skip to primary navigation skip to content

Cambridge Digital Library

Department A-Z

Longitude Essays

Royal Greenwich Observatory Archives > Longitude Essays

Josef (also José or Joseph) de Mendoza y Ríos

Josef Mendoza y Ríos was a Spanish astronomer and mathematician born on the 29 January 1761 in Sevilla and died in Brighton in 1816 aged fifty-five; Mendoza y Ríos published astronomical and nautical work in Spanish and English as well as French. Most significantly for his part in the Board of Longitude’s history, Mendoza y Ríos wrote a series of tables that were designed to make latitude and longitude calculations at sea easier and faster to perform. Using Mendoza y Ríos’ tables the latitude of a ship at sea could be calculated from two altitudes of the sun. Also the longitude of a ship at sea could be calculated from the distances of the moon from any celestial body listed in his tables, utilising the lunar distance method that was pioneered by Tobias Mayer’s in 1755. The lunar distance method works by measuring the position of the moon relative to the position of a star and comparing that ‘lunar distance’ to a prepared table of lunar distances like those made by Mendoza y Ríos. By comparing the local lunar distance with the tabulated values, the navigator finds the Greenwich Time for that observation and can then compare local and Greenwich Time, giving them the longitude of the ship. The Board of Longitude considered the lunar distance method of finding longitude at the same time that it supported the creation of timepieces that would maintain accurate Greenwich Time throughout the voyage as a way to determine longitude.

Both Mayer and Mendoza y Ríos were encouraged in their work on the lunar distance method by the Board of Longitude. In RGO 14/46 [RGO 14/46:66] we have the first letter that Josef de Mendoza y Ríos sent to the Board to recommend his tables for publication in 1796 along with the financial documents for the publication in RGO 14/18 [RGO 14/18]. The tables of Mendoza y Ríos were published with the help of the Board of Longitude but it took many years of editing and negotiation, which can be seen in the correspondence Mendoza y Ríos had with the Board’s Secretary William Wales and the Astronomer Royal, Nevil Maskelyne. This collection of tables, as edited by the Board, was finally produced in 1814 and a copy of “Forms for the ready calculation of the longitude by the observed distance of the Moon from the Sun or a star, with the tables published by Joseph de Mendoza Rios” was sent to The Admiralty with a covering note declaring that Mendoza’s tables were in general use in His Majesty’s Navy and the East India Company.

Four years after the publication there is an example of his tables being used by mariners at sea in the related materials of the Board of Longitude. John Franklin was a British Royal Navy officer and Arctic explorer and was in command of HMS Trent on an Arctic voyage attempting to go north from Spitsbergen in 1818. FIS/3 [FIS/3] is the copy of Mendoza y Ríos’s 1814 publication that Franklin took with him to the arctic. It contains printed forms that have blank spaces, left for the mariner to enter their lunar distance observations, followed by step by step instructions for the calculation that gives the time at Greenwich and then the longitude position for the ship using Mendoza y Ríos’s astronomical tables.

Josef de Mendoza y Ríos also appears in a private correspondence exchanged between Astronomer Royal Nevil Maskelyne and Andrew Mckay, another astronomer. Maskelyne attempted to assist McKay in his career as an astronomer and helped him to apply to become a fellow of the Royal Society and suggested that Josef de Mendoza y Ríos would act as a second signatory on the application. This suggestion from Maskelyne, along with the support that he received from the Board of Longitude, demonstrates the high level of involvement Mendoza y Ríos had in British scientific networks, both socially as well as with his endeavour to solve the problem of longitude at sea.

Sophie Waring
History and Philosophy of Science
University of Cambridge