, though it looks white, is really a mixture of all colours. An apple appears red because its skin reflects the red part of that light back towards us and absorbs the rest.

B The eye completes the process. At the back of the eye lie cells that respond to light, and among them are special cells called cones, which detect colour. Humans have three types of cone, sensitive to red, green and blue light. The brain combines the signals from these three types into the millions of different shades we are able to see.

C Not every creature sees the world as we do. Dogs, for example, see a far narrower range of colours than humans. Bees, on the other hand, can see ultraviolet light, which is completely invisible to us; this reveals hidden patterns on flowers that guide the insects towards the nectar. What counts as “colour” depends entirely on the eyes that are looking.

D Colour also carries meaning, and that meaning is far from fixed. The same colour can suggest quite different things in different places: red may stand for danger in one culture and for good fortune in another. Researchers have found that colour can even shape how we feel — influencing how warm a room seems, or how quickly people act.

E Getting colour right is big business. Screens must reproduce colours accurately, and paints and fabrics have to match perfectly from one batch to the next, or customers complain. Some companies invest heavily in owning a particular shade so closely linked to their brand that shoppers recognise it at a glance, long before they read the name.

F In the end, because colour is created by the eye and the brain, no two people see it in exactly the same way. A small number of people cannot tell certain colours apart at all — a condition known as colour blindness. Colour, then, is less a simple fact about the world than an experience built inside each of us.

Every time a rocket rises into the sky, the same question returns: can the enormous cost of space exploration really be justified while so many problems remain unsolved on Earth? To its critics, spending billions to reach distant planets while people go hungry at home is impossible to defend. Supporters, however, argue that the choice is far less simple than it first appears.

A common complaint is that money spent “in space” is somehow lost to us. In reality, almost all of it is spent here on Earth — on the wages of the scientists and engineers who do the work, and in the factories that build the equipment. Space programmes create highly skilled jobs and support entire industries, and the sums involved, though large, are only a tiny fraction of what governments spend in other areas.

Supporters also point to the everyday technologies that grew out of space research. The effort to solve the problems of spaceflight has led to advances in materials, medicine and computing that we now take for granted. Satellite systems first developed for exploration make possible the weather forecasts, communications and navigation that modern life depends upon.

A further argument is harder to measure. Watching human beings travel beyond our planet, supporters say, inspires young people to study science and reminds us of what we can achieve when we work together. Critics reply that inspiration is a poor return for such vast expense, and that the same money could do more good if it were spent directly on schools or hospitals.

Perhaps the fairest conclusion is that the real question is not space or Earth, but balance. Few people would seriously argue for abandoning exploration altogether; equally, few would defend it at any price. The genuine task is to decide how much a society can reasonably invest in looking outward while still meeting its needs at home.