Friday 11 June 1999
"Smoke" is used to describe a certain condition of visibility. When visibility is low, you assume that something is in the air to cause it. When it's water drops, then that's called mist or fog; when it's drier, it's called haze. And there are two types of haze. Dust is the most common, but you also get smoke haze resulting from smoke itself or pollutants. Most UK stations don't report smoke unless they're 100 per cent certain that the cause is smoke; however, the station on the Isle of Man (Ronaldsway station), an independent station, does often report smoke.
Q Why do the Sun and Moon (pictured) appear bigger when they're near the horizon?
This is a famous optical illusion that has no simple answer. If you take a ruler and measure the size of the Sun - or, more easily, the Moon - at the horizon, and then again when it's higher, you'll see that there is no difference. There has been much research into this topic. Many think that because we can see buildings and trees that we know to be not that far away, our brain tries to imagine the Sun or Moon at that distance too, and this makes them appear bigger. However, experiments in which people were asked to judge the size of the Moon with and without reference objects such as buildings suggest this is not the case.
Q Light slows down when it enters glass. Is it theoretically possible to see into the past when you look through glass with a very high refractive index?
Surprisingly, yes - but not very far. Put a piece of glass with an amazingly high refractive index over one eye, and nothing over the other. The light passing through the glass will be slowed down, and arrive at the eye later than the light from the same object that hasn't gone through the glass. So one eye would be seeing in "real" time, while the other would see the same view with a tiny delay on it. However, the delay would be fractions of a microsecond - it's not much of a time machine.
Q Why don't the constellations change, if the stars are moving so fast?
Although the stars are indeed moving very fast (at a speed of many kilometres per second) they are so far away (typically trillions of kilometres) that they have to travel for thousands of years before we can see a difference in their positions. In one lifetime we won't notice a difference in the constellations, but if you could come back in 50,000 years' time, the sky would look very different.
The Technoquest website is at http://www.sciencenet.org.uk
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