From mboxrd@z Thu Jan 1 00:00:00 1970 Path: news.gmane.org!.POSTED.blaine.gmane.org!not-for-mail From: Marcin Borkowski Newsgroups: gmane.emacs.help Subject: Re: Wrong times for sunrise/sunset? Date: Thu, 24 Jan 2019 18:04:36 +0100 Message-ID: <878sza7zrv.fsf@mbork.pl> References: <87pnssoeiz.fsf@mbork.pl> <20190119231504178820923@bob.proulx.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Injection-Info: blaine.gmane.org; posting-host="blaine.gmane.org:195.159.176.226"; logging-data="160415"; mail-complaints-to="usenet@blaine.gmane.org" User-Agent: mu4e 1.1.0; emacs 27.0.50 Cc: help-gnu-emacs@gnu.org To: Bob Proulx Original-X-From: help-gnu-emacs-bounces+geh-help-gnu-emacs=m.gmane.org@gnu.org Thu Jan 24 18:10:09 2019 Return-path: Envelope-to: geh-help-gnu-emacs@m.gmane.org Original-Received: from lists.gnu.org ([209.51.188.17]) by blaine.gmane.org with esmtps (TLS1.0:RSA_AES_256_CBC_SHA1:256) (Exim 4.89) (envelope-from ) id 1gmiW0-000fXt-9a for geh-help-gnu-emacs@m.gmane.org; Thu, 24 Jan 2019 18:10:08 +0100 Original-Received: from localhost ([127.0.0.1]:57179 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1gmiVy-0005Q9-A8 for geh-help-gnu-emacs@m.gmane.org; Thu, 24 Jan 2019 12:10:06 -0500 Original-Received: from eggs.gnu.org ([209.51.188.92]:58304) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1gmiRu-0003FD-2h for help-gnu-emacs@gnu.org; Thu, 24 Jan 2019 12:05:56 -0500 Original-Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1gmiRs-00051N-3K for help-gnu-emacs@gnu.org; Thu, 24 Jan 2019 12:05:53 -0500 Original-Received: from mail.mojserwer.eu ([195.110.48.8]:51152) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1gmiRo-0004ns-Cl for help-gnu-emacs@gnu.org; Thu, 24 Jan 2019 12:05:50 -0500 Original-Received: from localhost (localhost [127.0.0.1]) by mail.mojserwer.eu (Postfix) with ESMTP id 34EC8E6BE2; Thu, 24 Jan 2019 18:05:20 +0100 (CET) X-Virus-Scanned: Debian amavisd-new at mail.mojserwer.eu Original-Received: from mail.mojserwer.eu ([127.0.0.1]) by localhost (mail.mojserwer.eu [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id KL9B378vJVpu; Thu, 24 Jan 2019 18:05:13 +0100 (CET) Original-Received: from localhost (static-dwadziewiec-jedenpiec7.echostar.pl [109.232.29.157]) by mail.mojserwer.eu (Postfix) with ESMTPSA id BA816E6650; Thu, 24 Jan 2019 18:05:12 +0100 (CET) In-reply-to: <20190119231504178820923@bob.proulx.com> X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] X-Received-From: 195.110.48.8 X-BeenThere: help-gnu-emacs@gnu.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: Users list for the GNU Emacs text editor List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: help-gnu-emacs-bounces+geh-help-gnu-emacs=m.gmane.org@gnu.org Original-Sender: "help-gnu-emacs" Xref: news.gmane.org gmane.emacs.help:119150 Archived-At: On 2019-01-20, at 08:17, Bob Proulx wrote: >> Here are my settings: >> >> --8<---------------cut here---------------start------------->8--- >> (setq calendar-latitude 52.4) >> (setq calendar-longitude 16.917) >> (setq calendar-location-name "Pozna=C5=84, 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=E2=86=91 124=C2=B0 Southeast >> Sunset Today: 16:14=E2=86=91 237=C2=B0 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