NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: A simple question, I hope, on almanac SHA
From: Stan K
Date: 2013 Apr 13, 04:54 -0400
From: Stan K
Date: 2013 Apr 13, 04:54 -0400
Well, I found a source, but it should probably be considered second-hand or hearsay>
In the current edition of the Power Squadrons N Student Manual, referring to a piece of software that is used in the course, it says "...because the SHA is calculated for the entered day and time, rather than for 0000 on the middle day".
That just doesn't feel right to me, although it does agree with what Bowditch says about the SHA and meridian passage of the planets shown on the left-hand daily pages of the Almanac. Why 1/3 of the way into the three days rather than half way? I will have to inquire.
Stan
In the current edition of the Power Squadrons N Student Manual, referring to a piece of software that is used in the course, it says "...because the SHA is calculated for the entered day and time, rather than for 0000 on the middle day".
That just doesn't feel right to me, although it does agree with what Bowditch says about the SHA and meridian passage of the planets shown on the left-hand daily pages of the Almanac. Why 1/3 of the way into the three days rather than half way? I will have to inquire.
Stan
-----Original Message-----
From: Frank Reed <FrankReed@HistoricalAtlas.com>
To: slk1000 <slk1000@aol.com>
Sent: Fri, Apr 12, 2013 1:28 pm
Subject: [NavList] Re: A simple question, I hope, on almanac SHA
----------------------------------------------------------------
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
----------------------------------------------------------------
From: Frank Reed <FrankReed@HistoricalAtlas.com>
To: slk1000 <slk1000@aol.com>
Sent: Fri, Apr 12, 2013 1:28 pm
Subject: [NavList] Re: A simple question, I hope, on almanac SHA
Hi Stan,
I was giving someone else a hard-time recently about non-descriptive subjects, so I've modified this subject to provide just a hint of what your question was about. I hope you don't mind! :)
You wrote:
"The SHA of stars changes very slowly, so the Nautical Almanac uses a single value for the three days of a daily page. I would have to believe that this is the value for 1200 UT of the middle day (rounded to 0.1'), but I have not been able to find this, or something contrary, documented anywhere."
"The SHA of stars changes very slowly, so the Nautical Almanac uses a single value for the three days of a daily page. I would have to believe that this is the value for 1200 UT of the middle day (rounded to 0.1'), but I have not been able to find this, or something contrary, documented anywhere."
I would guess you're right, too. I was having dinner here on the island last night with Herbert Prinz (delicious seafood at Jamestown Fish), and a text message came in during dessert with your message so I offered it up for discussion. We spent hours talking about navigation and astronomy and NavList, so SHA trivia fit right in! We agreed that your suggestion is almost certainly correct. HP also pointed out that, of course, this is documented somewhere in technical publications on the Nautical Almanac, but good luck digging it up. I suggested that you could determine this by re-calculation. Find as many cases as you can where the SHA changes by 0.1 minutes from one page to the next. Then find some source that calculates SHA's for every hour (my site does) and as long as they're using nearly identical input data and nearly identical precession and nutation models, you should be able to find cases where the SHA flips its tenths digit on just the right date. Here's the link for my web-based software that does this: http://reednavigation.com/lunars/nadata_v5.html. Other NavList members, including Paul Hirose, have similar tools available.
-FER
----------------------------------------------------------------
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
----------------------------------------------------------------
