Text 901, 61 rader
Skriven 2006-10-17 22:52:02 av DAVID WILLIAMS (1:250/514)
     Kommentar till en text av Miles Maxted
Ärende: Re: Daylight Saving Time
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-> The dial sits outside what looks like an American Courthouse and  
-> was so installed correctly in 1926.  They rennovated the grounds  
-> about 5 years ago,  and reinstalled the plinth and its round plate  
-> correctly - but attached the gnonom back to front. 
 
-> Sprott spotted this and sought correction - finally getting it  
-> when he went to the media, generating the controversy I mentioned  
-> earlier.  And then some twit half-inched the gnonom - which still  
-> missing. 
 
-> The plate is correctly orientated to the solar south when I  
-> inspected it,  and has been beautiful engraved for its Auckland  
-> location with both proper times and a detailed set of  
-> calendarisations around the rim.  It is signed `J.Kenderdine', but  
-> undated. 
 
-> As the Librarian was most perturbed by the lack of action over  
-> replacement,  I phoned Sprotty (recently returned from Canada  
-> where he has a consulting practice - `TJ Sprott & Co' was his NZ  
-> consulting name years ago) and got a concise briefing on the  
-> status of the dial - hard copy to come in the mail. 
 
-> It seems that he made detailed measurements of both the dial and  
-> its gnonom,  and has designed a replacement ready for Council  
-> approval. 
  
I wrote a program some time ago that designs the plate for a sundial 
for use at any latitude. The spacings of the markings for the hours are 
latitude-dependent. At either of the earth's poles, the gnomon is 
vertical and the hour markings are evenly spaced around the plate. At 
low latitudes, the gnomon leans steeply, so it is almost horizontal, 
and the markings for the hours around noon are crowded together under 
it. The markings for the hours far from noon are widely spaced around 
the sides. However, the program was for an old Mac, and probably 
wouldn't be much use to anyone nowadays. 
  
An important feature for a sundial especially in the southern hemisphere 
is that it must have some way for people to compensate for the Equation 
of Time. Sundials are generally summer toys. Not too many people want to 
use them in wintertime. During the northern summer, the Equation of Time 
is never more than about five minutes, plus or minus, so if you only 
want to know the time to within five minutes or so, you can ignore the 
Equation of Time. But during the southern summer, the Equation of Time 
can exceed 15 minutes, so unless a correction is applied, a sundial 
can mislead you by more than a quarter of an hour. (I have a suspicion 
that this is why sundials are much less used in the southern hemisphere 
than in the north.) 
  
The computer program I posted here last week draws a graph of the 
Equation of Time throughout the year. Basically, it shows the 
correction that must be applied to a sundial reading to get "mean" 
(i.e. clock) time. I have seen sundials that have this graph engraved 
on the plate, so anyone who reads the dial can apply the proper 
correction. I would suggest that any dial for use in New Zealand should 
have it. 
  
Have fun! 
  
                          dow 
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