NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Brad Morris
Date: 2014 Apr 4, 12:57 -0400
Hello Greg
You asked about the sensitivity of the observation.
It depends on what you intend to do with the result.
Are you attempting an ocean crossing on a latitude line? Then a minute or so in time either way won't make any real difference, as the objective is to be following a latitude either to the south or north of your intended destination. Hit land and make the appropriate turn to the north or south and follow the coast to your destination.
If on the other hand you are attempting map making (like determining the latitude of a harbor within 5" of arc) then the exact time of meridian passage to the second is critical if you intend to use raw data. Knowing your longitude, you can calculate the exact moment of upper or lower meridian crossing.
Since computation is so inexpensive these days, you can see the sensitivity yourself directly. Manipulate time, longitude and latitude at the USNO website and observe the change. There are also a bunch of programs and spreadsheets here that also perform this task, created by others here on the list.
I'm sorry if this isn't a crystal clear answer! The issue is that your question isn't quite succinct. A better way to ask it is: Given a latitude tolerance (being within X seconds of arc of true latitude) what is my permitted tolerance in time from meridian crossing? Then there is a mathematical transformation which yields a direct result!
Brad
All,
I believe you have misunderstood my 2ed question, which had nothing to do
with the first Ho bias question ( but THANK YOU - lots of good info & insight )
In attempting to recreate the techniques of Lewis & Clark,
I am trying to understand how they measured latitude.
It is my understanding that when it was impractical or impossible to do a noon sun
sight they used some of the brighter navigational stars at meridian passage.
I know they did not use a Hg AH, but a mirror leveled with a spirit level.
What I don't know is how critical is meridian passage, angle wise;
i.e. ( 180 degrees +/- ? degrees is acceptable? what order magnitude is it?)
How did they know when they had meridian passage? On the surface, It seems
it would be the same as trying to do noon sun for latitude - but I get the feeling
there was more to it.
Thanks,
~Greg
On 04/04/2014 04:31 AM, Luc Van den Borre wrote:
On 4/04/2014 3:11, Brad Morris wrote: > Anon, like me, can see no use for the up arrangement of the pentaprism. > I still don't understand why Nicolas would do that. He's a bright > fellow, I'm sure he had a reason. It's a surveying sextant, meant to be used horizontally. If a sextant's normal range is -5° to 125°, the range with the pentaprism 'pointing down' will be 85° to 215°, so if the prism were fixed you wouldn't be able to measure angles smaller than 85° without unscrewing the prism housing from the frame. With the pentaprism in the other position the range will be -95° to 35°, returning the ability to measure angles smaller than 85°. Both the C&P and Freiberger pentaprism attachments can be rotated 90°, of course: http://www.cassens-plath.de/katalog/index.html?startpage=90 http://www.fpm.de/index.php?c=1&s=pentaprisma Luc
