Re: Wrong times for sunrise/sunset?

[ken]

On 1/19/19 10:25 AM, Marcin Borkowski wrote:
> Hello,
>
> C-u M-x sunrise-sunset (and today's date) says:
>
> --8<---------------cut here---------------start------------->8---
> Sat, Jan 19, 2019: Sunrise 7:54am (CET), sunset 4:13pm (CET) at Poznań,
> Poland (8:19 hrs daylight)
> --8<---------------cut here---------------end--------------->8---
>
> 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---
>
> (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 also noticed that other online services give yet other results.
>
> Anybody knows why the difference(s)?
>
> TIA,
>
> --
> Marcin Borkowski
> http://mbork.pl

Reading on sunrise-sunset a long time ago, I was given to understand
that, while the calculations are unambiguous, the perception of those
times is open to interpretation.  Especially when the sun (and other
astronomical objects) is low to the horizon, the Earth's atmosphere acts
as a lens, enabling us to see the sun (etc.) even before and after it is
in our straight line-of-sight.  That is, the atmosphere's lensing effect
allows us to see a little bit over the horizon.  The calculations
performed by emacs do not compensate for the atmospheric magic.  But
other sources, rather than being accused of inaccuracy by the public who
might look out the window, typically amend the purely physical reckoning
to accommodate earthly perceptions.

hth,
ken