NavList:

Just a few clarifications:

The method of determining latitude by observation of Polaris was developed in Portugal in the second half of the 15th century. It worked well until the Portuguese voyages of discovery were nearing the equator for the obvious reason that Polaris became less visible the farther south the voyages went. According to the “Décadas da Ásia“ of Joao de Barros the Portuguese king then appointed a „Junta de matematicos“ who developed the method of determining the latitude by meridian altitudes of the sun including the necessary tables.

The earliest tables giving the declination directly for every day of the year can be found in the “Regimento de Munich” (ca. 1509) which gives the values for one year only. It is supposed that there was an earlier printing in the last decade of the 15th century but there is no copy known. The tables were based on the “Almanach Perpetuum” of Abraham Zacuto as has been shown by Pereira da Silva about 100 years ago. Zacuto – by the way – was not an “astrologer” as has been said here but a bona fide astronomer. That these tables did not owe anything to Regiomontanus is apparent by the maximum declination used in the tables and by Zacut: 28deg 33min whereas Regiomontanus used the better value of 28deg 30min. The earliest tables for a 4 year cycle are in the “Tractado da Spera” printed ca. 1517; they may have been calculated for the voyage of Magellan. These tables were copied many times in Portuguese atlases and navigation manuscripts and printed manuals during the 16th century and even in an Italian publication in the 17th.

Regarding Columbus: It is not known that he used such tables, on the contrary: it is rather implausible. It is known, though, that he possessed the Ephemerides of Regiomontan, because the 1481 edition is in the Bibliotheca Colombiana. That book contains some marginal notes in Columbus’ hand concerning weather observations. The interesting thing is that the tables for 1488 through 1506 are missing. This may indicate that he took them along on his voyages and used them for his prediction of a lunar eclipse in 1504.

If you would like to know more about this and do not abhor printed (as opposed to Internet) sources you could read the following:

Albuquerque, Luis de; Astronomical navigation. Chap. VII in Cortesao, Armando, History of Portuguese Cartography, Vol. II, Coimbra 1971. (also printed separately Lisboa 1988)

David W. Waters (The Art of navigation…) and Eva Taylor (The Haven-Finding Art) also recount this period of navigation well – if not in the same detail as they did no original research.

Regards
Wolfgang
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