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## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Time
From: Ed Kitchin
Date: 2000 Nov 10, 7:23 AM

```Hello:
While I don't hold myself an expert, what I learned in a celnav
course was, that zone time was GMT +/- the distance from 0 longitude
(Greenwich) deviled by 15. To explain. Each time zone is 15 degrees, which
equals one hour of time plus or minus from GMT. Therefore, (example) the
eastern time zone is minus four hours (standard time) from GMT. To convert
zone time to GMT, add the required number of hours (time zones) to Greenwich
meridian. For example, eastern time zone is plus four. Local apparent noon
for any location, is found by adding or subtracting (west, Greenwich best.
East, Greenwich least) the zone difference from Greenwich, then adding or
subtracting (as appropriate) your longitude.
I hope this begins to help.

Ed
Watch time is simply local time. I believe ships time is the same.
----- Original Message -----
From  "Thomas Schmidt"
To:
Sent: Thursday, November 09, 2000 7:54 PM
Subject: Re: Time

> Dave Weilacher wrote:
> >
> > Anyone got concise definitions of various "time"s
> > and formulas for converting from/to.
> >
> > For example:
> > Zulu time
> > UCT
> > UTC
> > Zone Time
> > Watch time
> > Ship's time
> > etc.
> >
> > How about local apparent noon
> > Meridian passage of sun
> > ....
>
>
> TAI = International Atomic Time, a strictly uniform time scale,
>       based on atomic clocks. Unit is the SI second.
>
> UT  = Universal time, based on the rotation of the Earth and
>       therefore irregular. Important for us, however, since
>       day and night comply with UT. Unit is the mean solar
>       day.
>
> UTC = A practical compromise between TAI and UT. Unit is the
>       SI second, so the rate of UTC is strictly predictable
>       (in contrast to UT). Occasional leap seconds assure
>       that it doesn't get out of step with day and night
>       (which TAI does slowly but surely)
>
> Meridian Passage of the Sun = the moment when the sun is
>                               exactly due south.
>
> True Solar Day = the period of time between two meridian passages
>                  of the sun. Not a uniform measure of time
>                  because in the course of a year some true solar
>                  days are longer, some are shorter ("equation of
>                  time"). Can be observed directly.
>
> Mean Solar Day = the period of time between two meridian passages
>                  of the sun, with the effect of the equation of time
>                  averaged out over one year. Can not be observed
>                  directly, must be taken from appropriately
>                  regulated clocks.
>
>
> Local True Solar Time = is noon whenever the sun crosses the meridian
>                         as seen from your current location.  At the same
>                         instant, locations at different geographical
>                         longitudes have differnt LTSTs.
>
> Local Mean Solar Time = a succession of Mean Solar Days so that noon LMST
>                         is _on average_ when the sun crosses the meridian.
>                         (that is, LTST is always a bit faster or slower
>                         than LMST, but they agree on average).
>                         At the same instant, locations at different
>                         geographical longitudes also have different
>                         LMSTs.
>
>
> Local times are different at different longitudes. Zone times are the
> local time at a specific reference longitude, adopted by all people who
live
> in the vicinity of the reference meridian. Therefore, different locations
at
> different longitudes but within the same zone have the same time.
> If you live to the east of the reference meridian, zone time noon will be
later
> than your local meridian passage of the sun; if you live to the west of
the
> reference meridian, zone time noon will be earlier than your local
meridian
> passage.
>
> ZULU is the zone for the reference meridian "zero longitude" and has by
> definition UTC as time scale. All other zones differ by an integer number
> of hours (some exotic zones by half-integer hours).
>
> Therefore ZULU and UTC are the same, and they are practically the same as
> the antiquated and now obsolete "Greenwich time".
> The difference between UTC and Greenwich time is that UTC is based on
> atomic clocks while Greenwich time was based on the rotation of the Earth,
> with some corrections applied.
>
> --
>   -------------------------------------------------------------------
>   Thomas Schmidt                  e-mail:     schmidt@hoki.ibp.fhg.de
>
```
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