NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Longitude by altitudes. was Re: How Many Chronometers?
From: Frank Reed
Date: 2009 May 12, 12:55 -0700
From: Frank Reed
Date: 2009 May 12, 12:55 -0700
Marcel, you wrote: "It looks like there must have existed previously either accurate means of measuring the time OR an other mean of measuring longitude, like e.g. the one indicated by the new title of this thread "Longitude by altitude". " The method of longitude by lunar altitudes depends ultimately on the same precious tools as the method of longitude by lunar distances: an accurate ephemeris of the Moon's position (or, if you don't reduce the observations en route, an observatory making continuous, highly accurate records of the Moon's position), and a portable instrument comparable to a sextant capable of measuring the Moon's position in the sky with great accuracy from distant locations. There's no significant evidence for either of these. On the other hand, even in the Roman era it was well understood that you could map a whole hemisphere of the Earth in one night to +/- 3 or 4 degrees in longitude by observing a lunar eclipse. All you need is a fleet of ships and an army of well-trained observers... Yeah. That's all. :-) The method is simplicity itself: all of the observers (who have clear skies) observe the point in the heavens that is exactly in the zenith at the moment the Moon enters full total eclipse (which can be estimated by trained observers within a couple of minutes). Then you collect all the observations and plot those "zenith points" on a celestial globe. They will map out the geographic locations of the observers. If the observers were stationed all around the coast of a continent, each, let's say, 100 miles from the next, you would get a very fine outline map of that continent on your star globe. From my point of view, the biggest problem with any of these supposed pre-Columbian sea voyages to the New World, whether from Europe or the Near East or China, is the total lack of immunity to Eur-asian diseases among Native Americans which became tragically obvious shortly after the first Spanish voyages to the Americas. So unless those voyages took place many centuries earlier, when the complex of deadly diseases would have been significantly different (e.g. no "black death" in Europe before about 1347), or unless the voyagers who took those early pre-Columbian voyages were astoundingly disease-free (which is possible with small isolated groups, like the Norse in Newfoundland), plague, smallpox and the rest should have been common in the Americas well before 1500. -FER --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---