|
|
|
|
|||||
|
||||||
Subject: Re: It Works.
From: Noyce, Bill (William.Noyce@XXX.XXX)
Date: Wed Apr 03 2002 - 10:46:19 EST
George Huxtable says
> 1. For a lunar that uses altitudes that are calculated rather than
> measured, it is necessary to make a measurement for local apparent time.
> This should be done within a day or so of the moment of taking the lunar,
> depending on how well the timepiece can be trusted over such a period.
> :
> 4. The local apparent time, so derived, and corrected by the equation of
> time to give local mean time, can be checked against the ship's timepiece,
> and can now be used with the Almanac to establish precise altitudes for
> both the bodies used in a lunar observation, even though the GMT and the
> longitude are both still uncertain.
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
are 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.
I think, George, that if you go back to your example where you assumed
the watch was 30 minutes fast, and simply change the assumed longitude
to be 7.5 degrees east of the true position, then you'll come up with
results that match Bruce's. This assumed longitude is what a navigator
would have concluded with traditional celestial observations, if he
were misled by a watch that is 30 minutes fast.
|