How Windmills Work Should it be called a starmill?The windmill is driven by energy from the star which we call the sun. The sun is 71% hydrogen and 27% helium. The very high temperature and pressure inside the sun cause nuclear fusion: hydrogen and helium nuclei combine and produce vast quantities of heat. Fortunately, the sun is 150,000,000 kilometres away from us, and has enough hydrogen left to burn for billions of years.As the earth rotates and orbits the sun, radiation from the sun warms the atmosphere, the clouds, the surface of the ground, and the surface of the sea. As a result, different parts of the atmosphere are at different temperatures. This causes differences of pressure. The attempt to equalise the pressure in different parts of the atmosphere is known as wind.So a windmill could be called a starmill, a sunmill, or a nuclear fusion mill, but we call it by the energy source which is closest and most familiar to us, the wind. How the energy of the wind is capturedThe diagram on the left shows a board - seen from above - being held up in the wind. If you hold the board still, the moving air will mostly flow round the left-hand side. If you let the board go, the wind will push it back and to the right. If you only allow the board to move to the left or the right, the board will be pushed to the right. Windmills usually have four boards held at an angle to the wind.The sweep on the left is taken from the photograph on the Home Page. Its right edge is closer to you than the left edge. The wind blows from where you are, and will tend to flow round the left edge. The sweep cannot be blown backwards, because it is fixed to a central axle (called the windshaft) which points into the wind. If the windshaft is free to rotate, the sweep is pushed to the right. In other words, the windmill's sweeps or sails always rotate anticlockwise.In the Middle Ages, sails were made of cloth attached to lengths of wood. Later, they were made entirely of wood, and since the eighteenth century have often been called sweeps. Two problems To work efficiently, the sweeps must always face directly into the wind, the direction of which often changes. The sweeps must also be able to deal with changes in the strength of the wind, which are unpredictable.