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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: 60° Pendulum Astrolabe
From: Geoffrey Kolbe
Date: 2016 Jul 22, 06:54 +0100
From: Geoffrey Kolbe
Date: 2016 Jul 22, 06:54 +0100
There is a nice article here about the use of this type of instrument in WWII in general and about the 'Geodet' instrument used by the US forces in particular.
The article mentions the use of a stopwatch to determine the moment of coincidence of the two star images (reflected and direct). On the basis that a timing accuracy of around +/- 0.1 seconds is achievable with a manual stopwatch, then if 4 seconds is equivalent to a nautical mile (1852 metres), that implies an accuracy in longitude of around +/- 50 metres might be the best to be expected.
As for the accuracy of a *pendulum* astrolabe, I think it is fair to say that *all* modern levels (as used in surveying) and good reasonably modern theodolites use a pendulum to achieve a 'levelness' accuracy of around 0.2 seconds of arc. You just have to get the instrument level to within a few minutes of arc and the pendulum does the rest. If one minute of arc is equivalent to a nautical mile, then 0.2 seconds of arc is equivalent to about 6 metres, or 20 feet. Compared to the problems in determining the time of image coincidence, the inaccuracy of the pendulum mirror would not seem significant.
Geoffrey Kolbe
