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On Aug 25, 2005, at 7:02 PM, Bill wrote:

>> Yes.  A principle means of establishing latitude on land, where a
>> plumb
>> bob or spirit level can be used to establish one's "horizon."  At sea,
>> with only a short time at twilight when both the real horizon and
>> stars
>> are visible, tough to find a star undergoing meridian passage (and
>> wasting time better spent obtaining a 3-point or 4-point fix).
>
>> I believe by timing the actual moment of meridian passage,
>> longitude also can be determined, but I'm getting far out of my depth
>> here.
>
> Fred
>
> I think you are in your depth and spot on.  I recall toying with the
> idea,
> and looking for bright stars that would be making their meridian
> passage
> during prime viewing time to discover--as you pointed out--the
> opportunities
> are few and far between.
>
> Bill

Thanks Bill.  Aside from the plumb bob, land-based astronomers also had
the advantage of having accurate and precise pendulum clocks, from the
1500s or 1600s onward.  Additionally, they could mount their telescopes
to swing exactly north south, to observe meridian passage.  The latest
National Geographic has a very nice photo of such a telescope on the
prime meridian at Greenwich.  The telescope is housed in an observatory
with a standard angled roof, whose roof line follows the axis of the
meridian.  The crown of the roof folds back to give the telescope a
view of the skies along the meridian.  Unfortunately, I don't know how
such telescopes were aligned to begin with.

Fred

   

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