Book excerpt: For over 150 years the United States Nautical Almanac Office has published The Nautical Almanac, first as part of the American Ephemeris and Nautical Almanac, and then on its own, to provide the US Navy with a convenient form of the astronomical data used for celestial navigation. This book is still the standard resource for marine celestial navigation for the U.S. Navy. The Nautical Almanac provides essential astronomical information for every kind of sea-going vessel, from cruise liners to fishing boats. It is fully comprehensive, containing a set of concise sight reduction tables for the navigator, together with all the necessary information needed for use in determining the position at sea from sextant observations. Nautical Almanac is essential for astro-navigation and used by many astro-teaching courses.