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    Re: Suitable Sextants - Mirrors
    From: Herbert Prinz
    Date: 2005 Oct 14, 15:01 -0400

    Yourname Here wrote:
    > here is a link to photographs of page 108, Figure 6-4 of Bauer's " The
    > Sextant Handbook". [...] For those interested, Bauer goes into great
    > detail in the methodology of swining an arc on pages 105-108 which
    > are the same as my own.
    > http://www.villagephotos.com/pubbrowse.asp?folder_id=1485991
    I hate to burst your bubble, but Bruce Bauer has it all wrong, and so do
    you. His figure 6-4 does not show clearly how he is swinging. But he
    tells us on p.106, which you chose not to reproduce in your post:
    "Swinging the arc is also called rocking the sextant and simply means
    rotating the instrument from side to side around the line of sight to
    the horizon."
    I think that there has long been a consensus on this list (at least
    since Frank Reed brought the attention to the subject a while ago) that
    this is the wrong way of doing it. Actually, it's an impossible way of
    doing it (see below).
    Instead of trusting a particular book, let's use common sense. The goal
    of swinging the arc is to find the exact point where the vertical line
    through the star intersects the horizon. If you pick a point on the
    horizon arbitrarily and then start swinging the sextant around the line
    of sight to this point, how do you ever find the correct point??
    That this wrong way does not work is further evidenced by Bauer's own
    claim that he cannot make it work for any other than medium altitudes.
    See p. 108, "One more wrinkle on swinging arcs [...]", full quote on the
    page you posted. Bauer says  "At high altitudes the arc becomes hard to
    manage because of its sharp curve." Not at all. The very problem with a
    star at high altitude is that it is difficult to guess the azimuth
    because the curve is so _flat_! Of course, if you decide on the wrong
    azimuth prematurely,  then face that direction and start swinging the
    arc on a horizontal axis, the star shoots out quickly on eiter side of
    the mirror. Neither will its arc ever touch the horizon - it may
    intersect. Even a Venetian wall mirror on your sextant will not help you
    solve this problem.
    Why Bauer says that swinging does not work for low altitudes, because of
    the arc flattening out, is beyond me. It makes me wonder whether he ever
    swung a sextant. For sufficiently low altitudes one can actually swing
    the object full circle (!) without loosing its view.
    Herbert Prinz

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