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    Re: NA "v" correction & mercury observations
    From: Jeremy C
    Date: 2011 Jan 17, 02:24 EST
    I think that it is worth checking if you are shooting right before the top of the hour and the rate of change for the Sun's altitude is large.  I ran some theoretical numbers and it can make as much as 0.4' of difference in intercept with Gary's example.
    It is also worth checking if you are attempting to determine Longitude at transit. 
    In a message dated 1/17/2011 5:55:14 A.M. Central Asia Standard Time, glapook@pacbell.net writes:
    As long as we are talking about "v" corrections, nobody ever mentions the "v" correction for the sun. The Nautical Almanac states that it fudges the tabulated GHA of the sun to minimize the effect caused by not providing a "v" correction for it. However, by doing this, you can end up with a 0.4' error in the GHA. For example, looking at the data for Christmas, December 25, 2011, the tabulated 1200 Z GHA of the sun is 00° 01.6' and at 1300 Z it is 15° 01.2'. Ignoring the "v" correction and just using the increment table for the same 60 minute period would have you adding exactly 15° to the 1200 Z GHA coming up with 15° 01.6' instead of the tabulated value, a difference of 0.4'. Looking at it a different way, for a 59 minute and 59 second period you would add 14° 59.8' producing a GHA at 1259:59 Z of 15° 01.4' but if you interpolate down from the nearer 1300 Z GHA for one second by subtracting 0.3' you find a GHA of 15° 00.9' a difference of 0.5'. So, it appears to me, that if you want the maximum possible accuracy for the GHA of the sun you should look at the consecutive tabulated GHAs and determine the appropriate "v" value and make the correction for it.



    --- On Sun, 1/16/11, Anabasis75@aol.com <Anabasis75@aol.com> wrote:

    From: Anabasis75@aol.com <Anabasis75@aol.com>
    Subject: [NavList] NA "v" correction & mercury observations
    To: NavList@fer3.com
    Date: Sunday, January 16, 2011, 4:06 AM

    You will find the nuance of my writing to cover that very topic Kermit.  That is why I said it is "applied" rather than "added."   "v" is nearly always positive, but sometimes is negative for the planets.  Venus at this very moment, has a negative "v" correction.   It is -0.3 for today's (Jan 16) bonus Exotica exercise (although due to it being only 8 minutes past the hour, it didn't affect GHA).
    For those who don't know, the "v" correction is used to save space in the NA.  If you notice in the Increments and corrections page, the column isn't labeled "Sun" it's "Sun and planets"  Since the planets move at a slightly different rate there is an additional correction to be applied to get the proper GHA.  The moon also has a "v" correction that allows for more accuracy because the table in the I&C pages is for a "standard moon" but since the moon's rate of change for GHA is variable, you need to apply a fairly sizable "v" correction that varies each hour. 
    Mercury LOP's and/or Azimuths were shot on January 7,8, & 9 this month.  I would have gotten one on the 10th but the horizon was terrible at 109 degrees so the planet disappeared before the horizon appeared even remotely distinct.  Peter's spreadsheets and my starpath calculator allowed me to actually get LOP's and correct for the azimuth as I shot them instead of having to ask for data as I did in 2009.
    I just hope that shooting Mercury doesn't make me mad as a hatter. ;-)
    In a message dated 1/15/2011 3:52:29 P.M. Central Asia Standard Time, antoine.m.couette@club-internet.fr writes:

    RE : http://www.fer3.com/arc/m2.aspx?i=115290
    [NavList] Re: help with sun sights
    From: Anabasis75---com
    Date: 15 Jan 2011 03:00

    Hello Jeremy,

    You wrote :


    " 5) The "v" correction is similar to the "d" correction but is applied to the GHA of the body. You must use the number at the bottom of the column for all non-lunar bodies. "


    May I comment as follows ?

    Yes ! And if I remember well, the "v" correction is ALWAYS POSITIVE for the Navigational Planets, with one (exotic!) exception : VENUS for which it can be negative.
    I would also guess that the same holds for Mercury too ... but limited interest here.

    Best Regards


    Antoine M. Couëtte

    My following question is a bit late, I know ... since Mercury was beautiful in the morning sky just one week ago (i.e. by the 08th of January 2011), but ...

    ... do you belong to the "Mercury Shooters Club" ? :-))

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