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George:

Wonder if you've been smokin' something :-))

OK, I rose to your challenge.  I've got three top-notch sextants here: two
Plaths and a Tamaya, all with 4x scopes.

Focused on the white siding of a house about 200' away (very nice and distinct
horizontal lines).

Results:

Tamaya (Spica):  same readings exactly, right side up and upside down; I only
did the test twice, then figured I'd quit while ahead :-)

Plath#1 (Classic):  minor differences in a number of readings, never more that
1.5'.  There appeared to be no difference in right side up or upside down
reading repeatability errors, i.e., I could not find any persistent error
between the two positions of the sextant.

Plath#2 (Weems & Plath Classic):  again, very minor differences over a
number of
readings, without any repeatable error between sextant positions.  Errors never
exceeded 1.0'

Conclusions:

1.  my eyes aren't as good as they used to be (at age 62, whatdya want?);
2.  the minor measurement errors I was seeing with the Plaths were due to:

a. my own eyesight limitations;
b. the target line not being as fine as it could have been; and
c. possibly, the weight of the sextants...the Plaths are much heavier than the
Tamaya and maybe I wobbled a bit.

Perspective:  Some years ago I did a lot of sextant tests standing at the
waterline on a beach in Morocco using my Plath Classic sextant with a 6X scope
("a lot" being hundreds of shots taken over a couple of months...I lived near
the beach).  I was shooting the sun in the afternoons, LL.  I was able to get
repeated (plotted) sun lines with less than 0.5' variation from one another,
and sometimes as good as 0.2'.  I never remember an error greater than 1.5'.
At sea, of course, you can only dream about this kind of accuracy!

Soooo, I'd bet that the differences you're seeing with the plastic sextant
aren't due to gravity at all, but to other measurement errors (including human
and sextant errors).

But, in the larger scheme of things, what does it matter anyway?  Most of us
still take celestial sights from a vertical position!

Cheers,

Bill
S/V Born Free

At 12:38 PM 4/21/2001 +0100, you wrote:
>UPSIDE-DOWN SEXTANT TEST.
>
>I wonder if any of you owners of expensive metal sextants would like to
>participate in a simple little experiment.
>
>Look at the horizon through your sextant, preferably from a stable
>platform, using as much magnification as you can. If you're on land, a
>rooftop or hilltop or some other convenient horizontal target will do just
>as well. It doesn't matter whether it's distant or local. Nor does it
>matter whether the target is in a horizontal direction from you, but it
>shouldn't be higher than a few degrees. Avoid viewing through window-glass.
>Now adjust the screw until the two images of the target or horizon are
>aligned, just as you would when checking the index error. Now record the
>sextant reading, as precisely as you can. It will be somewhere close to
>zero degrees. For nearby objects, it will normally be a negative reading
>(i.e. off the arc).
>
>Next, standing at the same spot, invert the sextant, and make the same
>measurement again. Make sure that the final adjustment is made with the
>screw moving in the same direction each time (for me, always clockwise).
>For ultimate accuracy (though it won't make a lot of difference), for the
>inverted measurement, stand on a book of height h, where h is the vertical
>offset of your sextant from the telescope line to the pivot point. Volume 1
>of the 1977 Bowditch is just thick enough to match my sextant perfectly!
>This will put the two sightlines of the inverted sextant into exactly the
>same place as they were before, but interchanged.
>
>In theory, the two readings should be exactly the same. But a sextant will
>flex slightly, on account of the gravity stresses due to its own weight,
>which reverse with respect to the sextant body when it is inverted. In
>particular, a slight flexure of the mirror mountings may occur, or perhaps
>the whole frame or the index arm may flex slightly. It's not obvious, to
>me, which way the readings would be expected to change, and by how much.
>But the more rigid the sextant, the less the change should be, and such a
>test may be a useful way to evaluate this aspect of an instrument and
>testing for any looseness before taking it out of a shop.
>
>All I have to go on here is my cheap Ebbco plastic sextant. A plastic frame
>is of course much less rigid than a metal one, but on the other hand the
>weights involved are considerably less. When I check this against a nearby
>roof ridge, I find that because of the simple nature of the Ebbco (and the
>limited resolution of the ageing human eye) there's a scatter in each
>observation of plus-or-minus 1 minute or so. It's then necessary to average
>10 observations. In the normal orientation I get a mean-of-ten reading of -
>23.2  minutes. That is, 23.2 minutes away from xero, off the arc. With the
>sextant inverted, the average is -24.6, 1.4 minutes more negative. Not a
>big change, but a measurable one.
>
>If any list member with a more exotic sextant is prepared to make a similar
>measurement, I would be most interested to learn the result, and whether
>the difference is large enough to be measurable..
>
>Why am I interested in the upside-down behaviour of a sextant? Well, it's
>connected with an instrument for measuring the observed dip of the horizon
>at sea, the Blish prism. And inversion of the whole sextant-prism
>combination seems likely to provide a simple way of finding any zero error
>in the instrument, so that an absolute value of dip can be obtained. Any
>flexure of the sextant, on being inverted, would complicate this process
>and need to be corrected.
>
>More about the Blish prism will follow shortly.
>
>George Huxtable
>
>------------------------------
>
>george@huxtable.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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