Principles of Celestial Navigation

Supplemental Material » Determining Hc and Zn Using the publications » The Nautical Almanac and Pub 229

To obtain Hc and Zn without electronics, two publications are important, "The Nautical Almanac" for the given year, and "Publication 229." The Nautical Almanac contains, among other data, all of the information needed to compute the Greenwich Hour Angle and declination of the observed object at the precise moment the sextant measurement is taken. The Nautical Almanac contains these data for all navigational objects, be it one of the navigational stars, the Sun, Moon, Venus, Mars, Jupiter, or Saturn. The Nautical Almanac is produced jointly by the U.S. Naval Observatory (USNO) in the United States and Her Majesty's Nautical Almanac Office (HMNAO) in the UK. It is available in the U.S. via the Government Printing Office, and elsewhere via the United Kingdom Hydrographic Office distributors.

The second important book is "Publication No. 229, Sight Reduction Tables for Marine Navigation," usually just called Pub 229. It is also known as "NP 401" in the United Kingdom. Pub 229 is six volumes, and it is static; that is, it does not change from year to year. It contains the data to get Hc, Zn, given an object's local hour angle (LHA), Dec, and the observer's latitude. (Note, we have not discussed local hour angle in this module; just know it is easily calculated from the object's GHA and our longitude.) Pub 229 was developed collaboratively by USNO and HMNAO, and distributed by National Geospatial-Intelligence Agency (NGA). Copies can be found for free download on the NGA site

A major point to remember is that the two publications are used together. The Nautical Almanac, along with an estimate of your position, yields the object's LHA and Dec. Using these, we obtain the object's Hc and Zn from Pub 229.