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    Re: Chronometer Suggestions
    From: Gary LaPook
    Date: 2009 Jan 6, 15:45 -0800
    You did that the hard way. All you had to do was multiply 15.041' (since we are talking the rate of change GHA Aries, 15º 02.5' per hour; 15.041º per hour) change of GHA of all the stars shot for one minute of time by the cosine of 49.5º to calculate the distance the fix moved westward of 9.74 NM which is 15' (approx) of longitude at your latitude. Note the longitude changed by 15'.

    This points out a fact that many may have not have noticed. Any stellar fix always gives you your latitude and the time only needs to be accurate to the nearest month or so depending on the level of accuracy you are looking for. You can think of the fix being in terms of latitude and GHA, the GHA doesn't change but the longitude changes in step with the change in GHA of Aires. the only thing that ties the fix to longitude is time which fixes the relationship between GHA Aires and longitude. Every star fix (the sun and moon change declination so this is not true for them) moves westward at the rate of 15º 02.5' (15.041º) per hour but the latitude doesn't change. This may be more obvious if you work with H.O. 249, volume 1 for selected stars.


    --- On Tue, 1/6/09, RBEmerson <pavilion@pinefields.com> wrote:
    From: RBEmerson <pavilion@pinefields.com>
    Subject: [NavList 6951] Re: Chronometer Suggestions
    To: "NavList" <NavList@fer3.com>
    Date: Tuesday, January 6, 2009, 3:03 PM

    For laughs and grins, I ran some fix data from another thread here
    through StarPilot, with and without adding a minute to the timing.
    The raw data is:
    AP 49-16.1 N 123-07.1 W (in British Columbia, Canada)
    All times GMT on 27-Nov-08
    Ic = 31.5' OFF, He = 0.0 ft (or meters ;-) )

    Body WT Hs
    Capella - 03:52:46 - 39-35.7
    Vega - 03:54:45 - 32-06.9
    Altair - 03:56:11 - 22-37.0

    That results in a fix of either
    (Watch Time without change)
    49-30.6 N 122-57.3 W
    (Watch Time + 1 minute)
    49-30.6 N 123-12.3 W

    Note there is no change in latitude, only in longitude. Adding one
    minute gives a fix 9.75 nm west of the un-adjusted fix.


    On Jan 6, 4:32 pm, Alexandre E Eremenko <ereme...@math.purdue.edu>
    > Bill,
    > Of course. 1 sec of time = 15" =0'2.
    > (Same accuracy as a good sextant
    > measurement).
    > Alex.
    > On Tue, 6 Jan 2009, William Sellar wrote:
    > > 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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