NavList:

Frank, you said ......

"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:" ...........

And, if you took a morning or afternoon sight and ran the noon Latitide determined back or forward to the time of this sight and used this Latitude in a conventional time sight calculation you would determine a posion without any prior knowledge thereof - the so called "back and fill" method of which i have previously written - only requirement being the keeping oa a relatively accurate DR between the times of taking the sights. 

Also, if you have a means of accurately determining time, equal altitudes before and after noon can be used to determine the actual time of noon, at which time the sun' s GHA mut equal the vessel's Longitude, and again a position without any prior knowledge thereof .

Another, and practical use of circles of equal altitude may be employed when passing under or nearly under the sun when that body be at very high altitudes - say at least 88d 30m, when circles of equal alitude may be plotted on the average chart or plotting sheet. I have had several opportunities to employ this method on the  African Coast with reasonably good results - the difficulty being in chasing the sun around the horizon with the associated rapid change in azimuth and consequent inteference of ship structures

Admittedly, these are relatively simple solutions, however, they do prove that there is more than one way "to skin a cat".

Henry 

On Mon, Feb 23, 2015 at 9:56 PM, Frank Reed <NoReply_FrankReed@fer3.com> wrote:

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.

Frank Reed
ReedNavigation.com
Conanicut Island USA