NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: V-Correction for Sun?
From: Stan K
Date: 2016 Sep 23, 19:16 -0400
From: Stan K
Date: 2016 Sep 23, 19:16 -0400
Axcel.
Take a look at paragraph 24 on page 261 of the Nautical Almanac. (This has been discussed on NavList in the past, with several suggestions as to what the Nautical Almanac should do rather than adjust the hourly GHA value.)
If you compare the hourly GHA of the Sun with other sources like the USNO web site or on-line almanacs, you will find that the Nautical Almanac is in disagreement by as much as 0.2' for about half of the 8760 hours in a (non-leap) year.
(FYI, my program, Celestial Tools, has two modes for the Sun. In the "Accurate val. of hourly GHA" mode, it will generally agree with sources other than the Nautical Almanac. In the "NA val. of hourly GHA" mode, it agrees with the Nautical Almanac for almost 99% of the 8760 hours, and when it differs, it is only by 0.1'.)
Stan
-----Original Message-----
From: Axcel B. <NoReply_AxcelB.@fer3.com>
To: slk1000 <slk1000@aol.com>
Sent: Fri, Sep 23, 2016 7:00 pm
Subject: [NavList] V-Correction for Sun?
From: Axcel B. <NoReply_AxcelB.@fer3.com>
To: slk1000 <slk1000@aol.com>
Sent: Fri, Sep 23, 2016 7:00 pm
Subject: [NavList] V-Correction for Sun?
Hello everybody,
I have a question and don't know if this is actually the right forum but since I don't know where else to post this question, I am going to try it here on NavList:
I was wondering why the Nautical Almanac does not use a v-Correction for the Sun. The Sun's average motion is obviously 15°/h but the True Sun's rate varies slightly throughout the year. So does Venus for instance (and can even move "backwards" with respect to the Sun) but with Venus we use a v-Correction. Why not with the Sun? The position of the True Sun is accurately given for every full hour in the Almanac but when using the Increments we are always assuming a speed of 15°/h which most of the time is not the actual speed of the True Sun. I was comparing GHA values between two consecutevie hours and found that the difference can be up to 0.3' (by applying an Increment of 60min to the GHA of the full hour and compare it with the tabulated GHA value of the consecutive hour). This means that one could get an error/difference in the Sun's position of 18 seconds - isn't it? Is this generally acceptable for purposes of Celestial Navigation and that's why we don't use a v-correction for the Sun?
Thanks
Axcel
Axcel
