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    Re: A Practical Nav Problem
    From: David F. McCune
    Date: 2006 May 24, 10:47 -0500


    > Historically, navigators did not mind getting latitude  when the
    > Sun was very
    > close to the zenith. In the logbooks they even seem to get  a
    > kick out of it.
    > I think the modern aversion to high altitude sights is due to  a specific
    > method of "swinging the arc" which became popular around 1940
    > +/-10  years.
    > There's really no reason why a navigator should not be able to
    > get good  altitudes
    > even when the object observed is close to 90 degrees  altitude.

    Your comment got me to dig out my old log book from a trip to from
    California to Hawaii in June/July. In the period around June 28 to July 3 I
    was at 24d N (heading WSW) and the sun's declination was just over 23d N.
    So I had a zenith angle of less than a degree.  Indeed, on July 1 I recorded
    the sun's meridian passage at a zenith angle of 00d 00.0'.  (I do remeber
    thinking that was pretty cool!  Though there's not a single note to that
    coolness in my log.  I think I was tired and lonely by that time and just
    wanted to get on with the sailing, so the log just show's the numbers.) I
    did several sights each day, using the sun, moon, planets and stars.
    Looking back on the plotting sheets for those days, I seem to have had no
    trouble with my latitude.  On the other hand, I have a memory of being
    troubled.  But I can't really tell you why.  So it seems you are right.
    Somehow anxiety crept into my navigation when the zenith angle was less than
    a couple of degrees, but that anxiety does not seem to have been supported
    by the facts.  And anyway, on July 4 I found the Alenuihaha Passage right
    where it was supposed to be.

    (On reflection, I think some of my anxiety came from being a bit unsure how
    to plot those sun sights just around noon when the sun's GP was so close to
    my own.  After all, the LOP curves noticeably when the GP is just a degree
    or so away.  I vaguely remeber thinking I could draw the GP and then draw an
    arc.  But looking at the plotting sheets, I just drew straight LOPs,
    including straight E/W LOPs for the meridian passage sights, and went about
    my business.)

    So I suppose you are right.  My concern about the sun's declination being a
    few degrees from my latitude is unjustified.

    What is the "specific method" of swinging the arc that came into practice
    around 1940?  Prior to that, was there a different or better technique for
    assuring perpendicularity?


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