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The Sun's Light

  1. Aug 28, 2005 #1
    We know that the sun's light takes 8mins to reach the Earth.
    That means the sun we see now is actually what the sun was before 8 mins.
    We also know that Earth rotates on its own axis.
    It rotates 2 degrees in 8 mins.
    So when actually the light travelled frowm the sun, we moved 2 degrees East.
    So the actuall position of the Sun is 2 degrees west.
    Aint I right??? Or its just a misconception??
  2. jcsd
  3. Aug 28, 2005 #2


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    That's a misconception. The correct angular displacement would be approximately the speed of rotation/translation divided by the speed of light - which is negligible.
  4. Aug 28, 2005 #3
    why is it so???????????????? see I am still in lower classes and I dont know about this so kindly explain...
  5. Aug 28, 2005 #4


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    There are a couple of ways of thinking about this.

    First, imagine it's raining and the rain is falling straight down. You begin to walk, slowly at first. You will notice that the rain no longer appears to be falling straight down but is coming down at a slight angle. Now walk a little faster! You see the "slant" angle of the rain increase.

    The deviation from the vertical of the falling rain depends on both the speed at which the drops are falling and the speed at which you are walking or running. Whatever your speed, it is clear that the faster the drops are falling, the more nearly vertical their descent becomes. Imagine them coming down REALLY fast - say at the speed of light! The raindrops will appear to you as coming straight down which gives the "true" direction of the source.

    Another way of thinking about it is like this: Suppose the Earth is fixed and it is Sun that is moving through space at a speed equaling the actual sum of the Earth's speed in its orbit around the Sun plus its rotational velocity.

    The Sun would appear to be moving at something like 67,000 miles per hour. In 8 minutes or about 1/7 hour, the Sun will have moved almost 9,000 miles. That sounds like a lot but to find the angular deviation of the Sun from its "true" or "present" position, you have to compare that 9,000 miles with the Sun's distance, about 93 MILLION miles. That works out to about 0.0004 radians or about 0.006 degrees or about 20 seconds of arc!
    Last edited: Aug 28, 2005
  6. Aug 28, 2005 #5
    Thank you

    Thank you so much for clearing the doubt :smile:
  7. Aug 28, 2005 #6


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    I think one more factor; the bending of light due to the refraction of light through air is to be accounted. Isn’t it? Especially at morning and evening this affects a lot.
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