From: ara.t.howard@...
Date: 2006-08-09T03:23:22+09:00
Subject: solar local time


i'm working on a sticky issue:  i'm converting some times based on a utc time
and longitude - the goal is the express the time as solar local time.

the code i have is close.  it does something like:


     DEG_PER_HR = 15.0
     SECS_PER_MIN = 60.0
     SECS_PER_HR = SECS_PER_MIN * 60.0
     SECS_PER_DAY = SECS_PER_HR * 24

     # longitudes > 180 degrees are behind UT
     # longitudes <= 180 degrees are ahead of UT

     offset_seconds =
       if longitude > 180.0
         - ((360.0 - longitude) / DEG_PER_HR) * SECS_PER_HR
       else
         (longitude / DEG_PER_HR) * SECS_PER_HR
       end

     utc + offset_seconds


this in fact yields the correct time __except__ that's it's expressed in zulu
time and not the 'local' time zone.  the trick is that 'local' here means
'there' not 'here'!  ;-)  in otherwords it needs to be expressed in the local
time zone of that __longitude__ not mine and not that of greenwich.

to put it more consisely i need to be able to follow

   locatime_but_in_utc = utc + offset_seconds

with

   move_to_zone locatime_but_in_utc, longitude

or some other viable alternative.

thoughts?


-a
-- 
to foster inner awareness, introspection, and reasoning is more efficient than
meditation and prayer.
- h.h. the 14th dali lama