Pub. 249
Written for air navigators and widely adopted by small-boat sailors. Three volumes:
- Volume 1, selected stars. For each latitude and LHA of Aries it lists seven well-placed stars with Hc and the true bearing Zn directly: no declination, no interpolation. It is tied to an epoch and reissued every five years.
- Volumes 2 and 3 (latitudes 0° to 40° and 39° to 89°). For declinations from 0° to 29°, which covers the Sun, Moon and planets always, and the stars within 29° of the equator.
Hc is given to the nearest minute and the azimuth angle Z to the nearest degree. A small table (Table 5) handles the odd minutes of declination.
Pub. 229
Written for marine navigation. Six volumes, each covering 16° of latitude (0° to 15°, 15° to 30°, and so on to 90°). It covers every declination, so any body can be reduced. Hc is given to a tenth of a minute and Z to a tenth of a degree, with an interpolation table, and occasionally a second small correction (the double second difference) near the zenith.
Side by side
| Feature | Pub. 249 | Pub. 229 |
|---|---|---|
| Hc precision | 1′ | 0.1′ |
| Azimuth | 1° | 0.1° |
| Declinations | 0° to 29° (Vols. 2, 3); seven chosen stars (Vol. 1) | All |
| Volumes for the world | 3 | 6 |
| Working | Shorter | A little longer |
| Goes out of date? | Vol. 1 every five years; Vols. 2, 3 never | Never |
Which should you use?
From the deck of a small boat, a sextant altitude is rarely good to better than a minute or two. Pub. 249's whole-minute precision is not the weak link, and its working is quicker, with fewer places to slip. Most cruising sailors carry Volumes 2 and 3 for their latitudes, and Volume 1 if they shoot stars often.
Choose Pub. 229 if you want one method for every body, including stars far from the equator, or you want the extra precision for a steady platform or for checking work.
A third way is a calculator: the same answer from two formulas, straight from your DR, with no tables to carry. Many navigators learn the tables first, because they show what is going on, and keep a calculator as a check.
Both tables are free to download from the U.S. National Geospatial-Intelligence Agency. The British editions are NP 303 (the same tables as Pub. 249) and NP 401 (Pub. 229).
Try all three
In the course, the intercept method practice lets you work the same sight with Pub. 249, Pub. 229 or a calculator and see that the line of position lands in the same place. The real books shows how the pages are laid out.