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    Re: Practical difficulties
    From: Lu Abel
    Date: 2011 May 13, 14:18 -0700
    Welcome to the real world!   Unfortunately, too many "I'd never trust GPS, give me my good old sextant" advocates seem to lack experience in the real world.   Too many books about celestial also assume perfectly clear days and perfectly stable platforms.

    Or, as a friend of mine puts it so succinctly:  "In theory, there's no difference between theory and practice, in practice, there often is."


    From: Patrick Goold <goold@vwc.edu>
    To: navlist <NavList@fer3.com>
    Sent: Fri, May 13, 2011 9:46:14 AM
    Subject: [NavList] Practical difficulties

    I found recently that there is one obstacle to using celestial navigation that is not mentioned in the books I have.


    Early this week I was making a two-day jaunt across the Chesapeake Bay, the first of the season, on my 27 foot yawl, Restless.  I hoped to get my first real sights taken from a small boat underway.  There was a good chance that I would have a true horizon for noon sights and a good long fetch for morning and afternoon.


    In the event I got a good feel for some of the practical difficulties of celestial navigation for an ordinary person on a small boat.

    (1)        cloud cover made morning and afternoon sights impossible on the first day.

    (2)        The first day noon sight had to be taken quickly through a small window in the clouds but several things made this difficult.  (A) We were beating into a three foot chop that was very closely spaced.  There was a vigorous pitch to the boat and we were heeling between 15 and 20 degrees.  This made finding perpendicular, getting the sextant aligned with the eye and the sun so that there was an image in the mirror, and getting an accurate shot surprisingly difficult.  All my land practice had not prepared me for this.  (B) Nor had it prepared me for coping with the interference of the rigging and sails.  It took me several tries to find a  place on deck where the sail did not interfere with the reflected image.  The shot I got was well past the estimated local noon and pretty unreliable anyway. 

    (3)        When the cloud window closed and I put away my sextant, I had that slightly queasy feeling that signals the onset of seasickness.   I could not make myself sit down at the nav station and work out even a simple noon sight.  

    (4)        The next day the sky was clear all day.  But now we were on a broad reach with a steep and substantial quartering swell.  The boat was moving quickly through all six degrees of freedom.  I am not especially prone to seasickness but this kind of tack is the one most likely to bring it on.  Even if I have had a few days to get my sea legs (which I hadn't in this case), when the boat is yawing, swaying and surging in addition to pitching, rolling and heaving, I need to keep one eye on the horizon or go green.  The sextant never left the case.  Even had I taken a full set of sights, reducing and plotting them would have been beyond me.


    These were not storm conditions.  It was a rather typical spring day on the Chesapeake Bay.  The wind was NE at 15 knots.  The swell was no more than three or four feet but short and steep. 


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