A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: David Pike
Date: 2019 Dec 9, 17:01 -0800
Mike Freeman you wrote: I have often wondered why LOP's derived from star sights used in conjunction with AP3270 require a correction for precession and nutation whereas sights of Polaris do not require this correction.
I think I now understand but you may wish to correct or improve my understanding.
Polaris tables are published in the Nautical Almanac annually, they are also updated annually and are correct for 1 year only.
AP3270 Vol 1 is a different publication and lasts several years. A precession and nutation correction is required increasing as the years pass until a new edition of AP3270 Vol 1 is purchased.
Look forward to replies
Yes you are correct. You must remember that with Air Navigation speed of working is the main criterion, because when you might be travelling at 8nm/minute you can’t afford to waste time bothering about the odd 0.1/nm. You’d fly that far in less than a second. Therefore, everything must be simplified to speed up the time required to get the fix on the chart. Also, celestial in the air is probably an order of magnitude less accurate than on the surface because of acceleration errors. Three nm accuracy is very good, and 20nm accuracy is not uncommon. This is another reason why corrections might as well be simplified to the nearest 1nm. In the air, there’s no fancy correction for Polaris as per the Nautical Almanac, you simply apply one amount, Q. This can be found in two places, AP3270 Vol 1 and in the Air Almanac. The values of Q printed in Vol 1 are for the epoch year. 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.
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. DaveP