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I don't want to get involved in the L & C Lunars discussion, but in
response to Bruce's posting I did get down my 15" vernier octant to check
how the degrees were marked off. The arc is numbered in 10 degree
intervals, i.e., 0 - 10 - 20, etc, with the interim 5 degrees designated
only by an arrow. My octant probably dates to the mid 1800's, so may not
be representative of that used by L & C.

I do not recall the date of the L & C observations in question, and am
otherwise ignorant of the moon phase at the time, as well as the apparent
error involved. However, for what it's worth, one error sometimes made
near the full of the moon is to inadvertently measure the distance to the
wrong limb when the moon appears full but actually isn't - old pubs have
a set of rules to avoid making this mistake which was apparently not so
uncommon as one would think.

I have also come across a graphical method of clearing the distance, for
which some degree of accuracy is claimed, and which is advocated as a
check against other calculations. This graphical solution was being
touted circa 1822 in Arnold's Lunarian - I really don't know or remember
if you have actual calculations made by L & C.

On Wed, 21 Apr 2004 21:06:13 EDT Bruce Stark  writes:
> You may have found it, Ken.
>
> You wrote: "But it's just way too close to
> the moon; 15�, in fact. Exactly 15�. Now I can't quite figure out
> what error would lead to the captains recording distances that were
> off by 15�, but for some reason the fact that there are no minutes
> or
> seconds in the error makes me think it's possible."
>
> The vernier on Lewis's sextant read to 15" of arc, so I'm almost
> certain the
> numbers on the arc would have been at 15� intervals. Also, the index
> mark of
> the vernier was almost certainly near the right edge of the "window
> frame" of
> the index arm. The 30� or 45� engraving they should have been
> reading the
> additional degrees, minutes and seconds from would have been outside
> the "window
> frame." Inside the frame, where the serious reading was done, 60�
> would have
> appeared.
>
> Perhaps, given the difficulty of reading a vernier sextant by
> firelight, or
> candle lantern, they picked up on the 60�, and counted from there.
>
> Does anyone have a 15" vernier sextant to test this theory with?
>
> Bruce

   

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