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Subject: Re: Use of Sun Sights for Local time, and Lunars for Longitude
From: Arthur Pearson (arthurpearson@XXX.XXX)
Date: Wed Oct 23 2002 - 22:37:16 EDT
Thanks to Bruce, Herbert, Bill and George for this dialogue. This use of
LAT from sun sites and the elegant process of stepping to acceptably
accurate calculated altitudes opens the door for the navigator to use
the night time moon and stars for lunars, even though no horizon is
available for altitudes. Given the relatively few days when the moon
simultaneously has both a horizon and a comparing body, I would think
this was a very important advantage for traditional navigators without
accurate chronometers or GPS units. Is there documentation anywhere that
confirms this was common practice during the heyday of lunars?
Again, thanks to all who contributed to this thread. I need to sit
quietly with some diagrams and an outline of steps to be sure I have
absorbed the techniques.
Arthur
-----Original Message-----
From: Navigation Mailing List
[mailto:NAVIGATION-L@XXX.XXX] On Behalf Of Bruce Stark
Sent: Wednesday, October 23, 2002 6:37 PM
To: NAVIGATION-L@XXX.XXX
Subject: Re: Use of Sun Sights for Local time, and Lunars for Longitude
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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