The Astronomical Almanac is an annual reference work produced jointly by the United States Naval Observatory and Her Majesty's Nautical Almanac Office. It gathers precise tables and explanatory material used to predict the positions and apparent motions of Solar System bodies and selected stars, and to provide time and coordinate data required for observational and navigational practice.
What it contains
- Ephemerides and apparent places for the Sun, Moon, planets and selected stars, including rise and set times and positional coordinates.
- Predictions of eclipses and other significant celestial events.
- Orbital elements, conversion tables between time scales, and reference information useful for spacecraft tracking and astronomical observation.
- Explanatory sections describing methods, definitions, and the reference frames used in the tables.
History and publication
First issued in 1767, editions of the almanac have been produced continuously since the 18th century. In 1981 the separate American and British publications were merged into the single joint annual that is distributed today. The volume is revised each year to incorporate improved measurements, updated models, and changes in standards for time and reference systems.
Who uses it and why
The data in the almanac supports professional and practical work across several fields. It is consulted by astronomers for planning and reducing observations, by navigators for celestial navigation and route planning, and by surveyors, space mission planners and other technical specialists who need accurate ephemerides.
Accuracy and navigation
When combined with careful observations and proper computation, the ephemerides in the almanac allow traditional celestial navigation techniques to yield very accurate fixes. Under favourable conditions and with modern instrumentation, positions derived from these tables can be determined to within a few hundred metres; practical results depend on the observer’s skill, the quality of observations, and environmental conditions.
Further reading
For more detailed descriptions of the material and the institutions that produce it, consult resources on astronomy and historical accounts of almanac production.