# NavList:

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

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Re: When did "time sights" fade away?
From: Peter Hakel
Date: 2011 Jul 30, 15:41 -0700
Using the notation:
a: intercept distance  (a > 0 is Toward,  a < 0 is Away)
Zn: azimuth toward GP

LOP intersects AP latitude:      a / sin(Zn) from AP.
LOP intersects AP longitude:  a / cos(Zn) from AP.

For LOP orientations close to the cardinal directions one of these quantities becomes ill-defined but that poses no practical problem.

In the attached example I plotted the LOP across the two computed intersections first; I marked the three relevant azimuths and the intercept distance after the fact as a consistency check.

Peter Hakel

To: NavList@fer3.com
Sent: Monday, July 18, 2011 2:50 PM
Subject: [NavList] Re: When did "time sights" fade away?

Thanks Henry - beautifully put.
With me the concept has certainly not faded away, I invariably use it in the form of 'Longitude By Chronometer' with an Azimuth. Such a sight reduction produces an LOP more easily plotted than any intercept solution because the navigator can choose a Latitude, mark off on it a single computed Longitude, and draw the LOP through the tick mark using the computed Azimuth to align the plotter. Less marks on the chart/plotting sheet, just one line to draw, and much easier to do bouncing about on a 32 ft yacht.
Aside, I use the complementary method of my solution for what might be called an 'Ex Meridian Latitude' with an Azimuth, having chosen an easy Longitude for the single tick mark required for plotting the LOP.
Needless to say I've got it all running on low power IT hardware with my own software, so the sight reduction and plotting is a piece of cake to do on the yacht for any sight.
When I enquired about commercial software that could produce a Longitude by Chronometer and Azimuth sight reduction from of a well known broadline US Celnav specialist, there was none to be had. I did get from the other end of the phone an audible intake of breath of the type normally used in responding to mad people.
One senior member of the list has seen and used the software. He couldn't get out his rut that Long By Chron was intrinsically inaccurate, a notion that you have elegantly demolished: thankyou again Henry, the person concerned wouldn't or couldn't take the refutation from me.
I showed the software to a local Celnav instructor with his own practical sailing school - his response was that he 'quite liked to see a spider's web of lines on the chart'.
Having chipped in to the list once before to mention the ease of plotting arising from the Long By Chron and Azimuth method without a single response from the rest of the list, I'm happy to leave them to their 'not made here' prejudiced inflexibility. If it makes them feel better to plot more lines than I do - bless 'em all.
Must stop - sailing south tomorrow morning.
Michael
55 North
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