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    Re: Longitude by altitudes. was Re: How Many Chronometers?
    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. 
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