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Nautical almanac

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This article is about nautical almanacs in general. For other uses including other specific nautical almanacs, see Nautical almanac (disambiguation).

Two sample pages of the 2002 Nautical Almanac published by the U.S. Naval Observatory

A nautical almanac is a publication describing the positions of a selection of celestial bodies for the purpose of enabling navigators to use celestial navigation to determine the position of their ship while at sea. The Almanac specifies for each whole hour of the year the position on the Earth's surface (in declination and Greenwich hour angle) at which the sun, moon, planets and first point of Aries is directly overhead. The positions of 57 selected stars are specified relative to the first point of Aries.

In Great Britain, The Nautical Almanac has been published annually by the HM Nautical Almanac Office, ever since the first edition was published in 1767. ^{[1] [2]} In the United States of America, a nautical almanac has been published annually by the US Naval Observatory since 1852.^[2] Since 1958, the USNO and HMNAO have jointly published a unified nautical almanac, for use by the navies of both countries.^[2] Almanac data is now available online from the US Naval Observatory.^{[3] [4]}

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 marine 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.