# NavList:

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

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Re: Ex-meridian reasoning muddle?
From: Gary LaPook
Date: 2016 Jan 5, 05:35 +0000
Yes but here is nothing magical about having latitude, it is just another LOP that happens tor run straight east and west. to get a fix you still need to cross it with another LOP (perhaps advanced from the morning) and the east-west LOP is no more accurate or easy to use than any other LOP.

I think the reason for the ex-meridian process is that it dates from the days before LOPs in which latitude and longitude were determined separately and in the navigator's mind were also kept separately.

Just treat your sun shot near noon as a normal LOP shot. Any shot taken within two minutes of noon will produce the exact same Hc since the LHA of the sun will be the same whole number of degrees (since the AP will be within 30' of longitude of your DR) when you enter HO 249 or HO 229 or HO 214 or HO 218 or HO 208 or Weems LOP book. The Zn will always be 180 (or 360.)  Even if you take the shot more than two minutes from noon (within reason) the resulting LOP will accurate enough for practical navigation, any inaccuracy lost in the noise of the observation.

gl

To: garylapook@pacbell.net
Sent: Monday, January 4, 2016 8:09 PM
Subject: [NavList] Re: Ex-meridian reasoning muddle?

Mark,
The ex-meridian needs knowledge of longitude as you say. So then what happens if the DR longitude is out by lets say 15' then how will this affect the ex-meridian correction. Example: Lat. 34° N, Dec. 22° S, LHA t 6°, and LHA t 6.25°
[1.964(.8290)(.9272)/.8290](.267)(6)(6) = 17.5' correction added to Ho
[1.964(.8290)(.9272)/.8290](.267)(6.25)(6.25) =  19' correction added to Ho
A difference of 1.5' only. Not much to worry about.
The benefit of the ex-meridian is that it is easier to calculate than the intercept method and gives latitude directly without the need of plotting.
Greg Rudzinski
Date: 2016 Jan 4, 18:36 -0800
I had managed to miss the ex-meridian exercise in my past midday shots education.  I had learned a meridian shot is a nice way to get a good latitude and a so-so longitude without any sight reduction pain.
I understand the ex-meridian concept is if you miss your meridian averaging shots or "on the money” peak meridian shot, it gets you back to square one with a little mathematical hocus pocus.
The only thing is that bothers me is we are trying to get back to an accurate latitude using my estimated longitude to get LHA. In modern times with my accurate chronometers, if I have confidence in my longitude...I probably already have similar confidence in my latitude....so why not just take that sight as a LOP , bring up the morning LOP  if available on the DR track, then shoot another in the afternoon, and have a 3 sight St Hilaire party with advanced DR’d LOP’s.
If we are doing this in olden days with more suspect chronometers..and thus suspect longitude....pre St Hilaire, wouldn't I play the safe game and use my tools to zero up the more sure latitude rather than use possibly tweaked longitude to calculate?
I mean latitude I might catch from other star or planet bodies. I understand St Hilaire wasn't around yet....If I understand correctly Sumner got a lat based LOP in 1848 by using three latitudes...I assume because he was more certain of Lat than Lon.
With the ex-meridian I feel like I am using a single suspect variable to calculate a suspect variable, rather than using multiple LOP’s to slowly reign in all the suspectness........if you get what I mean...
This probably means I missed some subtle mathematical elegance or the point completely.... I am trying to figure out when this is the right thing to do.
MC
PS I have my Fathers slide rule he used for navigation at the USCG.  It is an intimidating looking Pickett N4-T, vector type LOG LOG.  I remember him zooming through Nav problems with it when I was young in the 60's and early 70's. I always marveled at his ability to do seat of the pants navigation in his head.  He  told me it was because he grew up with the slide rule and manual calculation (he also had a near photographic memory).  I treasure it for sentimental reasons, and would secretly like to learn to use it someday.  I grew up at the very end of the slide rule era, and switched to the calculator too young to remember the slide rule much.

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