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Subject: Re: Use of Sun Sights for Local time, and Lunars for Longitude
From: Bruce Stark (Stark4677@XXX.XXX)
Date: Wed Oct 23 2002 - 18:36:39 EDT
It seems to me we're all pretty much in agreement that calculated altitudes
are OK for clearing a distance. Now, perhaps, more lunars will be taken. On
land it's so much more convenient to used calculated altitudes, and it only
takes five or ten minutes to get a set of distances.
I appreciate William Noyce and George Huxtable taking time to explain where
the thirty-to-one reduction in error comes from. I also appreciate them
pointing out that in most cases there's no need to repeat the calculations.
Actually, there may be a better way of getting that thirty-to-one reduction
in the error of the moon's hour angle than the one we've been discussing.
Here's an excerpt from a posting William Noyce made last April:
>I don't think you need to make any special "local apparent time"
observations or calculations. Assuming the navigator >has been using
celestial observations all along, but has an incorrect clock, he will have
determined a celestial "fix" >whose longitude is off by almost exactly 15'
for every minute of time error. These two errors will cancel out to >reduce
errors in computed altitudes, the same way as Bruce Stark's procedure using
local time. The remaining errors >come from the change in declination (pretty
fast for the moon), and the difference in rate of change of GHA between >the
sun, planets, and stars.
Maybe some list members will check this out. Working from local time and
shifting back and forth between arc and time isn't everyone's idea of fun.
Bruce
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