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Re: Wrong times for sunrise/sunset?


From: Marcin Borkowski
Subject: Re: Wrong times for sunrise/sunset?
Date: Thu, 24 Jan 2019 18:04:36 +0100
User-agent: mu4e 1.1.0; emacs 27.0.50

On 2019-01-20, at 08:17, Bob Proulx <address@hidden> wrote:

>> Here are my settings:
>>
>> --8<---------------cut here---------------start------------->8---
>> (setq calendar-latitude 52.4)
>> (setq calendar-longitude 16.917)
>> (setq calendar-location-name "Poznań, Poland")
>> --8<---------------cut here---------------end--------------->8---
>
> How did you decide upon that longitude and latitude?

Wikipedia.

>> (BTW, if anyone is ever near here, please drop me a line - we might be
>> able to meet in person;-)).
>>
>> And C-u M-: calendar-time-zone says (correctly) 60.
>>
>> However, https://www.timeanddate.com/astronomy/poland/poznan (for today)
>> says:
>>
>> --8<---------------cut here---------------start------------->8---
>> Sunrise Today: 07:51↑ 124° Southeast
>> Sunset Today: 16:14↑ 237° Southwest
>> --8<---------------cut here---------------end--------------->8---
>
> I looked but could not see that page listing a longitude and latitude
> for that location.

Me neither...

>> I also noticed that other online services give yet other results.
>>
>> Anybody knows why the difference(s)?
>
> I do not know but I will guess.  Here are some ideas.
>
> The longitude and latitude of the two calculations were different
> enough to produce that difference.  If the two locations are not
> identical then the calculations will produce a different result.
>
> The times given were to the nearest minute.  Errors due to rounding or
> truncation may cause them to be closer together or further apart in
> result.
>
> The models used to calculate sunrise and sunset may be different
> between the two methods.  I didn't investigate but there are different
> approximations for the non-spherical shape of the earth.  The planet
> is somewhat pear shaped.
>
> If the two methods were different then they would produce slightly
> different results.  I would trust a calculation based upon the
> https://en.wikipedia.org/wiki/Nautical_almanac ( now known as The
> Astronomical Almanac ) for your location and altitude as
> authoritative.  (It has been a while since I have done the
> calculations myself however.  I would need a refresher.)  If that were
> known then we would know which model was more accurate for your
> location.
>
> Do you know if timeanddate.com uses civil twilight?  Or nautical?  Or
> astronomical?  Wikipedia has a good graphic for the differences.
>
>   https://en.wikipedia.org/wiki/File:Twilight_subcategories.svg
>
> One might wonder, what's the difference between them?  I am
> undoubtedly going to describe this wrong.  But hopefully it will be
> good enough to explain the concepts.
>
> If the upper limb of the sun (the upper limb is the top of the circle
> that is the sun) is below the horizon between 0 and 6 degress then it
> is civil twilight.  However it is still quite bright out due to
> refraction of the sun due to the atmosphere.  It is that refraction
> that requires the sun to be below the horizon 6 degrees before it is
> lost from visibility.  It is still too bright to see stars.  That
> makes it too bright for navigation by star sights.  But the sun is
> below the horizon.
>
> If the upper limb is between 6 and 12 degrees below the horizon, a
> number that as far as I have been able to determine is a practical
> number from observation and usefulness and not from any intrinsic
> constant of the universe, then it is dark enough that the bright stars
> are visible and also the horizon.  There are 57 bright stars typically
> used for celestial navigation and the brightest of those are visible
> when the sun is below 6 degrees of the horizon.  And it is also bright
> enough to see a clear horizon in order to observe by sextant the angle
> of the star above the horizon.  This is nautical twilight and is the
> period of time when celestial navigation by star sights are taken.
> When one can see both the horizon and one of the bright 57
> navigational stars in order to observe their altitudes.
>
> If the upper limb is more than 18 degrees below the horizon then it is
> too dark to see the horizon line.  It is not possible to observe by
> sextant the altitude angle of a celestial object.  But that is when
> the dim celestial objects can be observed.  Astronomers need the sun
> to have set or not yet risen in order to have good "seeing".
>
> And so we see that even a seemingly simple thing as defining sunset
> depends upon what we need to know it for!  Do we need to know if the
> car driver should have lights on?  Or if we need to take star sights
> with a sextant?  Or if we are going to be able to see interesting
> astronomical objects with a telescope?
>
> I will guess the difference is due to some combination of the above
> along with the possibilty of it being something else. :-)
>
> Bob
>
> P.S. Trivial: Ask random people what is the most important navigation
> star and most will pick Polaris the North Star.  However that is not
> one of the 57 bright stars usually used for celestial navigation.  It
> is not the brightest of stars.  Also in equatorial latitudes it is
> hard to see low stars through the haze.  It might not be possible to
> observe it during nautical twilight.  It isn't visible in the southern
> hemisphere at all.  While the North Star is by its position a useful
> star it arguably is not "the most important" by a lot.  Yet it has the
> best marketing department! :-)
> https://en.wikipedia.org/wiki/List_of_selected_stars_for_navigation

Thanks Bob, that was absolutely fascinating, even though I'm not into
astronomy at all!

Best,

--
Marcin Borkowski
http://mbork.pl



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