NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Meridian Transit of the Sun and Daily Pages
From: Stan K
Date: 2015 Oct 11, 16:00 -0400
From: Stan K
Date: 2015 Oct 11, 16:00 -0400
Peter,
Don't forget that the Nautical Almanac adjusts the Sun's GHA "by up to 0.15' to reduce the error due to ignoring the v-correction." This probably makes very little difference in that calculation of the time of LAN, but you will find that the hourly GHA of the Sun listed on the daily pages will disagree with a calculation, such as with sun.xls, by as much as 0.2' about half the time.
In Celestial Tools I have two modes for the Sun. In the "Accurate" mode, the hourly GHA almost always agrees closely with sources other than the Almanac. In the "Nautical Almanac" mode, the hourly GHA agrees with the Nautical Almanac almost 99% of the time, and when it does not agree, it is only off by 0.1'. (With the help of a friend, this was tested for all 8760 hours of 2014.) This was done for the sake of Power Squadron students, whose "bible" is the Nautical Almanac, but if you are looking for better accuracy, as most people would, the "Accurate" mode is the way to go. Or, of course, Navigation Spreadsheets.
For example, for 0h UT on 1 January 2015, the values of GHA of the Sun are given for several sources:
USNO web site 179º12.2'
Nautical Almanac 179º12.0'
Reed Navigation 179º12.1'
sun.xls 179º12.1'
Navigator Lite 179º12.2'
Nautical Almanac (Ruiz) 179º12.2'
Celestial Tools (Accurate) 179º12.2'
Celestial Tools (NA) 179º12.0'
Other sources have values of 179º12.22', 179º12.17', 179º12.169' (which all round to 179º12.2'), 179º12.2', and 179º12.1'. Rounded to 0.1', 11 of the 14 sources I checked say 179º12.2'.
Bottom line is, unless you are doing a textbook problem, any of these values is good enough.
Stan
-----Original Message-----
From: Peter Hakel <NoReply_PeterHakel@fer3.com>
To: slk1000 <slk1000@aol.com>
Sent: Sun, Oct 11, 2015 12:42 pm
Subject: [NavList] Re: Meridian Transit of the Sun and Daily Pages
From: Peter Hakel <NoReply_PeterHakel@fer3.com>
To: slk1000 <slk1000@aol.com>
Sent: Sun, Oct 11, 2015 12:42 pm
Subject: [NavList] Re: Meridian Transit of the Sun and Daily Pages
Steve,
My sun.xls spreadsheet is based on the VSOP87 planetary theory described in Jean Meeus’s book Astronomical Algorithms. I have been testing it against the Commercial Edition almanac every year, getting excellent agreement. The spreadsheets take input (in this case, UT) in the green cells on the top and the results are shown in the cyan cells below. The following demonstration video (made in 2009):
https://www.youtube.com/watch?v=CyHVqqVJQH0
shows the usage (for Moon, but others work the same way). In contrast with the video, the process is much faster on a desktop computer or a laptop with a hardware keyboard. For your application you need to iterate on UT - that is, you keep changing the “green” input until the calculated GHA matches your longitude.
Peter Hakel
From: Steve E. Bryant <NoReply_Bryant@fer3.com>
To: pmh099---.com
Sent: Sunday, October 11, 2015 12:42 AM
Subject: [NavList] Re: Meridian Transit of the Sun and Daily Pages
My sun.xls spreadsheet is based on the VSOP87 planetary theory described in Jean Meeus’s book Astronomical Algorithms. I have been testing it against the Commercial Edition almanac every year, getting excellent agreement. The spreadsheets take input (in this case, UT) in the green cells on the top and the results are shown in the cyan cells below. The following demonstration video (made in 2009):
https://www.youtube.com/watch?v=CyHVqqVJQH0
shows the usage (for Moon, but others work the same way). In contrast with the video, the process is much faster on a desktop computer or a laptop with a hardware keyboard. For your application you need to iterate on UT - that is, you keep changing the “green” input until the calculated GHA matches your longitude.
Peter Hakel
From: Steve E. Bryant <NoReply_Bryant@fer3.com>
To: pmh099---.com
Sent: Sunday, October 11, 2015 12:42 AM
Subject: [NavList] Re: Meridian Transit of the Sun and Daily Pages
Dear Frank, First let me say that I greatly appreciate the responses to my question. I stand in awe of the experience, education, and knowledge represented by the Navlist contributors. As I indicated to you over a year ago, I’ve been afraid to ask any of my elementary questions given the complexity of the questions usually asked on the Navlist for fear that I would not be capable of understanding the answers. Regarding your question, I have decided to be one of the instructors for our United States Power Squadron’s Junior Navigation class; more precisely, I have been permitted to be one of three instructors. I was a student in this class back in 2009 and thoroughly enjoyed the experience. I am familiar with the typical methodology for calculating the meridian transit; but, I did needed a memory jog for the method utilizing the rough but helpful daily pages ‘hint’ located on the far right, bottom of the daily pages. The reality for our class is that our sights require time to the nearest second. I note that Peter Hakel has offered a helpful Excel tool for solving the problem quickly but I don’t know how to make it work? I also am curious what the source is for the ephemerides used in the Excel program. The “Navigators” in our National Office who will be grading the students ‘sight book’ tell us that their evaluation of the handwritten sight reductions will require that the data be taken from the Nautical Almanac as copyrighted with the United Kingdom Hydrographic Office i.e., The Nautical Almanac(s) 2015-2016. Best regards, Steve PS, We did actually take sights on 27 September. -----Original Message----- From: NavList@fer3.com [mailto:NavList@fer3.com] On Behalf Of Frank Reed Sent: Saturday, October 10, 2015 3:52 PM To: steveebryant---.net Subject: [NavList] Re: Meridian Transit of the Sun and Daily Pages Hey Steve, how about a little context? Why are you using the *Nautical Almanac* for this task? As an exercise, a textbook puzzle (nothing wrong with that, if that's the reason)? If you really need to know when local noon occurred on that date for some practical reason, then why? The application helps decide the approach. There are many tools well-suited to this task that are far easier to use. And what kind of accuracy do you need? Nearest minute? Nearest second? Again *why*? What's your goal? Frank Reed [PLAIN TEXT VERSION OF MESSAGE AUTO-GENERATED. ORIGINAL MAY INCLUDE MORE CONTENT] ---------------------------------------------------------------- NavList message boards and member settings: www.fer3.com/NavList Members may optionally receive posts by email. To cancel email delivery, send a message to NoMail[at]fer3.com ---------------------------------------------------------------- : http://fer3.com/arc/m2.aspx/Meridian-Transit-Sun-Daily-Pages-FrankReed-oct-2015-g33023
