Publication of the First Nautical Almanac

Background. Nevil Maskelyne, the fifth British Astronomer Royal, was appointed on 26 February 1765 after the unexpected death of Nathaniel Bliss, making him ex officio a Commissioner of Longitude. He had trialled the lunar distance method of determining longitude at sea and reported the results to the Board of Longitude on 9 February 1765. This proposal led to the establishment of The Nautical Almanac, whose production Maskelyne oversaw as Astronomer Royal.

Content and Method. The full title was the British Nautical Almanac and Astronomical Ephemeris for the Meridian of the Royal Observatory at Greenwich, and Maskelyne created it using Tobias Mayer's corrections for Leonhard Euler's Lunar Theory tables. The distinctive feature of the inaugural issue for 1767 was the tabulation of lunar distances, facilitating longitude determination at sea from observations of the Moon. It was the first nautical almanac to contain data dedicated to the convenient determination of longitude at sea, and for 1767–1959 it carried both astro-navigational and general astronomical data, with ephemeris tabulations given in Greenwich apparent time for 1767–1833.

Significance. Because the observations feeding the almanac were made at the Royal Observatory, Greenwich, the Greenwich meridian became the reference for longitude measurements in the Royal Navy and on British Admiralty charts. The work was originally published from the Royal Greenwich Observatory in England, and a subsidiary volume of tables requisite for finding latitude and longitude at sea accompanied the ephemeris.