From mboxrd@z Thu Jan 1 00:00:00 1970 Path: news.gmane.org!.POSTED!not-for-mail From: Bob Proulx Newsgroups: gmane.emacs.help Subject: Re: Wrong times for sunrise/sunset? Date: Sun, 20 Jan 2019 00:17:23 -0700 Message-ID: <20190119231504178820923@bob.proulx.com> References: <87pnssoeiz.fsf@mbork.pl> NNTP-Posting-Host: ciao.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable X-Trace: ciao.gmane.org 1547968740 106084 195.159.176.228 (20 Jan 2019 07:19:00 GMT) X-Complaints-To: usenet@ciao.gmane.org NNTP-Posting-Date: Sun, 20 Jan 2019 07:19:00 +0000 (UTC) User-Agent: Mutt/1.10.1 (2018-07-13) To: help-gnu-emacs@gnu.org Original-X-From: help-gnu-emacs-bounces+geh-help-gnu-emacs=m.gmane.org@gnu.org Sun Jan 20 08:18:57 2019 Return-path: Envelope-to: geh-help-gnu-emacs@m.gmane.org Original-Received: from lists.gnu.org ([209.51.188.17]) by ciao.gmane.org with esmtps (TLS1.0:RSA_AES_256_CBC_SHA1:256) (Exim 4.84_2) (envelope-from ) id 1gl7Nh-000RaS-1G for geh-help-gnu-emacs@m.gmane.org; Sun, 20 Jan 2019 08:18:57 +0100 Original-Received: from localhost ([127.0.0.1]:36542 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1gl7Np-0006j9-Tj for geh-help-gnu-emacs@m.gmane.org; Sun, 20 Jan 2019 02:19:05 -0500 Original-Received: from eggs.gnu.org ([209.51.188.92]:39669) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1gl7MH-00069A-Ur for help-gnu-emacs@gnu.org; Sun, 20 Jan 2019 02:17:31 -0500 Original-Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1gl7MF-0006rr-QS for help-gnu-emacs@gnu.org; Sun, 20 Jan 2019 02:17:29 -0500 Original-Received: from havoc.proulx.com ([96.88.95.61]:59157) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1gl7MF-0006qq-EI for help-gnu-emacs@gnu.org; Sun, 20 Jan 2019 02:17:27 -0500 Original-Received: from joseki.proulx.com (localhost [127.0.0.1]) by havoc.proulx.com (Postfix) with ESMTP id 40BD82A3C for ; Sun, 20 Jan 2019 00:17:24 -0700 (MST) Original-Received: from hysteria.proulx.com (hysteria.proulx.com [192.168.230.119]) by joseki.proulx.com (Postfix) with ESMTP id 0709721244 for ; Sun, 20 Jan 2019 00:17:24 -0700 (MST) Original-Received: by hysteria.proulx.com (Postfix, from userid 1000) id E10D32DC7C; Sun, 20 Jan 2019 00:17:23 -0700 (MST) Mail-Followup-To: help-gnu-emacs@gnu.org Content-Disposition: inline In-Reply-To: <87pnssoeiz.fsf@mbork.pl> X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] X-Received-From: 96.88.95.61 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:119131 Archived-At: Marcin Borkowski wrote: > C-u M-x sunrise-sunset (and today's date) says: >=20 > --8<---------------cut here---------------start------------->8--- > Sat, Jan 19, 2019: Sunrise 7:54am (CET), sunset 4:13pm (CET) at Pozna=C5= =84, > 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: >=20 > --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? > (BTW, if anyone is ever near here, please drop me a line - we might be > able to meet in person;-)). >=20 > And C-u M-: calendar-time-zone says (correctly) 60. >=20 > However, https://www.timeanddate.com/astronomy/poland/poznan (for today= ) > says: >=20 > --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. > I also noticed that other online services give yet other results. >=20 > 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