Monday 03 February 1997
Q Why don't stars appear in pictures from the Apollo landings on the Moon?
A The lunar surface is very bright, and reflects a lot of light. The television cameras on the Moon compensated for this by reducing the amount of light let through the lens. As a result, stars were not bright enough to be seen.
Q Why, if you shut one eye, do you still see in 3-D?
A You don't, really, but your brain supplies the missing information, so you get the impression that you are still seeing an image with depth. Depth perception still isn't fully understood, but our brain can use pictorial clues such as the angle an object covers on our retina. Other clues include the brightness of the object; if it is brighter, it will usually be nearer, so light and shade can also be important. There are also physiological clues such as when you focus on something close, the shape of the actual lens changes. To check if you are really seeing in 3-D when you have one eye shut, try moving your head from side to side, or touching objects at varying distances.
Q How far does the Earth travel round the Sun?
A About 570 million miles (900 million kilometres). The first measurement was made by Aristarchus of Samos in about 270BC. He measured the position of the Sun relative to the Moon when the Moon was half full. From this, he worked out the distance to the Sun (since the Earth's orbit is nearly circular, the distance travelled is 2p multiplied by the radius). The number he got was 20 times too small, but very early astronomers often did worse.
Q Stars twinkle because of the Earth's atmosphere. Why don't planets?
A Stars are so distant that they appear as point sources of light, so any disturbance in the Earth's atmosphere is easily visible. Planets, being closer, appear more as a disc than a point of light. Any disturbance is less visible because if the central part of the image is distorted as it passes through the atmosphere, that distortion probably won't reach the edge of the disc - so the planet won't seem to twinkle.
Q What is the strongest plant fibre?
A A fibre called ramie is the strongest. Its fibres are eight times as strong as those of cotton.
Q Do fish blink?
A No. Like snakes, they don't have moveable eyelids. Instead, they have a transparent eye protector permanently in place. Fish have excellent eyesight and can see parts of the spectrum we can't. They rely heavily on visual signals for species recognition, choosing a mate and territorial defence.
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