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Arthur,

Thank you for sharing these data with us.

I calculated GMT for each one of your observations, using your known
position to calculate altitudes of the bodies, rather than the
observed altitudes you had for your first set of observations.  The
calculations were made using Young's method, as laid out by George
Huxtable on this list.

The mean and standard deviation of the difference from the lunar GMT
to the true GMT were 259 plus or minus 240 seconds for the first
lunar and 192 plus or minus 62 seconds for the second lunar.  The
GMTs from both lunars were fast.  When I omitted the first
observation, an obvious outlier, from the first set, its mean and
standard deviation decreased to 170 plus or minus 56 seconds.  So the
precision of both lunars was similar (and not all that shabby), but
they were about 3 minutes fast, implying a gap in contact of 1.5' of
arc.  It is hard for me to imagine what might be causing this error,
but it would be nice to track down.

Comparing the lunar GMTs of the individual observations from which
you made calculations,  for the first lunar, I had a GMT of 19:44:59
versus your 19:44:55.  The difference may be due to you using
observed altitudes to obtain refraction whereas I used calculated
altitudes.  For the second lunar, I had a GMT of 18:41:15 versus your
18:42:59.  By my reckoning, you were "only" 2 minutes, 28 seconds of
time fast, rather than 4 min, 12 secs, implying a longitude error of
37 minutes.

I also would like to note that my calculations indicated that the
moon would be changing relative to the sun by 28' of arc per hour
during the first lunar and 27' of arc during the second, which may be
different from what you indicated.

Again, thank you for sharing these data with us in such a gracious
manner.  I hope you see a need to return the Caribbean soon to gather
more data!

Yours Truly,

Fred Hebard

   

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