Celestial navigation (latitude sailing)

Assembled from 18 blocks · 2 evidence · 9 related

1. Story
2. 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.
3. Knowledge
4. Celestial navigation (latitude sailing)
5. Mariner's astrolabe
6. Almanach Perpetuum
7. Connections
8. Almanach Perpetuum
9. Mariner's astrolabe
10. The Manila galleon (Nao de China)
11. The Treaty of Tordesillas divides the ocean
12. The Magellan expedition sails from Sanlúcar
13. The Treaty of Zaragoza settles the Moluccas by sale
14. Celestial navigation (latitude sailing)
15. Celestial navigation (latitude sailing)
16. Evidence
17. The tabular basis of Portuguese latitude navigation in the decades of the Cape route voyages. V55 verification basis: NOT consulted in this session. The 1496 Leiria printing is stated from the author's knowledge and should be confirmed against a library catalogue; the relationship between Zacuto's tables and the navigational regimentos actually carried at sea is a specialist question this record does not settle.
18. The standard single-volume account of Portuguese expansion: Ceuta, the African coast, the Cape route, the carreira da Índia, the Estado da Índia and the institutions that ran them. V55 verification basis: NOT consulted in this session. Boxer is a mid-twentieth-century imperial history and its framing has been substantially revised by later work, particularly on Asian agency and on the limits of Portuguese control; this pack uses it for structure, not for interpretation.