# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

Message:αβγ
Message:abc
 Add Images & Files Posting Code: Name: Email:
Re: LOP's without DR Position
From: Hewitt Schlereth
Date: 2015 Feb 24, 11:44 -0800
Henry, you reminded me that I did a very high altitude sight of the sun during a delivery trip New York-Bermuda- BVI. Got an Ho of 89* 40' and drew a 20' circle around the GP.
It was the only one I ever had a chance at and it's on pages 71-72 of Celestial Navigation in a Nutshell.

Since last fall I've been giving a course at the Maritime Institute here in San Diego on latitude and longitude by the Noon sight. Early on, several students asked me if there was a less cumbersome term than"AM-PM sights each side of noon" and we've come to call them "bracketing sights."

Formally the course is Celestial Navigation for Cruisers, but there is a tendency for students to call it "last ditch celestial navigation. I.e., "for when the GPS failz."  :-)

Hewitt

On Feb 23, 2015, at 9:26 PM, Henry Halboth <NoReply_Halboth@fer3.com> wrote:

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 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
Conanicut Island USA

View and reply to this message

View and reply to this message

Browse Files

Drop Files

### Join NavList

 Name: (please, no nicknames or handles) Email:
 Do you want to receive all group messages by email? Yes No
You can also join by posting. Your first on-topic post automatically makes you a member.

### Posting Code

Enter the email address associated with your NavList messages. Your posting code will be emailed to you immediately.
 Email:

### Email Settings

 Posting Code:

### Custom Index

 Subject: Author: Start date: (yyyymm dd) End date: (yyyymm dd)