The lunar-eclipse instructions of 1577 and the longitude of the Indies
In the same packet as the famous 1577 questionnaire, Spain sent the Americas a second printed sheet almost nobody reads: instructions for observing eclipses of the moon in order to fix latitude and longitude. It was reprinted for each eclipse year, and it worked at least well enough that by February 1578 a cosmographer was asking the Council of the Indies for the observations that had come in from New Spain "porque hace mucho al caso para las longitudes".
Why this wins its question: Accounts of the longitude problem begin in England in 1714, or at best with Philip II's prize of 1567 and Galileo's Jovian satellites; the 1577 Spanish eclipse instructions, printed and shipped by the thousand to American towns together with the questionnaire, are missing from almost all of them. This object names the instrument, dates its circulation to 25 May 1577, records that it was reprinted for successive eclipse years, and quotes the Council of the Indies register of 18 February 1578 in which a cosmographer asks for the observations already arrived from New Spain because they bear on the longitudes - and then states, as its own claim, exactly what the surviving administrative record does not prove about the results.
Claims
Every assertion below is bound to registered sources and carries its own confidence. Weight them; do not treat the page as uniformly authoritative.
The instructions existed, and they were an instrument of measurement rather than of edification. The editor of the Relaciones Geográficas records that Juan López de Velasco drafted "sus ingeniosas Instrucciones para la observación de los eclipses de luna... y verificar por ellos las alturas y longitudes", and that these began to be circulated to the towns of the Indies on 25 May 1577.
They travelled with the questionnaire and were printed as its twin. The interrogatorio of two hundred chapters, recast to fifty under the name Instrucción y Memoria, was distributed on that same date "en unión con las Instrucciones para los eclipses"; and the editor notes that the two were printed in identical type, case, ornamental initial and paper. The survey of the Indies and the attempt to place the Indies on a coordinate grid were one operation, not two.
The questionnaire carried the low-technology version of the same problem. Chapter 6 of the 1577 Instrucción y Memoria asks for the elevation of the pole where each Spanish town stands "si estuuiere tomada, y si se supiere, o uiere quien la sepa tomar" - and, failing anyone who can take it, on what days of the year the sun casts no shadow at noon, which is a latitude determination a parish priest could make with a stick.
The eclipse sheets were reissued as the sky required. The editor states that the Instrucciones para los eclipses de luna were printed, from 1577 onward, as many times as the years in which the phenomenon presented itself demanded; that he had himself seen copies of two of those printings and had notice of a third; and that the instructions were remitted again with the letters and cédulas sent to viceroys, governors and audiencias.
Observations came back, and the Council of the Indies routed them to its cosmographers. In the register of petitions of the Council for 18 February 1578, the cosmographer Juan Bautista Gesio asks that he be given "las observaciones de eclipses que han venido de la Nueva España y de otras partes, porque hace mucho al caso para las longitudes"; the Council minuted the request "Júntese con Juan de Velasco y véalo". Within nine months of the instructions going out, American eclipse observations were in Madrid and being read for longitude.
There was a standing office to receive them. Juan López de Velasco was appointed cosmógrafo-cronista mayor de las Indias on 20 October 1571, after the aged Alonso de Santa Cruz was pensioned, and held the post until 1591; on 11 January 1577 the Council granted him four hundred ducados for making and ordering a Libro de geografía de las islas y provincias de las Indias. The eclipse campaign was the measuring arm of that book.
The motive was navigation and empire, and the same archival page shows it. In the identical register Gesio asks for the relaciones and derroteros of the navigations already made of the South Sea, of the islands of the Malucos and the Philippines, of the route by the Strait and of the ports of New Spain, including those of Villalobos, Juan Gaetán, the commander Loaísa and Juan Ladrillero, and for the route now used from Acapulco to the Philippines. Longitude mattered because the crown was arguing about where the line of demarcation fell and how to sail the Pacific.
What these documents do not show should be said plainly. They establish that instructions were drafted, printed, circulated and reprinted, that observations arrived from New Spain and elsewhere, and that a cosmographer was told to look at them with López de Velasco. They do not establish what longitudes were derived, how accurate they were, or how many towns observed; no computed result appears on the pages read for this object, and the evidence here is administrative rather than astronomical.
The reach of the whole operation was also uneven, and by the same measure as its twin. The relaciones themselves arrived from New Spain, the Nuevo Reino de Granada, Tierra Firme, Quito and Peru but never from Tucumán, Chile or the Río de la Plata, the editor concluding that the force of the 1577 cédula "fue remitiendo de Norte a Sur y se desvaneció hacia el estrecho de Magallanes". An instruction printed in Madrid is not the same thing as an observation made in America, and the gap between the two is part of the record.