Text 876, 44 rader
Skriven 2006-10-11 21:20:06 av DAVID WILLIAMS (1:250/514)
     Kommentar till en text av Miles Maxted
Ärende: Re: Daylight Saving Time
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-> Mmmm ... the letters to Editors here ocassionally feature one  
-> advocating that clocks be set twice a year by horologists to lose  
-> or gain time to match teh seasons. 
  
In my extreme youth, in WW2 England, clocks were set ahead twice during 
the spring, to "Double Summer Time", and back twice during the fall. 
This meant that in midsummer, people could be seen mowing their lawns 
at midnight or later. Of course, very few people used artificial light 
during summer, which made the "blackout" easy to enforce. 
  
-> I've not done the comparison, but I feel that a straightline  
-> zigzig would not fit your `equation of time' at all well;  has  
-> anybody made any formal comparison, do you know ? 
  
Assuming that fixed dates are used for setting the clocks ahead or back 
an hour, then, to make the morning rush-hour reasonably safe, the dates 
for the two changes should be ones on which the times of sunrise (but 
not necessarily sunset) shuld be equal. Ideally, the dates should vary 
with latitude, but since this is impractical, some average latitude 
should be used for the calculation. I have here a table of sunrise 
times for latitudes in North America. At latitude 35 deg. N, which is 
about average for the continent, the sun rises on March 10 at 6:18 
a.m., local mean time. On November 3, it rises at 6:22 a.m.. I chose 
those two dates to be typical of the second Sunday in March and the 
first Sunday in November, which are the dates, by the new rules, for 
the clock changes. So, within four minutes, the times of sunrise are 
the same on these dates, which means that the new rules will be pretty 
good in this respect. 
  
Incidentally, the times of sunset on the same two dates are 18:03 and 
17:04, respectively, nowhere near equal. 
  
I don't have the wherewithall right here to calculate sunrise times in 
the southern hemisphere. Maybe you can find them out, and see how good 
your DST rules are. 
  
Incidentally, the mnemonics "spring ahead" and "fall back" are often 
used here to remember the directions in which the clocks change. But 
the new rules will make the "ahead" change occur in what is still 
technically winter. "Winter ahead" somehow doesn't have the right ring. 
  
                            dow 
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