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How long does it take the sun to set? |
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| mckeonj
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| 218456. Mon Oct 08, 2007 6:58 pm |
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This question was prompted by the 'How long is a day?' thread, q.v.
A simple, easy question, yes, but the answer could be long and complicated.
How long does it take the sun to set?
That is, from the time that the bottom rim touches sea level, to the time that the top rim disappears.
The common sense answer for these latitudes, that is the British Islands, is about two minutes.
Can you be more precise? I doubt it; the only precise data available is that the value lies between 0 and ∞.
Have fun.
_________________
John McKeon |
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| djgordy
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| 218459. Mon Oct 08, 2007 7:04 pm |
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| Quote: | 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 φ seconds, where φ 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 φ) 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 seconds (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. |
http://www.astro.uu.nl/~strous/AA/en/antwoorden/zonpositie.html#v240 |
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