Nautical almanac

A yearly reference that gives predicted positions of the Sun, Moon, planets, and selected stars at specified times, chiefly for celestial navigation.

Celestial navigation: Navigation that determines position or direction by observing celestial bodies. The almanac supplies the predicted positions needed to compare with observed celestial altitudes.
Ephemeris: A table or model giving the calculated positions of celestial objects over time. A nautical almanac is a practical, navigation-focused kind of ephemeris.
Regiomontanus: A fifteenth-century German mathematician and astronomer who produced influential astronomical tables. His tables helped establish the printed astronomical data tradition later adapted for navigation.
Noon sight: A sextant observation of the Sun near its meridian passage, used chiefly to determine latitude. The almanac identifies the Sun's declination for reducing this classic navigation observation.
Air Almanac: An annual reference containing astronomical data formatted for air navigation. It serves navigators in aircraft rather than focusing on the needs of mariners.
Sextant: An optical instrument that measures the angle between a celestial body and the horizon. Navigators use it to measure the altitudes that almanac-based calculations require.
Right ascension: An equatorial coordinate measuring angular distance eastward from the vernal equinox. Along with declination, it describes celestial positions underlying many almanac entries.
John Flamsteed: The first Astronomer Royal of England, known for precise stellar observations and catalogues. His observations improved the stellar data available to navigators and almanac makers.
Lunar distance: An angular measurement between the Moon and another celestial object, historically used to determine longitude. Almanac data enables the calculation of Greenwich time from observed lunar distances.
Astronomical almanac: An annual reference of astronomical data, including celestial coordinates and events. The nautical almanac selects and formats astronomical information for navigation at sea.

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