Nordestear y noruestear — the compass variation in the carrera de Indias
By 1590 the fact that a magnetic needle does not point true north everywhere had a Spanish sailors' vocabulary — nordestear, noruestear — a rule about where the error vanished, and a printed warning that a navigator ignoring it will make landfall somewhere other than intended. The book recording all three is a Jesuit natural history: it adds that no philosopher or cosmographer could explain the effect, that the pilots knew it anyway, and that the informant was Portuguese.
Why this wins its question: Histories of geomagnetism in English begin the story with Robert Norman in 1581 and Gellibrand in 1635, and treat Iberian practice as background. This object works from the Seville 1590 sheets and quotes what a Spanish-language book was telling a general readership five years before Norman's tract reached its second edition: the two verbs the pilots used, the four points where the needle was said to stand true, the Azores landmark, the warning that ignoring a small deflection puts a ship in the wrong place, and the admission that no philosopher or cosmographer could say why. It also keeps what a nationalist telling would cut — that the informant was Portuguese, that Muslim sailors off Mozambique were using the compass before the Portuguese reached them, and that the author's stated reason for valuing the whole art was the carriage of missionaries.
Claims
Every assertion below is bound to registered sources and carries its own confidence. Weight them; do not treat the page as uniformly authoritative.
José de Acosta devotes chapter 17 of the first book of the Historia natural y moral de las Indias to the subject, under the heading "De la propriedad y virtud admirable de la piedra Iman para nauegar. Y que los antiguos no la conocieron", and opens it by making the lodestone the condition of the whole enterprise: "a la piedra Iman se deue la nauegacion de las Indias, tan cierta y tan breue".
His measure of how routine ocean navigation had become is worth having from 1590 rather than from a modern summary. Men are seen every day who have made the voyage "de Lisboa a Goa, y de Seuilla a Mexico y a Panama, y en estotro mar del Sur hasta la China, y hasta el Estrecho de Magallanes: y esto con tanta facilidad como se va el labrador de su aldea a la villa"; some have made fifteen voyages and even eighteen to the Indies, and of others he has heard that they have crossed the Ocean more than twenty times, "en el qual cierto no hallan rastro, de los que han caminado por el, ni topan caminantes, aquien preguntar el camino".
The phenomenon itself is stated exactly. Iron rubbed on the part of the lodestone that in its natural bed faced south takes the virtue of looking always to the north "y en todas partes: pero no en todas le mira por ygual derecho. Ay ciertos puntos y climas, donde puntualmente mira al Norte, y se afixa en el: en passando de alli ladea vn poco, o al Oriente, o al Poniente, y tanto mas quanto se va mas apartando de aquel clima." Acosta then hands the terminology to the seamen: "Esto es, lo que los marineros llaman, Nordestear, y Noruestear. El Nordestear es, ladearse inclinando a Leuante: Noruestear inclinado a Poniente."
The practical stake is stated in one sentence, and it is the sentence that makes this a piece of applied science rather than a curiosity: "Esta declinacion, o ladear del aguja, importa tanto sabella, que aunque es pequeña, sino se aduierte, erraran la nauegacion, y yran a parar a diferente lugar del que pretenden."
The working rule is reported at second hand and Acosta says so. "Deziame a mi vn piloto muy diestro Portugues, q̃ eran quatro puntos en todo el orbe, donde se afixaua el aguja con el Norte, y contaualas por sus nombres, de que no me acuerdo bien." One of the four he does remember: the waters off the Isla del Cuervo in the Azores, "como es cosa ya muy sabida"; passing thence into higher latitudes the needle noruestea, and passing the other way towards the equinoctial it nordestea. How much and how far, he says, "dirãlo los maestros desta arte" — the pilots, not him. The four-point scheme is therefore recorded here as a pilot's rule reported by a non-specialist who admits he has forgotten half of it.
What follows is the most quotable sentence in the chapter and an unusual one for a book of its date: Acosta says he would gladly ask the bachelors who presume to know everything to tell him the cause of the effect, since a little iron rubbed with the lodestone conceives so great a virtue of looking always to the north, "y esto con tãta destreza que sabe los climas y posturas diuersas del mundo, donde se ha de fixar, donde inclinar a vn lado, donde a otro, que no ay Philosopho, ni Cosmographo, que assi lo sepa." The instrument knows what the learned do not; the passage is a printed acknowledgement that seagoing practice had outrun the natural philosophy of the universities.
Acosta does not claim the art for his own nation, and the point is his, not this object's. Noble authors writing the history of the East Indies, he says, record that Vasco da Gama, the first to discover it by sea, "topò en el paraje de Mozambique con ciertos marineros Moros, que vsauan el aguja de marear, y mediante ella nauegaron aquellos mares"; from whom those sailors had learned the device, "no lo escriuen". His own argument that the ancients did not have the compass rests on philology — that for a thing so notable as the mariner's needle "no se halla vocablo Latino, ni Griego, ni Hebrayco", and that a thing so important would not have gone unnamed had they known it — and on the practice of steering from the prow, which is why the Greeks called their pilots proritas.
The motive is on the same page as the achievement, in the author's own terms: the carrera had to be frequented so that so many souls should come to the knowledge of Jesus Christ and attain their eternal salvation, and therefore heaven also provided "guia segura, para los que andan este camino, y fue la guia el aguja de marear, y la virtud de la piedra Iman". An object about a Spanish contribution to the science of navigation that omitted this sentence would be quoting the chapter selectively.
What this object does not claim. It does not credit Acosta with discovering magnetic variation; he reports what pilots already said and names none of them but the one Portuguese. It does not claim the four-point scheme was correct — the source itself declines to say how much or how far, and no modern geomagnetic literature is cited here. It says nothing about the secular drift of declination over time, which the chapter does not know about, and nothing about the attempts to turn variation into a method of finding longitude, which are not in the pages read.