FAQ - KNOWN PHENOMENA


On this page you can find answers to questions that SUFOI is often asked.
If you have your own questions you would like to see answered on this page, use the form at the bottom of the page.

Question Answer
1. Is the sound of a plane always audible? No; our experience is that aircraft lights are often deceptive for exactly that reason. People think they should always be able to hear the sound of the plane, but if the wind is in the wrong direction or the plane is far away it is quite common not to hear any sound at all.
2. Do shooting stars always last only a few seconds? No. Shooting stars or fireballs can last a long time; in rare cases as much as several minutes.
3. Why does a "new, bright star" suddenly appear in the sky? It is probably not a star, but a planet. The planets are not fixed in the sky in relation to the stars, but appear in different positions depending on the month and year. These positions can be found in almanacs or astronomical calendars.
4. Why does a satellite seem to curve slightly as it moves through the sky? When a bright light moves through the sky and passes another bright light (e.g. a star), our senses are deceived by an optical illusion. Suddenly there is a fixed point in the sky, so we automatically judge the distance to be greater than before. This is the same mechanism discussed in point 5.
5. Are the sun and moon larger when they are near the horizon? No, this is an optical illusion. Objects that are close to other, recognizable objects are automatically perceived as being larger than usual, just as when the moon or sun are close to the horizon near trees, houses or the natural landscape. You can check this yourself by measuring the moon with your little finger when it is high in the sky, and again when it is close to the horizon.
NB: Do not try to measure the sun in this way.
6. Are stars visible during the day? In certain circumstances it is possible to see stars at dusk and dawn, however the most common sights are planets. These are sometimes so bright that they are actually visible hours before sunset and after sunrise.
7. Is it possible to judge distance in the dark? It is very difficult for the human eye to judge distance in the dark, especially when looking at a light in the night sky. Research shows that it is in fact impossible people to judge the distance. Therefore when one has the impression that "the light was just behind the trees", it is usually an optical illusion.
8. Why do the stars twinkle with difference colours? Light from the stars passes through Earth's atmosphere. As a rule the atmosphere is turbulent and in motion, and it is this turbulence that makes the relatively weak light from the stars twinkle and change colour. The light from planets is brighter, and is therefore not affected so much by motion in the atmosphere.
9. Do the stars move? Because of the earth's rotation on its own axis, it appears that the stars slowly move across the sky during the course of an evening. In Denmark this movement will always be from left to right down towards the horizon. Only the Pole Star stays fixed; it is positioned directly over the North Pole and hence Earth's axis.
10. Is the Aurora Borealis (Northern Lights) visible in Denmark? The Northern Lights are actually a relatively common event in Denmark, even though only a few people have seen them. This beautiful spectacle can be astonishing, but it is actually caused by electrical discharges from the sun (ionised particles) moving through Earth's magnetic field around the poles.

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