NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Semi-diameter in the Nautical Almanac
From: Bill B
Date: 2009 Dec 25, 00:18 -0500
From: Bill B
Date: 2009 Dec 25, 00:18 -0500
Frank wrote: > Yep. Anything with clearly defined edges beyond roughly a mile (for minute of > arc accuracy) or ten miles distance (for tenth of a minute accuracy). In fact, > you can use two distinct objects with well-defined edges. This confuses me. My understanding is we use distant objects to minimize the parallax error created by the distance between the horizon and index mirrors. Considering parallax *only* please allow me to build a straw man. -- The object, say a power line, is 1 nautical mile (nm) away from the observer. -- The nominal distance between image placement on the index and horizon mirrors is 4". --For the sake of simplicity, allow me to use the tangent. The tangent of 4" (.3333 feet) / 1 nm (approx 6076.1 feet) = 0.00005486 or 0d 00' 11".3. Given similar triangles, <6 arc seconds at 2 nm. 0d 00' 01".1 at 10 nm. What have I missed? Thanks Bill B -- NavList message boards: www.fer3.com/arc Or post by email to: NavList@fer3.com To , email NavList+@fer3.com
