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Q. Why is your apparent speed from the passenger window of an aircraft not nearly as great as your actual speed?

A. When we judge the speed at which we are travelling, we do so by looking at how fast objects around us are passing by. We are used to doing this on the ground, where trees, houses, cars, etc are quite close to us. In the air, the ground is very far away and there are no close reference points to judge our speed against. This whole problem might be an evolutionary relic - our brain has not been constructed to cope with very fast flight at large distances from the ground, and so we underestimate the true speed. In time, we might overcome this problem, but in the meantime, if you look at the speed of the plane's shadow on the ground, you have a more accurate reference frame for judging speed.

Q. Why is the Sun the colour it is?

A. The Sun is just one of many stars in the universe. The colour of each of these stars depends on the energy they give off, and this in turn depends on how hot they are. This is similar to a coal fire - the hotter the coals, the whiter they glow. As the sun cools over the next 5 billion years, it will appear less bright and yellowish white, and will slowly become orange and finally red.

Q. What is the main cause of gravity? Why is there an attraction between two masses?

A. The mathematics of general relativity used to describe what physicists call

"space-time" demonstrate that the geometry of the universe is curved in the vicinity of large masses. The best analogy is that of a trampoline with two heavy sacks placed on it. The two sacks will slightly depress the mattress in their vicinity, and if they are sufficiently close then they will tend to roll together.

Q. Why are some parts of the Earth hotter than others?

A. On a global scale, the temperature of a piece of land is related to the angle at which the Sun's light hits it. A square metre of land at the Equator is being hit head-on by sunlight, but towards the poles the light is hitting it at an angle. So each square metre of light hitting the Equator has to hit more than a square metre at the poles. In other words, the energy hitting a square metre at the Equator gets spread out over a greater area at the poles. It also has to get through more atmosphere because of the angle. Both of these factors mean that less energy reaches a square metre of land at the poles than at the Equator.

Q. How do solar panels produce electricity?

A. There are two main types of solar heating system. In the first, the heat energy from the Sun is used to heat and sometimes vaporise a liquid, which then carries the heat energy to where it is wanted. The second type is photovoltaic: photons from the sun interact with thin layers of silicon and negative charge jumps across a one-way junction and then cannot return. The resulting potential difference can then be used to drive an electrical current round a circuit, thereby generating power.

Questions and answers are provided by Science Line. Its Dial-A-Scientist service can be contacted on 0345 6000 444.