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    Re: lat/long from meridian passage
    From: Gary LaPook
    Date: 2011 Jan 23, 20:55 -0800
    Well this is embarrassing, I should have proofread my message!

    I wrote:
    " so your noon would have been 12:16:56 EST, 17:16:29 Z. "


    Obviously 12:16:29 EST is 17:16:56 Z not :29!So the time of his noon should have been 3 seconds later to account for the change in the equation of time making it 17:16:59 Z, not the 17:16:32 Z I had written.

    Another way to calculate the time of local noon is to find the time when the GHA of the sun equals you longitude, in this case 76° 21.7' west. At 1700 Z the sun's GHA was 72° 36.8' which subtracted from the longitude leaves a difference of 4° 14.9'. We then look in the increments table in the NA to find out how long it takes for the sun's GHA to change by this amount and we find that it is between 16 minutes and 59 seconds and 17 minutes. We then add this extra time to 1700 Z to find the time on the meridian of 17:16:59.5, not surprisingly, the same result as when doing the computation using the equation of time.


    --- On Sun, 1/23/11, Gary LaPook <glapook@pacbell.net> wrote:

    From: Gary LaPook <glapook@pacbell.net>
    Subject: [NavList] Re: lat/long from meridian passage
    To: NavList@fer3.com
    Date: Sunday, January 23, 2011, 4:41 PM

    Just looking at the equation of time issue.

    The NA gives it to the second. It is added or subtracted to 12:00 to find the time of Local Apparent Noon. So you do not add it to the time of meridian passage as it has already been added to 12:00 to derive the time of meridian passage. The NA gives the time of meridian passage at the Greenwich Meridian rounded to the whole minute only but the actual time of passage of the Greenwich Meridian was actually 12:11:29 on January 22nd.

    Since this is the time of noon LAT (local apparent time), the time that the sun crosses the meridian that you are actually on, you must account for the difference between your meridian and the standard meridian for your time zone to determine the zone time of noon at you meridian, LAN. The standard meridian for your time zone is 75° west so your meridian is actually 1° 21.7' further west so it takes 5 minutes and 26 seconds for the sun to move from the standard meridian to your meridian so your noon will be 5 minutes and 26 seconds later on your watch so your noon would have been 12:16:56 EST, 17:16:29 Z. But the equation of time keeps changing and it is 11 minutes 45 seconds on January 23 so you must interpolate between the two dates. The equation of time of time increased 16 seconds in 24 hours so it increased about 3 seconds in the period between noon at Greenwich and noon at you location so your noon occurred at 17:16:32 Z.


    --- On Sun, 1/23/11, Frank Reed <FrankReed@HistoricalAtlas.com> wrote:

    From: Frank Reed <FrankReed@HistoricalAtlas.com>
    Subject: [NavList] Re: lat/long from meridian passage
    To: NavList@fer3.com
    Date: Sunday, January 23, 2011, 3:35 PM

    For any who haven't opened the attachments, here is Patrick Goold's message, slightly edited for formatting, with a table replaced by commas:

    I. Determining the time of the meridian passage. Norfolk, Virginia. 22 January 2011.

    I was confident it was sometime well after noon local watch time but I found I could not make sense of how to use NA�s equation of time. NA gives mer. pass. as 12 hrs. 11 m and the equation of time for noon as 11 m 29 sec. (I inferred from Bowditch that I should add these two. But adding them gives a value outside the range of possible variation.) The website timeanddate.com put LAN at Norfolk, Va at 12:17. I used this as my initial estimate.

    I recorded five shots, all lower limb sun shots. The first two I made at set times and recorded the angles. The last two I waited for a return of the second and then the first altitudes, recording the times when this occurred.

    Sight(WT), Hs, Ha, Ho
    noon, 66� 16�, 33� 08�, 33� 22.8�
    12:15, 66� 36�, 33� 18�, 33� 32.8�
    12:17, 66� 35.6�, 33� 17.8�, 33� 32.6� !?!
    12:19:23, 66� 36�, 33� 18�, 33� 32.8�
    12:34:20, 66� 16�, 33� 08�, 33� 22.8�

    Because I am using an artificial horizon I made no dip or altitude corrections. Ha is just Ha divided by two. Ho is Ha plus 14.8 for lower limb correction.

    The noon sight and its mirror altitude sight gave an average of 12:17:10. The 12:15 sight and its mirror average to 12:17:12. The average of those two averages is 12:17:11. My time zone is GMT plus five. So I made out meridian passage at my site as occurring at 17:17:11 GMT.

    II. Determining Latitude

    Zenith distance = 90� - 33� 32.8� = 56� 27.2� N

    Dec (tab) = S 19� 38.5�
    d = .06 d corr = 0.4�
    Dec = S 19� 38.9�

    Latitude = z - Dec = 36� 48.3� N

    III. Determining Longitude

    GHA (hr) = 72� 06.8�
    GHA m/s = 4� 17.8�
    GHA = 76� 24.6�

    Longitude = GHA = 76� 24.6� W

    According to my handheld GPS, a Garmin GPSmap 76Cx, my location was actually

    36� 50� N
    76� 21.7 W

    for a difference of 1.7� of latitude and 2.9� of longitude.

    Conditions were somewhat less than ideal and accuracy may have suffered. Obviously there is something wrong with the middle of my five sights. It was a very windy day and the glass covers on the Davis artificial horizon permit some drafts to intrude and disturb the reflective surface. It was a very cold day (-4 C) and this made for some fogging of the AH and perhaps of sighting scope. Finally, there was a haze becoming clouds and overcast as the sighting progressed.


    Just to reiterate, everything between the "BEGIN" and "END" above is from Patrick Goold.


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