This volume provides the essential astronomical data required for celestial navigation during the 1991 calendar year. The text serves as a standardized reference tool for mariners, navigators, and astronomers who rely on precise solar, lunar, and stellar positioning. It compiles complex mathematical tables and ephemerides to facilitate the calculation of a vessel's position at sea.

Experts recognize the Nautical Almanac as a foundational technical reference for traditional maritime navigation. Users frequently note the high density of numerical data and the requirement for specific training to interpret the tables correctly.