Re: VMs: Identifying VMS stars, and the longitude problem

[elvogt@xxxxxxxxxxx]

Zitat von Jorge Stolfi <stolfi@xxxxxxxxxxxxx>:

>...> 
> My guess is that Ptolemy and other ancient astronomers could have
> determined the relative longitude of cities by comparing the local
> times (or the moon's position over the horizon) when a lunar eclipse
> was reported to have reached its maximum. Obviously this method is
> useless for sea navigation (unless one has very accurate predictions
> of lunar eclipses, which presumably were not available until
> recently).

...and unless you've got plenty of time on your hands, considering that 
eclipses only occur in six-month intervals... ;-)

> ...
>   > Say, a lunar eclipse is taking place at night. One person in an
>   > Eastern location sees the eclipse occur on the Western horizon.
>   > ... calculations can be taken to show the time of the eclipse in
>   > different locations, and the differences can be noted.
> 
> Yes, that is precisely what I meant.  Was this method actually used?
> How accurate could it be? 
> 

This seems to be the only feasible method, but considering that the Earth's 
shadow is very "fuzzy" compared to what is seen during a solar eclipse, I guess 
it would be fairly difficult to measure it with an accuracy of better than, 
say, 15 minutes. That translates as roughly 250 km the two observers have to be 
apart in east-west direction to notice _any_ effect at all.

(But we're getting into one of those off-topic discussions I ranted so much 
about lately, so perhaps we should continue this off-list!)

Cheers,

   Elmar


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