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I am unconvinced by two recent postings from Frank Reed on this topic.

He said-



>George H wrote:
>"Menzies tells us that the key event that had to be timed (with a star), was
>what he calls U3, at the first signs of emergence from full umbra. This seems
>surprising, as U2, the entry into full umbra, would have been equally useful,
>and the combination of the two more useful still."
>
>The combination of U2 and U3 would require a basic clock and more detailed
>recording. That's not a showstopper, but obviously if we can live without a
>clock, it's an easier observation.

The Chinese had water-clocks, so that wasn't a strong argument against. But
surely, if U3 could be timed in terms of a star crossing the meridian, then
U2 could just as well be timed the same way. And the two combined together
would cancel some of the errors of estimation.

I can think of a couple of reasons to prefer U3
>over U2. You get to see U2 first. It's a practice run for U3. You're prepared
>for the change in lighting that will occur when the events play out "in
>reverse" as the Moon emerges from the umbra, and you get to see the unique
>shading
>phenomena for this particular lunar eclipse. In addition, the observer's eyes
>would be fully dark-adapted by the time of U3, and he would have had plenty of
>time during the full phase of the eclipse to locate faint stars near the
>meridian.

I think those are weak arguments.

===========================

In another posting on the same topic

George H wrote:
"I think Trevor is wrong here. " etc.

Hmmm. I don't (on that particular point you quoted). I think you may have
read what he said differently. I understood him to be disagreeing with the idea
that you need timing info from a clock of some sort. And that's correct: you
don't. All that's required is the local sidereal time of some specific event (U2
or U3) in the eclipse recorded at various different sites, and that "LST" is
nothing but the identity of the star on the meridian.

Frank doesn't explain the point on which he defends Trevor Kenchington, so
let me do so here.

Menzies had said-

> "When the astronomer returned from his voyage, he and his colleagues in
> Beijing compared their data. Using their time keeping device, calibrated
> from the gnomon, they timed the interval between the transits of the star
> observed in the new territory at the time of the eclipse and the star seen
> by the astronomers in Beijing at the same moment.

Trevor commented-

That seems a rather unnecessary step. All they needed was a star
catalogue, with angular measures equivalent to SHA or Right Ascension --
the sort of thing that the astronomers back home should have been
working on anyway.

and I replied-

I think Trevor is wrong here. The longitude difference can be deduced from
the difference in time (measured by star positions) of a common observed
event (Moon eclipse). Without accurate predictions of the Moon's position,
it was, presumably, impossible to predict the exact moment of the lunar
eclipse. All that could be done was to go back to Beijing, after the event,
and find out at what time (measured by those same stars) it had occurred
there (hoping it had been a clear night). It wasn't the star-timings that
were unpredictable, it was the Moon event.

Frank argues that Trevor was correct. In Frank's own words, what's needed is-

"the local sidereal time of some specific event (U2 or U3) in the eclipse
recorded at various different sites."

Exactly. That was what I said was required, from the Beijing astronomers,
for that eclipse, after the event. What I disagreed with was Trevor's claim
that "all they needed was a star catalogue, with angular measures
equivalent to SHA or Right Ascension." Yes, that was necessary to
interpolate the timings of different meridian stars. But the reason they
had to refer back to the astronomers back home was to compare their local
time observations of that unpredictable lunar event.

George.

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