Re: Calendar > Moon

["B. T. Raven" <nihil@nihilo.net>]

Bauke Jan Douma wrote:
> B. T. Raven wrote on 19-05-07 14:26:
>> i)
>> It's obvious that sunrise-sunset times are dependent on the observer's 
>> longitude and (to a smaller degree) latitude but is the same true (to 
>> any degree)of the times of phases of the moon? There is a two minute 
>> discrepancy between the times reported by the Naval Observatory and by 
>> emacs 21.3
>> Can this be explained by lat. - long. differences among the observers? 
>> I understood that phases should be dependent only on the relative 
>> positions of the centers of the sun, earth, and moon. My settings are:
>>
>>  (setq calendar-latitude 45)
>>  (setq calendar-longitude -93)
>>
>> ii) Astronomy question
>>
>> In the context of describing a storm and catastrophic flooding of the 
>> North Sea coast, an English medieval chronicler says that Dec. 26, 
>> 1287 (Julian, or 1-2-1288 Gregorian) is the ninth (day of the) (i.e. 
>> two days +/- after first quarter). Emacs says it's the 13th (almost 
>> full). I was under the impression that celestial positions could be 
>> extrapolated many millenia backwards with great accuracy. Without 
>> instruments it's hard to precisely determine new and full moon but 
>> easy to tell the difference between quarter and full. Does any of you 
>> have any ideas to explain this discrepancy?
> 
> i.  yes it could be explained by diff. in lat./long. I don't know
>     which position either of them uses to calculate the times, nor
>     the algorithms used.
> 
> ii. how is Dec. 26, 1287 as you say 1-2-1288 Gregorian?  If it's anything,
>     it's 6 jan. 1288 'gregorian' (+11 days).
> 
> bjd
> 
> 
> 
> 

The 11 day correction was made in 1582 because that's how far the vernal 
equinox had gotten out of synch with the civil calendar. Apparently Emacs 
prorates this amount backward in time, as if the correction had been made in 
(e.g. 1287) some earlier year. If Caesar had used the Gregorian calendar 
starting in 45 B.C. the errors wouldn't have accumulated.
Btw, I can't find the site where I noticed the moon-phase discrepancy. Other 
sites agree with Emacs to the minute.

Thanks,
Ed