NavList:

Mike you wrote:  Hi, Just studying Celestial Navigation. The book I have uses the Nautical Almanac and AP3270 for its examples. I mention this as I have become aware there are different methods/tables. I have just become aware of precession and nutation and realise all celestial navigation will have to take this into account, either by including it in the almanac or by the use of some other special tables. I am just curious to discover how precession and nutation is taken into account for all the heavenly bodies. Aries, Stars, Sun, Moon, Planets & Polaris. The only reference made to precession & nutation is for Polaris. I look forward to replies. Thanks Mike

Mike.  If your book uses excerpts from AP3270, I would guess that it’s a British book aimed at aspiring ocean yachtsmen intending to use a marine sextant at sea.  Such books tend to concentrate upon Sun, Moon, and planet sights, because by the time you can see the remainder of the stars it’s often difficult to see the horizon.  However, they do usually include a section on Polaris, even though it’s not a very bright star, because it provides such a quick way of obtaining latitude in the Northern Hemisphere.

Turning to AP3270, the Sight Reduction Tables for Air Navigation are a joint UK/USA publication (Yet another example of that ‘Special Relationship’ we in the UK at least hear so much about).  In the UK they’re known as AP3270.  In the USA they’re known HO 249.  As Gary says, they actually come in three volumes.  Volume 1, traditionally known to RAF Navigators as the ‘Red Band’, because it had a red band around the top, so you could pick up the correct volume in poor light, tabulates altitudes and azimuths for selected navigational stars.  It is entered with LHA Aries and the observer’s closest whole degree of latitude.  Because the calculations by the publishers to produce the tables require the SHA and declinations of the stars, and these change ever so slightly from one year to the next because Precession and Nutation of the Earth’s spin axis, Volume One is dated.  So that they don’t have to produce a new Volume One every year, the volume is ‘epoched’.  That is, they produce a new volume every five years and include a set of P&N Tables to make a small correction to you fix position for years either side of the epoch year. 

Volumes 2 and 3 with Green and Yellow bands respectively work differently and last forever.  This is because you enter them with the observer’s closest latitude, the LHA of the body, and the declination of the body.  These latter two you take from the current air or nautical almanac, so they are effectively already corrected for P&N.  However, Volumes 2 & 3 are only published for declinations up to 29 degrees, and are meant mainly for Sun, Moon and Planet sights.

Polaris provides a quick and easy way of getting approximate latitude in the Northern Hemispheres, because its approximate position over the North Pole turns the PZX triangle into a straight line and simplifies the math.  However, if you want greater accuracy, you need to allow for the fact that Polaris isn’t exactly over the North Pole. You do this with a Q correction for altitude obtained by entering Table 6 in Vol 1 with LHA Aries and an Azimuth Correction obtained by entering Table 7 in Vol 1 with LHA Aries.  If you use tables 6 & 7 in Vol 1 for these corrections, then they are also date dependent, so you would have to move any Polaris position line obtained in any year other than the epoch date of your Vol 1  by an amount obtained from Table 5, the P&N Correction Tables.  That’s probably why your book only mentions P&N Corrections with respect to Polaris.  Hope this helps.  DaveP  

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