Nautical almanac

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A nautical almanac is a publication describing the positions and movements of celestial bodies, including the sun, moon, planets, and 57 stars chosen for their ease of identification and wide spacing. The Almanac specifies for each whole hour of the year the position on the Earth's surface at which each body is directly overhead. The Sun, Moon and Planets 'move' independently and so are specified separately, but for the stars only Aries is specified, the other stars having a set angular distance from that. The navigator can extrapolate by means of difference tables to obtain the position of each object for each minute of time.

In Great Britain a nautical almanac has been published annually by the HM Nautical Almanac Office, ever since the first edition was published in 1767.

Also commercial almanacs were produced that combined other information. A good example would be Brown's - which commenced in 1877 - and is still produced annually, its early twentieth century subtitle being "Harbour and Dock Guide and Advertiser and Daily Tide Tables". This combination of trade advertising, and information "by permission... of the Hydrographic Department of the Admiralty" provided a useful compendium of information. More recent editions have kept up with the changes in technology - the 1924 edition for instance had extensive advertisements for coaling stations.

The "Air Almanac" of the United States and Great Britain tabulates celestial coordinates for 10 minute intervals. The Sokkia Corporation's annual "Celestial Observation Handbook and Ephemeris" tabulates daily celestial coordinates (to a tenth of an arcsecond) for the Sun and nine stars.

To find the position of a ship or aircraft by celestial navigation, the navigator uses a sextant to take a 'sight' to measure the apparent height of the object above the horizon, and notes the time from a chronometer. The object's position is then looked up in the Nautical Almanac for that particular time and after allowance for refraction, instrument error and other errors, a position circle on the Earth's surface is calculated.