nautical almanac

nautical almanac, a periodical publication of astronomical and other, primarily ephemeral, information intended for the navigator and nautical astronomer. The earliest nautical almanac was the Connoissance des temps ou des mouvements celestes published in Paris under royal patent by the Bureau des Longitudes in 1679. The publication included full tables of the moon's motions and a long list of the establishment of the port for French harbours.

Towards the middle of the 18th century the improvement in instruments and tables had made the determination of longitude at sea by lunar distance a practicable possibility. The Astronomer Royal, Nevil Maskelyne, was therefore authorized by the Board of Longitude to compile a publication which would enable the mariner to compare the astronomical information and data contained in it with observations made at sea. The Nautical Almanac and Astronomical Ephemeris for the Year 1767 was the first English nautical almanac and the earliest by far to give essential data for the practical determination of longitude at sea. It has, with periodic modifications of function and title, continued publication ever since.

Towards the end of the 19th century it was recognized that much of the matter contained in the almanac was of little interest to seamen and it was decided to issue the first part, consisting mainly of the ephemeris and lunar distances, separately. In 1914 on the recommendation of the Royal Astronomical Society The Nautical Almanac Abridged for the Use of Seamen, as it then was, became an entirely separate publication. In 1948 the newly formed (and later Royal) Institute of Navigation, at the suggestion of the Admiralty, was asked to advise on the complete redesign of the Abridged Nautical Almanac. As a result, from 1952 the revised almanac giving, as in the Air Almanac, Greenwich Hour Angle (GHA) of the sun, moon, planets, and the first point of Aries tabulated against Greenwich Mean Time was published. From 1958 the Abridged Nautical Almanac and the American Nautical Almanac were ‘unified’ and became identical in content, reproduced from identical material which has been made available to other national nautical almanacs. The title of the purely astronomical part of the Almanac was in 1960 changed to The Astronomical Ephemeris.

The Nautical Almanac tabulates the two coordinates declination and GHA of the sun, moon, and planets at tabular intervals of 1 hour of GMT and, for the stars, GHA Aries, declination, and sidereal hour angle (SHA) at intervals of three days. In this way the almanac, in effect, determines the true direction, as it would be seen from the centre of the earth, of any object at the time of observation. By means of reduction tables, or a calculator or computer, this calculated altitude is compared with the observed altitude to give an intercept from which a position line may be plotted.

There are in every maritime nation a number of commercially produced nautical almanacs containing a wide range of navigational information outside of that normally included in the official publications, such as tidal information, light lists, navigation tables, pilotage information, and so on. Some reproduce, with permission, matter from the official almanacs. For the most part such almanacs fulfil a very useful role for yachtsmen and other small craft navigators including fishermen who wish to cut down the number of publications carried.

Mike Richey