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From: Frank Reed (no email)
Date: Mon Jul 05 2004 - 21:13:45 EDT
Bill wrote (quoting the NA Moon correction table):
"For bubble sextant observations ignore dip, take the mean of upper and lower
limb corrections and subtract 15' from the altitude."
Yeah, those instructions are a little bit too succint, aren't they?
There are two ways of looking at the calculation:
1) Find the Moon's complete LL correction. Find the Moon's complete UL
correction. Average those two numbers. This yields the Moon's center correction.
2) Find the Moon's upper table correction. Get the LL and UL corrections from
the lower table. Average those two numbers and add to the upper table
correction. Then subtract 15 minutes.
An example:
Suppose the Moon is 75d 00' in altitude and its HP is 57.0'.
By approach 1 above)
The LL correction would be 25.3+4.7 or 30.0 minutes. The UL correction would
be 25.3+3.6-30 or -1.1 minutes. The average of those is 14.45 minutes [=(30 +
-1.1)/2]. That's the correction you need.
By approach 2 above)
We average the two lower table corrections and get 4.15. Add this to the
upper correction. I get 29.45. Now subtract 15 minutes. The result is 14.45.
Either way will work. Myself, I prefer method 1 since I can remember it by
knowing why it works.
And:
"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?"
It includes refraction, parallax, semidiameter, and augmentation of
semidiameter, all rolled up in one! Refraction is large near the horizon and decreases
rapidly with altitude. The parallax correction is also large near the horizon
but it decreases rather slowly with altitude. The competition between these
two is the main reason why the Moon's correction peaks at 15 degrees.
And also wrote:
"While updated many times the Howell original was published in 1979 and uses
1976 tables."
It has been printed quite a few times, but it's basically not updated. I
don't know if you're familiar with the story. Sue never had the chance to update
the book since she was lost at sea twenty years ago last month in the sinking
of the sail-training vessel Marques off Bermuda. I presented the first Susan P.
Howell Memorial Fund lecture (on lunars navigation) two weeks ago in Mystic.
Some money from that fund may eventually be used to update her still-popular
"Practical Celestial Navigation".
Frank R
[ ] Mystic, Connecticut
[X] Chicago, Illinois
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