[...]
Post by Tom PotterPost by WhiskersGiven an accurate and reliable table of eclipse predictions, an astrolabe
could be used to help estimate relative longitude between two places;
here's a treatise from the 7th century trying to explain how
<http://www.ccel.org/ccel/pearse/morefathers/files/severus_sebokht_astrolabe_01_trans.htm#C14>.
The accuracy would have been 'pretty low' though; within one degree would
be quite impressive, I think (that's about 70 miles at the equator).
Thanks for the reference.
If one uses an astronomy program like Cybersky,
they will see that
the stars background provides a great clock,
and that the Moon subtends different angles
at the same "star" time
at various places on the Earth's surface.
I don't think they had access to Cybersky in the 15th century. What they
did have, though, was the actual sky. But their almanacs weren't very
complete or reliable - and a mariners astrolabe would not be capable of
sufficient accuracy, even if the navigator understood the principle and
was able to perform the mathematics.
Post by Tom PotterI suggest that if one analyses a triangle
formed by the Moon and a couple of stars,
they can determine their location,
both latitude and longitude.
how accurately could sailors do this
using "a typical 15th Century astrolabe"?
They couldn't have done it at all.
You're correct that 'lunar distances' are theoretically useable to
calculate longitude, given a sufficiently good almanac and mathematical
knowledge. This was one of the methods proposed in the 18th century for
finding the longitude at sea, but even though they had good almanacs and
sextants with optical magnification built in by then, the method still
proved too complex and difficult in practice.
<http://en.wikipedia.org/w/index.php?title=Lunar_distance_(navigation)&oldid=335691590>
<http://en.wikipedia.org/w/index.php?title=John_Harrison&oldid=361972753>
[...]
In 1736, Harrison sailed to Lisbon on HMS Centurion and returned on HMS
Orford. On their return, both the captain and the sailing master of the
Orford praised the design. The master noted that his own calculations
had placed the ship sixty miles east of its true landfall which had
been correctly predicted by Harrison using H1
[...]
Notice that there is a 60 mile error reported there by a skilled navigator
using the best 18th century methods and equipment but without using a
marine chronometer ("H1" was the first to be tried at sea). And that was
on a fairly short voyage, travelling mostly north and south - Portugal and
England are in the same time-zone. If that ship had relied on the
Master's calculations in poor visibility, she would have run aground.
Three hundred years earlier navigation was even more of a hit-or-miss
business. Quite literally.
Post by Tom PotterAnd how accurately could one do this today
using a digital, zoom lens, camera?
You mean by some process of photogrammetry?
<http://en.wikipedia.org/w/index.php?title=Photogrammetry&oldid=364068913>
I doubt if a 'zoom compact' is built or calibrated with enough accuracy
for the exercise to be useful. A camera with a lens of precisely known
focal length, and producing a large photochemical negative or transparency,
might produce an image of sufficient accuracy, but it would be a laborious
process. Quite apart from the difficulty of recording enough light to
register a star, from the deck of a ship at sea.
That's probably why compass, sextant, almanac, and chronometer, are still
the preferred method if GPS can't be relied on ;))
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-- ^^^^^^^^^^
-- Whiskers
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