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Subject: Re: Direction and alternate methods
From: Bruce Stark (Stark4677@XXX.XXX)
Date: Tue Jun 10 2003 - 15:31:14 EDT
Guess it goes without saying that I'm reasonably happy with the drift of the
List. But I'd like to see more posts such as Fred's recent one on timing
sights. We ought to share more of the practical problems we've run into, and the
solutions, if any, we've found.
Mike Burks brought up the use of slide rules for interpolation. Just for fun,
I learned to use a slide rule, but for navigation use it only to check the
interpolation when working with the1804 and 1805 Almanacs. Although Nathaniel
Bowditch called them slide rules, the old British navigation manuals called them
"Sliding Gunters" after the Gunter's scale. Guess that's how that telescoping
rig for dinghies got named (more of that history stuff).
On the use of altitudes only to get Greenwich time from the moon, check back
on the List. I weighed in on Feb. 26, '02, and said:
"I just got signed on to the List, and probably shouldn't jump in so soon.
The idea of comparing the altitude of the moon with that of one or more other
bodies to get Greenwich time has been around almost continually for at least
two centuries. It's such an appealing idea that many of the best navigation
authorities have been blind-sided by it.
First: Consider what you know about the reliable accuracy of altitudes taken
from the sea horizon, especially from a small boat. You will be combining the
errors of several such altitudes.
Second: Consider what it is you are trying to do. You are trying to measure
the moon's position in her orbit, NOT the rotation of the earth. If your
latitude is less than 30° there may be times when you'll actually see the
moon when her orbital motion is perpendicular to the horizon. At higher
latitudes, never.
The moon's is always within about 5° of the ecliptic. Her enlightened limb
points to the sun and shows the path of her orbit. How often do you see the
moon when her horns are pointing straight up or straight down?"
I don't see why working a lunar need be so complicated, Doug. Seems to me
anyone who can do simple addition and subtraction should be able to clear a lunar
in ten minutes, once he's familiar with my Tables. No puzzles involved. And
if Steve Wepster will continue his list of Precomputed Lunar Distances, and
extend it to one-hour intervals (so we don't have second difference problems with
the shorter ones) we won't have to calculate comparing distances.
If you use my tables 7 and 8 for the three-hour interval you should multiply
the minutes and seconds you get by three.
Bruce
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