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  1. #1
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    Question How fast does the sun set?

    Here's a question I was just asked... apparently my cousin has a bet on and I suspect he's going to lose.

    From the moment the sun first touches the horizon at sunset, how long does it take to fully set? (Horizon meaning, for this question, as seen at sea.) Apparently my cousin reckoned 1 1/2 minutes!

    Here in Vic it's almost impossible to go and actually measure, 'cos there's always land to the west. Bloody westerners, always getting in the way...
    I may be weird, but I'm saving up to become eccentric.

    - Andy Mc (AKA "Ghost who posts." )

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  3. #2
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    I think it would also depend on where you are as well

  4. #3
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    its fairly quick but i dont think that quick. it would vary slightly each day wouldn't it?
    S T I R L O

  5. #4
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    Skew - did a google and found this...


    The amount of time that passes between the moment when the bottom of the solar disk touches the horizon and when the top of the solar disk touches the horizon is not constant, but depends on your latitude, on the season, and on how quickly the conditions in the atmosphere change. We'll assume a quiet and unchanging atmosphere.
    The fastest sunset (or sunrise) at any given latitude occurs at the equinoxes (near 21 March and 23 September). The sunset then lasts approximately 128/cos phi seconds, where phi is the latitude. The slowest sunset occurs at the solstices (near 21 June and 21 December). For latitudes up to 60 degrees, the sunset then lasts approximately 142/cos(1.14 phi) seconds. For latitudes greater than 60 degrees, the length of the sunset rises steeply with latitude, until you get to the latitudes where there is a polar night or day, when the Sun doesn't rise or set at all for days or months. For arbitrary dates, there is no simple formula to calculate the length of sunset. The full procedure involves several of the formulas from the relevant Calculation Page.
    For example, at a latitude of 40 degrees (either North or South), the fastest sunset takes about 128/cos(40) = 167 seconds (2 minutes 47 seconds), and the slowest one about 142/cos(1.14*40) = 203 seconds (3 minutes 23 seconds). At a latitude of 50 degrees, the sunset lasts approximately between 199 and 261 seoonds (3 minutes 19 seconds and 4 minutes 21 seconds). At the equator, the sunset lasts between about 128 and 142 seconds (2 minutes 8 seconds and 2 minutes 22 seconds).
    The duration of sunset and sunrise is independent of the refraction by the atmosphere that slightly lifts up things near the horizon so that they appear to be higher in the sky than they would have done without any refraction, because you compare two instants of time when different parts of the Sun are at the same altitude in the sky. The same altitude means that the refraction is equally strong (except if the conditions of the atmosphere in that direction have changed in the meantime), so both instants are delayed by the same amount and their difference remains the same.


    Ian

  6. #5
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    Ya, about wot I wuz gunna say...


    The beatings will continue until morale improves.

  7. #6
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    Quote Originally Posted by scooter View Post
    Ya, about wot I wuz gunna say...
    What? or
    Cliff.
    ...if all you have is a hammer, everything looks like a nail...

  8. #7
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    Google's on my banned list... the redirection problem has reared it's ugly head again and I'm not interested in playing their game.
    I may be weird, but I'm saving up to become eccentric.

    - Andy Mc (AKA "Ghost who posts." )

  9. #8
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    Quote Originally Posted by Skew ChiDAMN!! View Post
    Google's on my banned list... ...I'm not interested in playing their game.
    So what do you do now?
    Ask here & wait for one of us to look it up for you.
    Cliff.
    ...if all you have is a hammer, everything looks like a nail...

  10. #9
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    Good guess! :biggrin:


    Now, for my next question...
    I may be weird, but I'm saving up to become eccentric.

    - Andy Mc (AKA "Ghost who posts." )

  11. #10
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    Quote Originally Posted by Ian Smith View Post
    Skew - did a google and found this...


    The amount of time that passes between the moment when the bottom of the solar disk touches the horizon and when the top of the solar disk touches the horizon is not constant, but depends on your latitude, on the season, and on how quickly the conditions in the atmosphere change. We'll assume a quiet and unchanging atmosphere.
    The fastest sunset (or sunrise) at any given latitude occurs at the equinoxes (near 21 March and 23 September). The sunset then lasts approximately 128/cos phi seconds, where phi is the latitude. The slowest sunset occurs at the solstices (near 21 June and 21 December). For latitudes up to 60 degrees, the sunset then lasts approximately 142/cos(1.14 phi) seconds. For latitudes greater than 60 degrees, the length of the sunset rises steeply with latitude, until you get to the latitudes where there is a polar night or day, when the Sun doesn't rise or set at all for days or months. For arbitrary dates, there is no simple formula to calculate the length of sunset. The full procedure involves several of the formulas from the relevant Calculation Page.
    For example, at a latitude of 40 degrees (either North or South), the fastest sunset takes about 128/cos(40) = 167 seconds (2 minutes 47 seconds), and the slowest one about 142/cos(1.14*40) = 203 seconds (3 minutes 23 seconds). At a latitude of 50 degrees, the sunset lasts approximately between 199 and 261 seoonds (3 minutes 19 seconds and 4 minutes 21 seconds). At the equator, the sunset lasts between about 128 and 142 seconds (2 minutes 8 seconds and 2 minutes 22 seconds).
    The duration of sunset and sunrise is independent of the refraction by the atmosphere that slightly lifts up things near the horizon so that they appear to be higher in the sky than they would have done without any refraction, because you compare two instants of time when different parts of the Sun are at the same altitude in the sky. The same altitude means that the refraction is equally strong (except if the conditions of the atmosphere in that direction have changed in the meantime), so both instants are delayed by the same amount and their difference remains the same.


    Ian
    What the????
    Speeka de England MAN??

    you said what??
    100% of all non-smokers die

  12. #11
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    geez you must be bored !! Get out to your sheds and do something worthwhile like build a sundial or something .
    It is very much quicker in the Philippines than here in Tasmania
    uhm , where am I ?

  13. #12
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    Vic, a relaxed few hours of sunset and pleasant fishing time, couple of hours of subtely reduced light levels, Queensland, about ten minutes.
    Stupidity kills. Absolute stupidity kills absolutely.

  14. #13
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    It looks like Ian has got it all nutted out for you, but I just thought I'd add this cool pic of a sunset from space, which reminds me that I'm still eagerly waiting for a teskelope that I bought of ebay a few days ago. I can't wait to do some stargazing. :biggrin:


  15. #14
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    John, have a look at http://www.heavens-above.com/ used this many times and find it is excellent.
    Stupidity kills. Absolute stupidity kills absolutely.

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