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    Re: A Practical Nav Problem
    From: Greg R_
    Date: 2006 May 24, 18:00 -0500

    Frank wrote: "There are two ways of swinging the arc, and they both  work  well up to about
    50 or 60 degrees altitude."

    Can you possibly  differentiate those 2 methods a little better? From reading your description, I  take it as being pretty much the same procedure (i.e. treat the view through the  sextant as if the celestial body were at the bottom of a pendulum, and swing the  sextant back and forth on the horizontal axis that points to the horizon until  you find the lowest point, which means the sextant is perpendicular to the  horizon). Hope *I've* described that so it makes sense.   ;-)


    ----- Original Message ----
    From: FrankReedCT@aol.com
    To: NavList@fer3.com
    Sent: Wednesday, May 24, 2006 3:39:54 PM
    Subject: [NavList 310] Re: A Practical Nav Problem

    David, you wrote:
    "Indeed, on July 1 I  recorded
    the sun's meridian passage at a zenith angle of 00d 00.0'.  (I  do remember
    thinking that was pretty cool! ...)"

    Sounds pretty cool to  me! Myself, I've never been in tropical latitudes.

    "(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.)"

    It's interesting to contemplate that if  you had noted GMT at the exact
    instant the Sun was overhead, your LOP would have  shrunk down to a dot --a true
    single-observation fix. This sight only works if  you have a fairly good idea
    where the meridian is.

    "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?"

    There are two ways of swinging the arc, and they both  work well up to about
    50 or 60 degrees altitude.
    1) The original method  which you'll find described in books on sextant use
    up until c.1940: you swing  the arc by keeping the Sun (or other body) centered
    in the field of view of the  instrument. The sextant is rotated about the
    axis that points to the Sun. The  horizon sweeps back and forth across the field
    of view.
    2) The later method  which seems to be easier to describe and so it's taught
    more often: you swing  the arc by making the Sun sweep across the field of
    view while the horizon  behind it stays put. In this case the sextant is being
    rotated about the axis  that points to the horizon (same as the axis of the

    The  problem with method 2 is that there is almost no curvature of the Sun's
    path in  the field of view when the Sun's altitude is high. So it really just
    doesn't  work for high altitudes. Many, many modern navigators have mistakenly
    concluded  that high altitudes are worthless for celestial navigation.
    Meanwhile, method 1  always works, but it's a bit harded to describe verbally. This
    is the sort of  thing where a couple of short video files could clear up a lot
    of  confusion.

    42.0N 87.7W, or 41.4N  72.1W.

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