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Frederick V. Hebard, PhD                      Email: mailto:Fred@acf.org
Staff Pathologist, Meadowview Research Farms  Web: http://www.acf.org
American Chestnut Foundation                  Phone: (276) 944-4631
14005 Glenbrook Ave.                          Fax: (276) 944-0934
Meadowview, VA 24361
On Aug 26, 2005, at 10:31 AM, george huxtable wrote:
> However, Fred expanded the subject to longitude determination, in a
> later
> posting-
>
>> Oops.  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.
>
>> to which Bill replied-
>
>> 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.
>
> and Fred added-
>
>> 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.
>
> That's an important difference. Because their telescopes could be
> arranged
> to swing precisely in a North-South plane, about an East-West axis,
> meridian passage could be timed very precisely, as an object passed the
> crosswires, travelling sideways, without recourse to observing
> altitude or
> seeing the horizon. So transits at stars could be accurately observed
> at
> any time of night. Indeed, at Greenwich, a special telescope was fixed
> immovably to look only at Sirius, as it passed, day (Sirius being so
> bright) or night, for regulating the observatory's clocks. All that was
> needed was a clear sky at the right moment.
>
> It's different for us at sea,


George,

I was referring to land-based observations when mentioning longitude by
meridian passage in the excerpt quoted above, which you explained very
nicely, also explaining very nicely the difficulties at sea.  In this
post, I'm returning to the land-based observations, repeating the
following question:

How did astronomers arrange their telescopes to swing precisely in a
North-South plane?  What technique did they use to achieve the
alignment?  I know it's off topic, although similar technique also was
used by surveyors, who might be regarded as navigators on land, to
bring it slightly back on topic.

Thanks,

Fred

   

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