NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Greg B
Date: 2013 Dec 27, 20:27 -0500
I had no idea they could resolve down to 10' !
I used to have a cross-staff that I made - it was more show than operational,
I used to do a lot of medieval / Renaissance reenacting - now I do mostly Rev. War,
I portray a surveyor in the Huntington Militia, so when I'm not on the canon
crew or demonstrating a musket vs a rifle I'm showing people how to do
period trig math, use a Gunther's Chain, or draw a survey map. I have also
had fun making a period log-line, sounding lead, and my octant. I have even
demo 'ed optics but I find you can't hold peoples attention for more than 5 min.
at a outdoor event.
~Greg
On 12/27/2013 07:30 PM, Frank Reed wrote:
Greg,
You wrote:
"Please help me understand the 'instrument time line'. What is the angular resolution of a cross-staff or back-staff? I always thought it was about +/- a degree"I would say a bit better than that is reasonable. Half a degree is probably normal while +/-10' is possible with a good instrument and a skilled observer. Do you have a cross-staff in your collection of reproduction instruments, Greg?
And you asked the key question:
"and if that's true why would you care about 1 arc minute?"Previously I characterized these tables as "advice for future navigators". You're quite correct that they had no real practical value before the first double-reflecting instruments were widely available, starting around 1740. So that early dip table is far ahead of its time. This is a common story in the history of navigation. You can find advice from well-meaning shorebound mathematicians that greatly exceeds the observational limits of the era. Even today we have to worry about this. But, you know how it goes... you make a lovely table of dip values calculated from some very clever mathematics... how can you resist publishing it?!
-FER
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