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Thread: sun set

  1. #1
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    sun set

    When does the Sun set? Not the time as that varies all over the world, but when does the sun actually set, when it touches the horizon or when it disappears from view? The obvious answer is when it disappears but is that correct?

  2. #2
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    Yes, thatís correct. The time is the moment when the trailing edge goes below the horizon, so when the sun disappears.


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    As above, so below

  3. #3
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    Here is what the US Naval Observatory has to say about it.

    Sunrise and sunset conventionally refer to the times when the upper edge of the disk of the Sun is on the horizon. Atmospheric conditions are assumed to be average, and the location is in a level region on the Earth's surface.
    Technical Definitions and Computational Details
    Sunrise and sunset. For computational purposes, sunrise or sunset is defined to occur when the geometric zenith distance of center of the Sun is 90.8333 degrees. That is, the center of the Sun is geometrically 50 arcminutes below a horizontal plane. For an observer at sea level with a level, unobstructed horizon, under average atmospheric conditions, the upper limb of the Sun will then appear to be tangent to the horizon. The 50-arcminute geometric depression of the Sun's center used for the computations is obtained by adding the average apparent radius of the Sun (16 arcminutes) to the average amount of atmospheric refraction at the horizon (34 arcminutes).
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  4. #4
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    Notice, that 34 arcminutes due to average amount of atmospheric refraction is wider than the *diameter* of the sun itself.

  5. #5
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    Variations in atmospheric conditions along the light path can alter the refraction enough to make a (to me) surprising difference in the observed sunset time, so calculating and tabulating it to better than a couple of minutes doesn't improve the match to what one observes, even with a clear horizon free of clouds and haze.

  6. #6
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    The Novaya Zemlya effect can lift the sun into view from five degrees below the horizon, although its appearance is strongly distorted. That gave Willem Barentsz's expedition a polar "sunrise" that came two weeks early.

    Grant Hutchison

  7. #7
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    Quote Originally Posted by grapes View Post
    Notice, that 34 arcminutes due to average amount of atmospheric refraction is wider than the *diameter* of the sun itself.
    So atmospheric refraction does this:

    sunsetcaribbean2.jpg

    Grant Hutchison

  8. #8
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    Quote Originally Posted by ngc3314 View Post
    Variations in atmospheric conditions along the light path can alter the refraction enough to make a (to me) surprising difference in the observed sunset time, so calculating and tabulating it to better than a couple of minutes doesn't improve the match to what one observes, even with a clear horizon free of clouds and haze.
    Doesn't that high value seem a bit hinky? The 1.678 deg. is a one-off from all the other 116 sunsets. Voclano eruption? I thought it might be a typo but the time difference is over 4 minutes later than either the day before of after. The simple average is ~ 0.54 deg.
    We know time flies, we just can't see its wings.

  9. #9
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    Thanks then QI was wrong

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    Quote Originally Posted by speach View Post
    Thanks then QI was wrong
    What did they say?

    Grant Hutchison

  11. #11
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    What is QI?

  12. #12
    It is comedy and trivia show combined, you can enter but may never come out.
    https://youtu.be/56kwrIauOB8
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  13. #13
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    QI stated that the sun set as the lower limb touched the horizon

  14. #14
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    Quote Originally Posted by speach View Post
    QI stated that the sun set as the lower limb touched the horizon
    Maybe they were reporting from Australia.

    As above, so below

  15. #15
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    Quote Originally Posted by speach View Post
    QI stated that the sun set as the lower limb touched the horizon
    Was the question multiple choice? How was it phrased?

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    The show actually asks panelists to state when the sun is 'below the horizon'. It does not ask for the formal definition of sunset. The moderator explains that because of atmospheric refraction, the point when you see the sun touch the horizon is about when the sun is actually just moving below the horizon. He explains that the amount of refraction is approximately equal to the diameter of the sun. Whether that's correct or a good approximation, I don't know. But the question wasn't 'when does the sun set?'

    https://www.youtube.com/watch?v=vvmq66op0G8

  17. #17
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    Quote Originally Posted by geonuc View Post
    The show actually asks panelists to state when the sun is 'below the horizon'. It does not ask for the formal definition of sunset. The moderator explains that because of atmospheric refraction, the point when you see the sun touch the horizon is about when the sun is actually just moving below the horizon. He explains that the amount of refraction is approximately equal to the diameter of the sun. Whether that's correct or a good approximation, I don't know. But the question wasn't 'when does the sun set?'

    https://www.youtube.com/watch?v=vvmq66op0G8
    Yes, I wondered if that was the episode speach was referencing. QI got it right, because refraction at the horizon lifts the sun by slightly more than its own diameter. That's what my diagram earlier in the thread shows.
    Quote Originally Posted by grant hutchison View Post
    So atmospheric refraction does this:

    sunsetcaribbean2.jpg
    The circle is the sun's true position below the horizon, with the effects of refraction indicated.

    Grant Hutchison

  18. #18
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    Quote Originally Posted by grant hutchison View Post
    That's what my diagram earlier in the thread shows.
    Yes, just so. Apologies - when I stated I didn't know, I should have referred to your earlier post.

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