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    Re: Polaris
    From: David Pike
    Date: 2019 Dec 10, 14:54 -0800

    Frank you wrote: 

    David, you wrote:
    "The values of Q printed in Vol 1 are for the epoch year."

    They are also specifically calculated for latitude 45°.

    Thanks for the reminder.  Cf the Nautical Almanac, they must have had to pick a latitude somewhere, and 45 seems to show constant values for a1 in the Nautical Almanac.  If anyone ever told me, I’d forgotten.

    You also wrote: 

    That's the traditional advice. A modern navigator can also easily generate excellent Q correction values with an inexpensive calculator:
       A = LHAaries + SHApolaris,
       Q = -pd·cos(A) - 0.21'·tan(Lat)·sin2(A), 

    I did work the first term out once many years ago.  It’s just the resolution of Polaris onto the observer’s meridian.  The second term must be a correction for the tangency of the observer’s line of sight to the ring Polaris makes around the Pole, but it’s a bit beyond my pay grade.

    Then you wrote: "Have you looked at a recent edition? The introduction of AP3270 Vol.1 was completely re-written some years ago. The similar introduction to Pub.249 Vol.1 (the US equivalent to AP3270) is being re-written for the next epoch dates."

    My last hard copy of AP3270 was 1980 Epoch.  I keep it mainly because the red band of Vol1 stands out nicely against the green and yellow bands of Vols 2 and 3 in my bookshelf.  Out of epoch, it’s not a lot of use for anything else.  My current soft copy of HO249 is the 2015 Epoch.  It’s almost word for word the same as my 1980 hard copy of 3270, only the dates in the examples have been changed, and there are a couple more examples.  I look forward to the 2020 Epoch becomming available free online. DaveP

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