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Subject: Lunar Distances v. Meridian Angles
From: Chuck Griffiths (griffiths_chuck@XXX.XXX)
Date: Tue Feb 26 2002 - 13:07:16 EST
OK, I know George H. promised he'd cover using moon altitudes in place of lunar
distances in his next part but I'm going to go ahead and jump the gun by asking
a question. Now that George has helped me start to understand lunar distances I
can't help but consider an alternative approach to finding GMT. Why can't we
observe the altitude of the moon and one other body and, using our assumed
latitude, solve for the meridian angle of both bodies. The difference between
the two angles should change by the rate at which the moon moves through the sky
faster than another body. If that's true, can't we find the meridian angle
between the two bodies for the even hours, say on either side of what time we
think it is, and use the same inverse linear interpolation approach to find the
time of our sight?
Of course, I can think of a couple issues with this approach worth discussion.
First, this only works when the altitude of the moon and the other body change
reasonably with time, i.e., we can't do it when either body is close to being a
meridian sight. Second, we need both altitudes simultaneously. I think this
could be solved by alternately observing one body then the other several times
and graphing the sights so that we could derive an averaged simultaneous
altitude from the graph.
Lastly, why bother when the other methods thus far described work? It seems to
me that if this is a workable solution it provides a method of checking time
using techniques that are already in most navigator's "bag of tricks". That is,
we get to correct for refraction, horizontal parallax, augmentation, etc. using
the tabular methods we use for other sights.
Chuck Griffiths
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