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Jan Kalivoda asks:

> would be the retardating effect of the parallax changing with altitude
observable
> only in apparent distances (as I suppose) or in true ones, too?

Effects of parallax (and refraction, if they matter) affect only the
apparent distance,
surely.  But choosing a comparing body that is well off the Moon's path
affects the
true distance also.

But the observational errors are made against the apparent distance,
aren't they?
Or to put it another way, clearing the apparent distance to get a true
distance
has the effect of magnifying observational errors, in the situation
where the Moon's
apparent motion is retarded.

Now that you mention it, though, I'm wondering about the data you've
presented.
It is listed as a number of seconds (of arc, I presume) by which an
observation
differed from the best-estimate GMT.  I didn't pay enough attention to
how it
was computed.  Is it the difference between (cleared observed distance)
and (distance
from almanac and chronometer)?  If so, it would be interesting to
un-clear the
distance from almanac and chronometer to see what the range of errors is
in the
apparent distances.  I also wonder whether your budding astronomer
happened to
choose to measure his distances at moments when the retardation was
small.

        -- Bill

   

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