The Astronomical Almanac

An annual reference published jointly by the U.S. Naval Observatory and the UK Hydrographic Office, containing astronomical data and ephemerides for practical use.

Ephemeris: A table or dataset giving the positions of celestial bodies at specified times. Ephemerides are the Almanac’s core predictions of celestial positions.
Celestial navigation: Navigation that determines position from observations of celestial bodies. Navigators use published celestial positions to reduce observations into a location.
United States Naval Observatory: A U.S. scientific institution that provides astronomical data and timing services, including support for navigation. The observatory is one of the Almanac’s joint publishers.
Conventions of the International Astronomical Union: Internationally adopted definitions and models for astronomical reference systems and calculations. The Almanac’s standardized calculations rely on internationally agreed conventions.
Celestial coordinate system: A reference framework for specifying positions on the celestial sphere. The Almanac reports positions using standardized celestial coordinates.
Nautical almanac: A yearly reference of celestial data and procedures designed for marine navigation. It is a navigation-focused companion to the broader astronomical reference.
UK Hydrographic Office: A United Kingdom agency that supplies hydrographic and navigational information. The office shares responsibility for producing the annual volume.
JPL ephemeris: A numerical planetary and lunar ephemeris produced by NASA’s Jet Propulsion Laboratory. It is a major computational ephemeris source distinct from the Almanac’s published tables.
Julian date: A continuous count of days and fractions of a day used in astronomical timekeeping. Julian dates provide a uniform time scale for astronomical calculations.
Astronomical twilight: The interval when the Sun is between 12 and 18 degrees below the horizon. Twilight predictions help observers and navigators plan around changing sky conditions.

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