Changing Times in The Nautical Almanac Over 250 Years

Nelmes, Susan

250 years ago, in 1766, the first edition of The Nautical Almanac and Astronomical Ephemeris was prepared, detailing astro-navigational and general astronomical data for the year 1767. Today, fulfilling the same purpose, The Nautical Almanac, serving the marinerís celestial navigation needs, and The Astronomical Almanac, a worldwide resource for fundamental astronomical data, are still published annually. Since 1981 both books have been jointly published by HM Nautical Almanac Office in the UK and the Nautical Almanac Office of the US Naval Observatory.

Time has played a fundamental role in these publications throughout their 250 year history. From the first edition in 1767 which used Greenwich apparent time as the basis of the tabulations, the 1834 edition seeing the change to Greenwich mean time, the adjustment of the beginning of the day from noon to midnight implemented for 1925, and the later introduction of universal and ephemeris time.

Studying the changes in the timescales used in The Nautical Almanac and Astronomical Ephemeris and its successor publications provides an insight into the interface between the ever changing frontiers of scientific knowledge of time and its practical use by mariners and astronomers.

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