North is at the top of the diagram the circumference is the horizon the center is overhead each ring is 30º of altitude. These diagrams take some explanation, which is presented in-depth in a separate article, comparing this method of sight planning to the several options available. Sky Diagrams from the AA: Evening sky at Lat 25N, July 15, 2019, 17h left and 19h right, LMT. The AA includes a few tables unique to aircraft sextants and some unique to high latitude flying, neither of which are relevant to marine navigation. Both books include a list of places on various time zones and who uses daylight savings, and both include similar Planet Location diagrams, with the AA versions, if anything, easier to use. The AA has a better list of symbols and abbreviations, or at least it can be considered a nice supplement to the NA data. Sun and moon rising and setting data as well as twilight and LAN times are about the same in both books, although they are laid out differently. It is often presented on its own, because it uses navigational star id numbers, so we just note that the source of this well known star chart is indeed the AA. The AA does not include the polar view and tropical Mercator star charts included in the NA, but it does have a star chart of its own that all navigators can benefit from. (We use this method in our book GPS Backup with a Mark 3 Sextant.) This Q-method is a bit quicker to implement. The latitude by Polaris correction in the AA is presented as a single Q correction, which combines the a0, a1, and a2 corrections used in the NA. But for all practical purposes, the precision of the AA is plenty adequate for routine cel nav on any vessel, and indeed the AA does include a Polaris azimuth table accurate to the tenth of a minute, just as the NA does. It could also affect gyro bearing calibrations where we do indeed want azimuths accurate to the tenth. This could be a handicap for those with good sextant skills, taking the right sights in good conditions, in which case they could lose a few tenths of a mile accuracy in some sights. This discussion in the AA, however, is more pessimistic regarding final fix accuracy because they are assuming sights from an aircraft. The overall guaranteed uncertainty of the values are slightly better in the NA than in the AA, both of which have sections discussing Accuracy. In a sense, then, the AA is a bit more user friendly to navigators as it includes this extra bit of information.Ī big difference in format is the AA only gives moon and planet data to the nearest whole minute, which effectively is rounding from 0.5'. So the AA just does not list it.įor the other planets, the AA will likewise not present data when they are not useable for cel nav, as shown below. To be a useful evening or morning star, it has to be far enough from the sun that is is not buried in bright twilight a point we look into below. The reason it is not shown in the AA is it is not a practical target at this time, because it is too close to the sun-compare sun and Venus GHA and dec in the NA data above. In short, it extends the sight taking time period. Normally whenever Venus is available it is a good object to shoot, because it is so bright (planet magnitudes are listed next to their names) we see it early evening twilight before the stars are visible, or later in morning twilight after the stars have faded. Notice that Venus is listed in the NA but not the AA. There is some subtlety here, however, in the presentation. DOY is convenient for figuring watch errors and for ETA computations over a long ocean crossing.
Notice too that the day of the year (DOY) is listed on each daily page of the AA, whereas in the NA, DOY values are in a separate table. Both make the same level of corrections values for the sun in each book will be the same ± 0.1'. In the AA, we get this data every 10 minutes, and make the minutes and seconds corrections using the AA's Interpolation Table. In the NA, we get GHA and declination every hour, and then we correct for the minutes and seconds using the NA's Increments and Corrections Table.
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