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

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Re: Chronometer Suggestions
From: Gary LaPook
Date: 2009 Jan 6, 12:48 -0800

```You don't have that exactly right, the LOP moves westward one minute
of arc every four seconds, not one nautical miles. Since the length of
one minute of longitude varies as the cosine of the latitude the
distance the LOP moves also varies by the cosine of the latitude. At
the equator the length of one minute of longitude is one nautical mile
but at 60� latitude it is only one half of a nautical mile.

In addition to this, the amount of change in altitude also varies with
the sine of the azimuth so you have to combine these two factors. Go
to :

which contains two tables called "motion of the body" or "M.O.B."
tables that are used in flight navigation to allow for the motion of
the body. Look at the table for four minutes. Since four minutes of
time is 60 times four seconds of time just divide the tabulated values
by 60 to obtain the change of altitude in four seconds of time. (Or
you can just consideer the tabulated values as seconds of arc.)For
example, the first value listed is 60' for latitude zero and azimuth
90�. Go accross the top line to 60� latitude and you will find that
the change in altitude is 30' exactly one half of the change at the
equator and which would result in a change of 30 NM in the intercept.

Go to my July 30, 2008 post on the "Celestial up in the air" thread
for further explanation at :

gl

On Jan 6, 12:20�pm, "Federico Rossi"
wrote:
> Lu,
>
> If I�ve understood well, this error doesn�t depend on your latitude on
> earth, i.e. it�s a maximum of 1 nm for every 4 seconds (for bodies due east
> or west) whether you are on the equator or far from it, does it?
>
> Federico
>
> Da: NavList@fer3.com [mailto:NavList@fer3.com] Per conto di
> Lu Abel
> Inviato: marted� 6 gennaio 2009 20.05
> A: NavList@fer3.com
> Oggetto: [NavList 6941] Re: Chronometer Suggestions
>
> Irv and Bill:
>
> It's a MAXIMUM of 1 NM for every 4 seconds, not a minimum. �
>
> If the body you're sighting is directly north or south of you, even a fairly
> significant time error would result in a very minimal shift in the LOP
> produced by the body (the extreme example is Polaris). � On the other hand,
> if the body you're sighting is directly east or west, then it's Geographic
> Position is moving by 1 NM every four seconds and any LOP developed from
> that sight would be off by 1 NM for every four seconds of clock error.
>
> Lu Abel
>
> Irv Haworth wrote:
>
> Minimum of 1 NM for every 4 seconds..( a quick answer)..
>
> Irvin F Haworth
>
>
> � _____ �
>
> From: NavList@fer3.com [mailto:NavList@fer3.com] On Behalf
> Of William Sellar
> Sent: January 6, 2009 5:05 AM
> To: NavList@fer3.com
> Subject: [NavList 6931] Re: Chronometer Suggestions
>
> As a beginning celestial navigator, I am wondering how much time and watch
> accuracy is actually required for practical navigation. �Can we predict how
> many miles off one would be for every second of time error?
>
> Bill
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```
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