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Author
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Topic: How do i find daylight saving time of an event using SPA
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David Burch
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posted May 19, 2005 06:17 PM
The SPA computes local mean times of events, just as presented in the paper nautical almanac. so the conversions would take place in the customary cel nav procedures.
Here is the procedure: compute the LMT of (say) sunrise from the SPA almanac, for lat 32° 41' N on May 19, 2005 which is = 04:58:45 LMT
then for your longitude (for example) = 117° 10' W, use the Arc to Time table (last link in the bookmarks of the SPA Help file) convert this arc to time, ie
117° = 7h 48m 00s
10' = 40s
Longitude to time = 7h 48m 40s
so GMT = 04h 58m 45s + 7h 48m 40s = 11h 106m 95s
and since PDT is ZD = +7h, we have PDT = 11h 106m 95s - 7h = 4h 106m 95 s = 5h 47m 35s PDT
summary look up time in the almanac, use arc to time to convert lon to time, if lon is west, add that time to the almanac time to get gmt, then subtract your time zone to get local time. pacific zone description is +8 in winter and +7 in summer.
If you will be in a fixed location for some period of time, you can make a short cut, by just correcting from the local time meridian, in this case 120° = 8 x 15°. in this case you are 2° 50' east of the meridian, which (arc to time) is 11m 20s, so you can just subtract the correction... the time given is for 120 w which occurs later, or 04h 58m 45s - 11m 20s = 4h 47m 35s, which would be correct in the winter, but you have to add an hour for daylight saving time.
to compact more, in daylight saving time at this location, just take the output from SPA and add 1h and subtract the correction = 59m 60s -11m 20s = 48m 40s.... ie in the summer add 48m 40s and in the winter subtract 11m 20s
Note that we have programs (StarPilot) that do all of these corrections automatically, but we have not added that functionality to the SPA since it is supposed to represent a digital version of the Nautical Almanac, not a working tool for solving problems.... just in case you might ask this perfectly logical question of why we have not included that.
From: Starpath, Seattle, WA
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