|
|
|
|
|||||
|
||||||
From: Frank Reed (no email)
Date: Mon Jul 05 2004 - 01:09:37 EDT
Bill wrote:
"I understand why dip would be ignored in either case. Howell suggests
ignoring temp/pressure corrections while the almanac says to include them."
Sue was wrong on that point. You should include corrections for non-standard
temperature and pressure when correcting any sextant observation. Note though
that they are very small in most cases. By the way, since you're inland, be
advised that you should use "sea level" barometric pressure. This differs a
little from the pressure usually reported in weather reports (but not very much
unless you're at Denver-like altitudes).
And:
"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?"
Yes.
And:
"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'."
That's correct. The Moon tables in the almanac have a "built-in" 30 minutes
to make the calculations less prone to error.
And:
" 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."
For either the Sun or the Moon, when using a bubble sextant or an artificial
horizon (and superimposing both images) you want to AVERAGE the corrections
for the upper and lower limbs. So let's suppose a Sun altitude by bubble sextant
of 10 degrees. Opening up the front of the almanac you'll find that the LL
correction is 10.7 minutes while the UL correction is -21.1 minutes. The average
is 5.2 minutes (add 10.7 and -21.1 and then divide by two). This will differ
by a tenth of a minute from the corresponding correction for the stars because
the Sun has a small parallax correction which is included in the table.
That's why it's slightly better to use the average from the Sun table instead of
taking the corresponding number from the star table. But remember, for ordinary
celestial navigation, a tenth of a minute is a very minor correction so you
won't go wrong with either approach.
Frank R
[ ] Mystic, Connecticut
[X] Chicago, Illinois
|