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Why stars blink and planets don't

  1. May 6, 2010 #1
    Why do starts blink and planets do not. There are some stars that are just as bright as a planet(unless my observation is wrong).
    I know the light from the star enters Earth's atmosphere and due to the variation in density(which means change in dielectric constant of the medium in which light travels), the intensity varies which appears as blinking. Kinda of low frequency modulation.
    Why doesn't this affect the light from the planet.
  2. jcsd
  3. May 6, 2010 #2


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    Stars twinkle because the atmosphere bends the light so one moment it's hitting your eye the next moment it isn't

    Planets are large enough that they are bigger than the size of the atmosphere twinkle patch (with apologies to Fried!) - so when one bit of the planet moves out of your eye the chance is that another bit moves in. On average the planet appears constant - although if you looked at a feature on the planet that feature would twinkle.
  4. May 6, 2010 #3
    Ok. Thanks. Why does the start light bend? Refraction?

    I couldn't stop laughing at your irony definition.
  5. May 6, 2010 #4


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    Picture the atmosphere as the top of a pan of boiling water.
    There are bubbles (called cells) of stable air which act rather like lenses.
    When you look through one of those you get a good image, when the edge of one passes over you see a twinkle, typically in the visible this happens many times/second.

    You can use this effect to take twinkle free images from the ground - http://www.ast.cam.ac.uk/~optics/Lucky_Web_Site/index.htm

    ps. the example we used to use (before the university introduced rules about this sort of thing) is imagine the obscured glass in a bathroom window. If you look through it at a distance object - the image would move around randomly as your head moved and you were seeing it through a different bit of glass. But if you pressed a body upto the glass - although each bit would be distorted you would accurately see the overall shape of the body.
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