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    Re: lunars hard to shoot?
    From: George Huxtable
    Date: 2000 Sep 07, 3:59 PM

    Cal Herzog  wrote-
    
    
    >Lunars are definitely a bigger challenge than shooting a horizon, but not
    >because of the brightness of the bodies. As you surmised, Paul, shades
    >take care of that pretty easily.
    >
    >I've shot a few lunars and I've found the biggest challenge is keeping
    >the bodies lined up accurately while I shoot -- particularly in a rolling
    >sea. The wider the angle, the more difficult it is. I shot a couple with
    >angles of well more than 100 degrees, with the rig of the ship in
    >between. Those are difficult.
    >
    >Assuming you have a sextant with shades on both the index mirror and
    >horizon glass, it doesn't matter which body is viewed through the
    >telescope and which is viewed through the index mirror. My decision
    >usually depends on which choice requires less of a contortionist act from
    >me. Lying on my back on deck is often the best way to get it. I try to
    >shoot the moon's lower limb (that is, the one closest to the other body).
    >Depending on the azimuth to the other body and the phase of the moon, you
    >may need the further limb. When it's possible, the sun and the moon make
    >a good combination, and it's probably the easiest to shoot.
    >
    >I use a modern whole horizon sextant with shade glasses for both the
    >index mirror and horizon glass. I've never shot lunars with a traditional
    >octant, but I hope to. I suspect the tools available in the mid-18th
    >century made this a more difficult task, although I've seen ingenious
    >solutions from the time that I would love to try out.
    >
    >I should also point out that all my experience doing this has been on
    >large sail training ships -- schooners and square rigs of 100 ft. or
    >more. I've never bothered to try it on small yachts, where I'm certain
    >the impact of the seas would make it even more difficult.
    >
    >I've been pretty happy with some of my sights, but I confess that more
    >than a couple have come out utter nonsense. I haven't done them regularly
    >enough for the paperwork to become second nature, so some of my errors
    >could have been there.
    >
    >-- Carl Herzog
    
    ==================================
    
    My response to Carl's interesting contribution is this-
    
    I hope he will let us know how he gets on when he has the chance to shoot
    lunars with a traditional octant.
    
    The original quadrant (invented by Hadley and simultaneously by the
    American Godfrey) was so named because it could measure an angle of a
    quarter of a circle (90 degrees), which was insufficient for many lunar
    measurements. The instrument was later known as an octant, because its
    scale subtended an angle of an eighth of a circle (45 degrees), but this
    was a change in name only. To allow a greater lunar distance to be
    measured, the sextant was later developed (for angles in the sky up to 120
    degrees) and then the quintant, as advocated by Lecky (angles to 144
    degrees).
    
    What I find specially interesting about the Hadley quadrant is this- In
    many books on the history of navigation, the Hadley quadrant is shown,
    either as a photo, an old engraving, or a modern diagram. In all cases, a
    second peep-hole and horizon mirror is shown, placed below the normal line
    of sight and facing in the opposite direction to the "normal" horizon
    sight-line, with the second horizon mirror tilted to look up at the index
    mirror. Yet in none of these modern texts that I am aware of will you find
    any description, or even any mention, of this second sight-line.
    
    What it's for is to observe a horizon in the opposite direction to the
    azimuth of a celestial body such as the Sun. The observer turns his back on
    the Sun, which he views via the index mirror which can see light from the
    Sun arriving over his head, owing to the great height of the quadrant
    (typically 18 to 20 inches radius) and the low placing of this second sight
    line. By making two measurements, of the altitude of the Sun above the two
    horizons, fore and aft, the dip angle could be corrected out.
    
    It would, however, be quite a complicated matter to achieve a proper zero
    adjustment of the index relative to both forward and backward measurements.
    
    The second sight line could also be used to measure lunar distances which
    exceeded 90 degrees.
    
    Such uses of the quadrant are described, in excruciating detail, in a
    pamphlet by William Ludlam "Directions for the use of Hadley's Quadrant",
    of which I have seen the 1790 edition.
    
    When smaller versions of the octant, and then the sextant, were introduced,
    it was no longer possible for the index mirror to view over the observer's
    head, and the advantage of the second sight-line was lost forever.
    
    George Huxtable
    
    
    
    
    ------------------------------
    
    george---.u-net.com
    George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    Tel. 01865 820222 or (int.) +44 1865 820222.
    ------------------------------
    

       
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