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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 May 1, 21:06 -0400
From: Stan K
Date: 2013 May 1, 21:06 -0400
I heard back from Catherine Hohenkerk. Here is the significant sentence from her e-mail, which discussed some other items:
" For your information, however, you are correct the SHA & Dec of the stars is for 12h (UT1) on the middle day of each page."
We also discussed why 00h on the middle day was chosen for the SHA and Meridian Passage for the planets, but she could not come up with a definitive answer.
Stan
" For your information, however, you are correct the SHA & Dec of the stars is for 12h (UT1) on the middle day of each page."
We also discussed why 00h on the middle day was chosen for the SHA and Meridian Passage for the planets, but she could not come up with a definitive answer.
Stan
-----Original Message-----
From: Herbert Prinz <herbert@heavenlysoftware.org>
To: slk1000 <slk1000@aol.com>
Sent: Sun, Apr 21, 2013 4:08 am
Subject: [NavList] Re: A simple question, I hope, on almanac SHA
From: Herbert Prinz <herbert@heavenlysoftware.org>
To: slk1000 <slk1000@aol.com>
Sent: Sun, Apr 21, 2013 4:08 am
Subject: [NavList] Re: A simple question, I hope, on almanac SHA
After a few spot checks on two stars I convinced myself that the chosen epoch for the tabulated SHA is noon of the middle day. Theoretically, the value given could also be the average found by integration over the entire 3 day interval. The latter would be the perfectly correct method as it minimizes the average look up error, but I doubt that it has been used. It would be overkill. There might not be a single day where it makes a difference for any star. Consequently, statistics cannot decide whether the method was applied.
My spot checks refute the hypothesis that the epoch is 00:00 of the second day. In other words, I found counterexamples.
The analysis is based on a critical table of SHA values generated with MICA 2.0. The current version is apparently 2.2.2. I could not find out from the USNO website what the updates are and whether they are crucial to the validity of my test.
Does anyone know what the updates are? Do they affect the ephemeris data?
Herbert Prinz
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