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Subject: Re: Time
From: Richard B. Emerson (navsys@XXX.XXX)
Date: Fri Nov 10 2000 - 11:14:24 EST
Ed Kitchin writes:
> 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.
While basically correct, not all time zone borders lie on the expected
15 degree limits. To make things more interesting, many states lie in
two time zones (in the US, check the front of your phone book for a
map of US time zones or, under Windows, look at the time zone tool).
Bottom line: time zones aren't always as intuitive as one might like.
For this reason, I prefer to do all of my celestial and plotting work
in UTC, marking those times with a "Z" suffix (e.g., 2310Z).
Local apparent noon (LAN) is not simply the local hour angle from 0
deg longitude. It's the time when the sun reaches the highest
altitude (meridian transit or the point at which the sun's GP lies on
the local meridian - a line of longitude projected into the sky and
passing over the observer's position - projected to the earth's
surface) and the bearing is either due south in the northern
hemisphere or due north in the southern hemisphere. Keep in mind the
sun's maximum altitude varies with the time of year; the altitude at
LAN doesn't often reach 90 deg and never does north or south of the tropics
of cancer and capricorn (which lie on lines of latitude equal to the
earth's inclination or tilt from the plane of the solar system -
roughly 23 deg. north and south).
Rick
S/V One With The Wind, Baba 35
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