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From: Bill (no email)
Date: Sun Jul 04 2004 - 19:09:16 EDT
BACKGROUND
After attending an excellent seminar by Ken of Celestaire at Chicago's
Strictly Sail show this winter, I decided it was time to quit toying with
the idea and actually add celestial navigation to my bag of tricks.
My first step was to peruse astronomy books and sights and get a feeling for
how SHA/RA, declination, GHA etc. fit together while learning the celestial
bodies. My assumption--if I can't identify it, I can't shoot it. A 2102-D
star finder is on my wish list when I take the show on the water, but for
backyard practice David Chandler's planisphere and the Navigator Star Finder
on Omar Reis's superb web site are more than adequate.
The next step in getting my feet wet was to acquire HO229 sight reduction
tables, a Nautical Almanac, Susan Howell's Practical Celestial Navigation,
and the AstroMedia/Wurzburg cardboard sextant with a bubble horizon from
Celestaire. With a little tweaking (added a horizontal slit over the
eyepiece to stabilize the eye position and bring the horizon line into
focus) and practice I am getting within 15-20' of the reference Hc taken
from Omar's sight. My location is determined by my Garmin GPS 76 and
plugged in along with GMT.
For sight-reduction-form practice I find I can take Omar's HC for the body
and use it as the Ho, and then proceed as usual. If done correctly, the
plotted LOP is dead-nuts on in reference to my plotted "DR" position, so
errors are immediately evident. So far I am encouraged enough with my
progress to consider a real sextant and 2102-D before my summer cruise(s) on
the Great Lakes.
PROBLEM AREAS
Now the other shoe drops. I am observing the center of the sun and moon as
it is easier with the cardboard sextant and bubble horizon and marker line,
especially before I added the slit to bring the marker horizon line into
focus. Given the slop of a bubble horizon and ± 5 to 10 minutes accuracy I
am not able to resolve the issues attendant to using a bubble horizon
empirically; and don't know enough about how parallax, semidiameter, and
refraction are employed to arrive at the Sun and Moon altitude corrections
and HP tabular values, so can't work it out logically.
To add to the confusion, Howell and the instructions in the Nautical Almanac
Moon Corrections page seem to be--for the neophyte--at odds. (Excerpts from
both follow.). Given, Howell is mainly addressing an artificial horizon and
the almanac a bubble horizon, but it strikes me there is something missing.
I am guessing the almanac gives no instructions for bubble horizon
observations of the Sun as they are assuming you are using a real horizon
with the Sun, while you may be using a lighted bubble for the Moon.
I understand why dip would be ignored in either case. Howell suggests
ignoring temp/pressure corrections while the almanac says to include them.
Perhaps the artificial horizons double angle cancels that out while a direct
shot with a bubble would not?
The almanac does not state which limb of the moon is observed with the
bubble horizon. Are bubble observations traditionally made on the center of
the body? I am guessing the center of the moon, as they have the reader
average the UL and LL corrections and subtract 15', while and UL sight would
subtract 30'.
Howell says that since you are observing the center of the sun, no
correction is needed except for refraction taken from the star table and
moon parallax. No mention of 15'. It strikes me in the case of the Sun the
total of UL and LL corrections (UL treated as a positive) is greater that 2x
the semidiameter, and that the UL correction is usually greater than SD, and
the LL correction is less than the SD, so the actual center of the Sun is
lower than its observed center by the difference of LL corrections - LL
corrections. That value differs from the refraction correction for stars.
Suffice it to say I am deeply confused about what corrections to make to the
Sun and Moon observations made from the center of the body with a bubble
horizon. I need to rely on the kindness of strangers to solve this
conundrum.
1. Can anyone provide a cookbook version on what corrections I need to make
or ignore? Temp and pressure, altitude and/or HP correction for the Moon,
and star refraction or UL/LL difference for the Sun? And nothing or -15' for
the moon?
2. If time and temperament allow, a bit of theory as to why in terms a
beginner might grasp?
Thank you
Bill
EXCERPTS
From Susan Howell's Practical Celestial Navigation
In correcting the altitude of an artificial horizon sights first apply the
index correction. There is no dip correction or temperature correction
necessary. The sextant altitude with I.C. applied is then divided by two
after which the main correction is applied. The main correction is figured
as for a normal sight except in the case when the Sun or Moon are
superimposed upon themselves. Here the center of the body has already been
observed so there is no correction needed for semidiameter, phase or
augmentation. For the Moon, there is still a parallax correction needed.
The values for upper and lower limb corrections are averaged. Refraction
correction is still necessary for the superimposed Sun and Moon sights, this
refraction value taken from the star altitude correction table.
Therefore, for most artificial horizon sights Ho = (Hs ± I.C.)/2 ± main.
Ho is the corrected sextant altitude.
From the Nautical Almanac
Moon Correction Table
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 corrections for pressure and temperature see page A4.
For bubble sextant observations ignore dip, take the mean of upper and lower
limb corrections and subtract 15'from the altitude.
App. Alt. = Apparent altitude = Sextant altitude corrected for index error
and dip.
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