Celestial navigation (latitude sailing)

technology

A navigator of 1500 could find latitude and could not find longitude, and almost everything strange about the period follows from that asymmetry. Latitude was obtained by measuring the altitude above the horizon of the Pole Star at night, or of the sun at local noon, and correcting the reading with a table: the Pole Star correction for its offset from the true pole, and the sun with a declination table giving the sun's position for each day of the year. Portuguese practice codified this in the regimentos — the rules of the astrolabe and quadrant — and leaned on the astronomical tables of Abraham Zacuto, whose Almanach Perpetuum was printed at Leiria in 1496. Longitude had no equivalent, because it is a measure of time difference and no clock of the period kept time at sea; it was estimated by dead reckoning from course, speed and elapsed time, and the error accumulated with every day out of sight of land. So a ship could reliably find a latitude and then run along it until land appeared, but nobody could say with authority where in the world a mid-ocean position actually was. That gap is not a footnote. It is why a treaty line drawn through the Atlantic could not be located, and why the ownership of the Moluccas was arguable for thirty-five years.

A technology is not a place. Drawing it on a map would assert something about the world that no stored fact supports.