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George wrote:

> That's an interesting variation. Presumably, the peep was mounted on the
> right arm of the octant's A-frame, looking across it to the left, rather
> like the normal peep, but somewhat shifted in direction. Can Trevor recall
> whather that was the arrangement? I have never seen, or read about, such an
> octant.


A bit more detail: I was idly passing through the public galleries of
the museum and only paused to look at a fine instrument on display --
when I realized with a shock just what I was seeing and what its
geometry implied. Not being truly familiar with quadrants, I did not
know whether what I now saw was, in fact, normal. Thus, I did not think
to take any special notes. The label in the display case did not suggest
that this quadrant had any special features.

The second peep was just like the primary one but mounted a short way
down the right arm, away from the index mirror and closer to the scale.

> The big problem with any such arrangement, which didn't allow the scale to
> be set to read 0 deg, was in determining the index error. [snip]
> An alternative arrangement, which could be used for the Salem instrument as
> well as the back-observation instrument, would be to measure the
> angle-in-the-sky between two known stars that come within its working
> range, rather like taking a lunar. This angle is, roughly-speaking,
> constant. Any discrepancy, after a complex correction for refractions,
> would be the index error. It would need a lot of astronomical knowledge and
> skill.

> Alternatively, altitudes of an object such as the Sun, when it was in a
> range where it could be measured in either mode (which would be a nice wide
> band of 30 to 90 degrees in the Salem instrument) could be taken
> alternately in the two modes, and a bit of graph-plotting would show the
> index error. [snip]


I don't think matters need have been so complex. The index error could
be determined in the normal way, using the primary peep. Any pair of
stars (or other objects) that subtended an angle of a bit less than 90
degrees at the observer's eye could then be observed through both peeps.
The observed angle through the primary peep, plus or minus its index
error, would be the actual angle subtended at the time (including all
refraction and whatever other factors). The difference between that
angle and whatever was observed through the second peep would be the
index error of that peep.

I'd not want to have to attempt the frequent IC checks needed with a
plastic sextant but the process shouldn't be really demanding.

Or am I missing something?


Trevor Kenchington


--
Trevor J. Kenchington PhD                         Gadus@iStar.ca
Gadus Associates,                                 Office(902) 889-9250
R.R.#1, Musquodoboit Harbour,                     Fax   (902) 889-9251
Nova Scotia  B0J 2L0, CANADA                      Home  (902) 889-3555

                     Science Serving the Fisheries
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