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From: Bill (no email)
Date: Mon Jul 05 2004 - 18:16:59 EDT
Thank you for sharing your knowledge. Some of it was a bit advanced for a
would-be practitioner that is still working his way towards beginner. <G>
Thanks to your collective help, I think I have the Sun bubble-horizon
observation under control and the suggestions given pass my common sense
test.
The moon is still a bit of a puzzle to me, despite your collective efforts.
Following up on the Nautical Almanac's instructions (below) and George
Huxtable's advise, "The Moon has an immense parallax of up to a degree,
which can't possibly be neglected!" my question is:
If using the almanac's method, should the Moon Altitude Correction be
factored in for a Moon-center bubble-horizon observation as well as the
average of the HP value? The second paragraph below is not clear (in my
mind) about that.
NAUTICAL ALMANAC INSTRUCTIONS
The correction is in two parts; the first correction is taken from the upper
part of the table with argument apparent altitude, and the second from the
lower part, with argument HP, in the same column as that from which the
first correction was taken. Separate corrections are given in the lower part
for lower (L) and upper(U) limbs. All 30' degrees is to be subtracted from
the altitude of the upper limb.
For bubble sextant observations ignore dip, take the mean of upper and lower
limb corrections and subtract 15'from the altitude.
RELATED QUESTIONS
It appears to me that the tables favor LL Sun/Moon observations and then
adjust to give the center of the body. I note the -30' (approx. 2X SD)
correction for UL Moon observations. So the almanac's -15' for a center
observation makes sense in that light. Howell's suggestion that you have
already observed the center so no corrections are need except for parallax
(HP average) and refraction also make sense. Guess it all depends on the
table's reference point. While updated many times the Howell original was
published in 1979 and uses 1976 tables.
Another puzzle to me are the Moon's Altitude and HP (parallax?)correction
tables. For the planets, Sun and stars refraction correction gets lower as
the body approaches the observer's zenith, ranging from 38' to 0'. Howell
states parallax also declines as the Moon moves towards the observers
zenith. She states the change in semidiameter of the Moon is only 0.3' from
horizon to zenith, and the range of possible semidiameters is 14.7' to
16.8'. That above makes sense to me. But in the moon altitude correction
tables we start with a 34.5' correction on the horizon, climb to maximum of
62.8' at approx. 15 degrees, and then move down to 10.9' at 90 degrees.
What is lumped into this correction that causes it to be so large and behave
as it does?
I apologize if the level of my questions on celestial navigation are an
order or two of magnitude below the rather amazing level of knowledge the
members of this forum demonstrate. However, I assume you all had to start
somewhere and a kind soul or two helped you to make sense out of what
appeared to be magic at the time.
Thanks
Bill
FOR WHAT IT'S WORTH
Believe I am developing a feel for the lack of precision with the bubble
horizon, both academically and experientially. I am able to cheat a bit
here with photo gear. Level up a tripod head, mount the sextant to the head
with an articulated arm and clamp, and mount a small LED to light the bubble
on another articulated arm. I can also ensure the sextant is perpendicular
to the horizon. (Swinging the sextant to arc the body on the horizon line
while handholding AND keeping the bubble level is an art form I have yet to
master.) A Rube Goldberg transit if you will. By selecting a star or planet
in the 30 to 60 degree range and noting its Hc for my time and position from
Omar's site, and then working backwards from Ho to Hs, I can preset the
sextant and make adjustments to the bubble level. By adjusting the level so
the bubble end aligns with one of the limiting lines I can get very good
repeatability. Testing it without the bubble by measuring angles of distant
objects with known heights at known distances and doing the trig, I find its
accuracy to be *much* better than the designer's claim of a possible 8'.
Closer to 3' with IC accounted for. Best to date: within 5 ft of the stated
height of a 10-story building. (Better lucky than good!) So I have a pretty
good feel for the slop this bubble introduces.
Once I have the instrument calibrated it gives me feedback on various
stances and positions that I am experimenting with for handholding. Even
though the cardboard model is no match for the real deal, it is teaching me
a great deal about stable shooting positions, and turning a mental exercise
(Let me think. The mirror image is too high so I need to move the index arm
forward to increase the angle the sextant sees therefore lowering the
image...) into a reflex action. If I had to stop and think about which way
to turn the focusing ring on a camera lens to follow focus as subjects move
toward or away from me, I doubt I would have many shots in focus.
I understand that some of the minor corrections are superfluous under my
conditions, given the accuracy of the sextant and the lack of precision of
my bubble horizon, but will be good to know in the long run. With a large
band of sailing buddies, it is not out of the realm of the possible that one
day a sextant with a $900 adjustable/lighted bubble horizon will find its
way into my sweaty little palms.
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