Wrong times for sunrise/sunset?

Wrong times for sunrise/sunset?

[Marcin Borkowski]

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

Re: Wrong times for sunrise/sunset?

[Pierpaolo Bernardi]

Il giorno 19 gennaio 2019, alle ore 16:27, Marcin Borkowski <mbork@mbork.pl> ha scritto:

>Hello,
>C-u M-x sunrise-sunset (and today's date) says

>I also noticed that other online services give yet other results.
>Anybody knows why the difference(s)?

See https://lists.gnu.org/archive/html/help-gnu-emacs/2016-07/msg00321.html

Hth

Re: Wrong times for sunrise/sunset?

[Marcin Borkowski]

On 2019-01-19, at 20:17, Pierpaolo Bernardi <olopierpa@gmail.com> wrote:

> Il giorno 19 gennaio 2019, alle ore 16:27, Marcin Borkowski <mbork@mbork.pl> ha scritto:
>
>>Hello,
>>C-u M-x sunrise-sunset (and today's date) says
>
>>I also noticed that other online services give yet other results.
>>Anybody knows why the difference(s)?
>
> See https://lists.gnu.org/archive/html/help-gnu-emacs/2016-07/msg00321.html
>
> Hth

Yes, that helps, thanks.

Out of curiosity - Michael Heerdegen writes there:

--8<---------------cut here---------------start------------->8---
AFAIK the Emacs calculation is exact - it respects all astronomical
effects I know of.  But it doesn't try hard to round to whole minutes
correctly, so the result may differ from the actual event by an epsilon,
where epsilon < 1 min I think.
--8<---------------cut here---------------end--------------->8---

But the differences I can see are a few minutes.  Why, after takign care
of all astronomical effects, is "rounding to whole minutes correctly"
even an issue?

(I understand that I know very little abot time calculations and such,
and I understand they are difficult.  I'd just like to know.)

Best,

--
Marcin Borkowski
http://mbork.pl

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

Re: Wrong times for sunrise/sunset?

[Marcin Borkowski]

On 2019-01-20, at 06:52, ken <gebser@mousecar.com> wrote:

> 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.

Thanks, that would explain a lot.

Best,

-- 
Marcin Borkowski
http://mbork.pl

Re: Wrong times for sunrise/sunset?

[Bob Proulx]

Marcin Borkowski wrote:
> 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---

I am a little further south and get 9:40 of daylight.

> 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?

> (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.

> 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

Re: Wrong times for sunrise/sunset?

[Óscar Fuentes]

Gmane ate my reply, so reposting by e-mail...

Marcin Borkowski <mbork@mbork.pl> writes:

> C-u M-x sunrise-sunset (and today's date) says:
>
> Sat, Jan 19, 2019: Sunrise 7:54am (CET), sunset 4:13pm (CET) at Poznań,
> Poland (8:19 hrs daylight)
>
>
> Here are my settings:
>
> (setq calendar-latitude 52.4)
> (setq calendar-longitude 16.917)
> (setq calendar-location-name "Poznań, Poland")
>
>
> (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:
>
> Sunrise Today: 07:51↑ 124° Southeast
> Sunset Today: 16:14↑ 237° Southwest
>
> I also noticed that other online services give yet other results.
>
> Anybody knows why the difference(s)?

One possible reason is refraction. A cursory look at solar.el gives the
impression that refraction is not considered, but the astronomy website
you referenced does. This can cause a difference of several minutes.

Another possible reason is not using the same geographical point. 20
kilometers to the west or east can result in minutes of difference.

However, the difference you observe in sunrise (3 minutes) is quite
larger than the diference in sunset (1 minute). At this time of year,
this can mean that your position and the position used by the website
differ on the N/S axis.

Just guessing.

Re: Wrong times for sunrise/sunset?

[Marcin Borkowski]

On 2019-01-20, at 16:00, Óscar Fuentes <ofv@wanadoo.es> wrote:

> One possible reason is refraction. A cursory look at solar.el gives the
> impression that refraction is not considered, but the astronomy website
> you referenced does. This can cause a difference of several minutes.
>
> Another possible reason is not using the same geographical point. 20
> kilometers to the west or east can result in minutes of difference.
>
> However, the difference you observe in sunrise (3 minutes) is quite
> larger than the diference in sunset (1 minute). At this time of year,
> this can mean that your position and the position used by the website
> differ on the N/S axis.
>
> Just guessing.

Thanks, one learns something every day!

Best,

-- 
Marcin Borkowski
http://mbork.pl

Re: Wrong times for sunrise/sunset?

[Marcin Borkowski]

On 2019-01-20, at 08:17, Bob Proulx <bob@proulx.com> 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

Re: Wrong times for sunrise/sunset?

[Van L]

> (I understand that I know very little abot time calculations and such, 
> and I understand they are difficult. I'd just like to know.)

Metwo.

Can those calculations tell the diameter of the Sun? without; observation.


     In his astronomical writings, Proclus described how the
     water clock invented by Heron could be used to measure the
     apparent diameter of the Sun. Proclus's method can be used
     at the equinox. Water is collected from the clock in a
     container while the sun rises. As soon as the Sun has risen
     the water is collected in another container and this
     measurement continues until sunrise the following day. Then
     the ratio of the weights of water in the two containers
     gives the apparent diameter of the Sun. [1]



Footnotes
─────────

[1] The link is:
<http://www-history.mcs.st-and.ac.uk/Biographies/Proclus.html>