From: ara.t.howard@...
Date: 2006-08-10T06:09:18+09:00
Subject: Re: solar local time

On Wed, 9 Aug 2006, Ara.T.Howard wrote:

> too funny.  this is a sketch of my code so far:

<snip my own broken code>

for anyone following the thread or searching archvies here's the answer i ended
up with, it's correct afaik:


     harp:~ > cat a.rb
     require 'date'
     require 'time'
     #
     # returns a datetime object defined at a certain location on earth.  in
     # otherwords a certian time 'there' not 'here' or 'utc' - a time in an
     # arbitrary other timezone.
     #
       def solar_localtime time, longitude
         deg_per_hr = 15
         secs_per_hr = 60 * 60
         longitude = Float longitude

         t =
           case time
             when Time
               time.utc
             when DateTime
               Time.parse time.to_s
             else
               Time.parse time.to_s
           end

         if longitude > 180.0
           off_sec = - ((360.0 - longitude) / deg_per_hr) * secs_per_hr
           tz = - ((360.0 - longitude) ) / 360.0
         else
           off_sec = (longitude / deg_per_hr) * secs_per_hr
           tz = (longitude ) / 360.0
         end

         t += off_sec

         y, d, h, m, s = %w( year yday hour min sec ).map{|m| t.send m}

         DateTime.ordinal y, d, h, m, s, tz
       end
     #
     # demo
     #
       now = Time.now.utc # you could also do: now = DateTime.now
       puts(now.iso8601)

       require 'yaml'
       [15, 30, 45].each do |longitude|
         y longitude => solar_localtime(now, longitude).to_s
         y -longitude => solar_localtime(now, -longitude).to_s
       end


     harp:~ > ruby a.rb
     2006-08-09T21:06:33Z
     15: 2006-08-09T22:06:32+0100
     -15: 2006-08-09T20:06:33-0100
     30: 2006-08-09T23:06:33+0200
     -30: 2006-08-09T19:06:32-0200
     45: 2006-08-10T00:06:33+0300
     -45: 2006-08-09T18:06:33-0300


the trick is use DateTime - you can't do it with Time.

regards.

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