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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.

-FER

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