A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2019 Dec 10, 13:23 -0800
David, you wrote:
"The values of Q printed in Vol 1 are for the epoch year."
They are also specifically calculated for latitude 45°.
"The values in the Air Almanac are for the year of the almanac. Therefore, Air Navigators are advised that if they have a choice, they should use the Q correction from the current Air Almanac."
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),
where pd is the polar distance of Polaris for the current year, which is 39.1' for 2020, declining just about a quarter of a minute of arc annually for the next ten years at least, and SHApolaris is 315.7° in 2020 declining 0.4° per year for the next ten years. Clearly the second term in Q can be ignored, especially in lower latitudes. The first term on average provides over 99% of the required correction:
Q = -pd·cos(A).
LHAaries can be calculated, found in tables, or generated by an app. Note that LHAaries can also be estimated visually to the nearest few degrees. Find a navigational star on the observer's meridian and look up its SHA. Call that SHAmer. Or estimate SHAmer by looking at a pair of navigational stars on either side of the meridian. Then
LHAa = 360° - SHAmer.
If the observed star is below the pole (for example Dubhe or Kochab might be directly below Polaris), then add 180°.
You also wrote:
"Incidentally, I doubt if many people read the Introduction to AP3270, because it’s in such small print, and it looks really boring, but they should try to one night when they can’t sleep, because it’s a mine of useful information such as this. "
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.