A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2015 Feb 23, 18:43 -0800
The traditional noon sight for latitude is a simple example of what Antoine is talking about. If I observe the altitude of the Sun at local noon, in just a few short steps I can calculate my latitude without prior knowledge of it:
- correct the sight for the usual IC, dip, refraction, SD,
- subtract from 90° to convert to zenith distance,
- get the Sun's declination for the approximate GMT (within an hour is normally close enough),
- then add (with appropriate signs) to get latitude: Lat = ZD + Dec.
The point here is that you get latitude with zero a priori knowledge of your position. Contrast this with the modern sight reduction of the same sight. Even for a noon Sun sight, you must select a comparison position (it's an "AP" and it may also be a "DR" position). In the intercept approach, we calculate what the altitude ought to be from this comparison position, and then we subtract our observed altitude from it. The difference is the intercept: the distance from the comparison point to the LOP that includes our actual position. In the first case, the traditional noon Sun latitude calculation, we get the positional information directly by calculating from the observed altitude. In the other case, the intercept method, we get positional information by comparing our observed altitude with a simulation --an altitude calculated to be correct from some arbitrary location which must be relatively close to our actual position in order to be useful.
Here's a puzzler: by this classification, is the traditional time sight, and by extension the Sumner line, in the former category, with no knowledge of position, or the latter, with knowledge of position essential? Note that this is just a matter of semantics and "botany". The sight and the algorithm do what they do, and it doesn't change anything where we put it in our collection of potted plants.
Conanicut Island USA