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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 :
    
    http://navlist.googlegroups.com/attach/c09c132c9a92fad1/HO+249+extracts+.pdf?view=1&part=4&hl=en
    
    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 :
    
    
    
    http://groups.google.com/group/NavList/browse_thread/thread/a270bc3d6aeb66d4/a86eed546f8313d7?hl=en&lnk=gst&q=celestial+up+in+the+air#a86eed546f8313d7
    
    
    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
    >
    > W, Van BC Canada
    >
    > � _____ �
    >
    > 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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