A nautical almanac is a publication describing the positions and movements of celestial bodies for the purpose of enabling navigators to use celestial navigation to determine the position of their ship while at sea 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 use difference tables to extrapolate 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.

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