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Go Back   SailNet Community > Skills and Seamanship > Seamanship & Navigation
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Seamanship & Navigation Forum devoted to seamanship and navigation topics, including paper and electronic charting tools.


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  #1  
Old 04-05-2011
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Local Noon

Does anyone know where within a time zone does the local noon correspond with the clock? Is it half way across the zone? The eastern edge? Or are the lines totally arbitrary and it just falls "somewhere" in the zone?
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Old 04-05-2011
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As with so many questions, the answer is, "It depends."

The extra factor here is the Equation of Time, which is the difference between apparent solar time (what a sundial tells you) and mean solar time (what a sundial would tell you if the sun moved at a constant rate across the sky every day over the course of a year).

If the equation of time is zero, then local apparent solar time is equal to local standard zone time at the standard meridian for that zone. The prime meridian and every 15 degrees of longitude east and west of the prime meridian are standard meridians for the corresponding time zone.

So towards the end of December when the equation of time is zero, and we're not on daylight savings time (the navigator's bane), then at 120 W longitude (the standard meridian in my time zone), it will be local apparent noon when my clock says 12:00.
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Old 04-05-2011
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I always set myself up for those "it depends" answers!

Thanks, that makes sense. I forgot the earth's rotation and revolution aren't perfect circles.
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Old 04-05-2011
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In GMT, 0 degrees, its definitely on the meridian that passes through Greenwich Village, London.
Here in Dublin, 6,15" west, local noon occurs approximately 1225 GMT.

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Old 04-06-2011
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Hilariously, the Greenwich meridian has moved several meters since the days of Flamsteed and the founding of the Royal Observatory, so if you go there and stand on the line they have marked, you will actually be in, iirc, a few arcseconds east. The new prime meridian is in the parking lot.

Some say it's a result of a more precise geoid. I subscribe to the "England is tipping over towards Ireland" theory.
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Old 04-06-2011
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It is the center of the time zone with seven and a half degrees of Longitude on each side.
But you will need to check with the Nautical Almanac to determine true local noon for that time of the year.
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Old 04-06-2011
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So, this has to do with what exactly? Celestial navigation?
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Old 04-06-2011
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Quote:
Originally Posted by AdamLein View Post
Hilariously, the Greenwich meridian has moved several meters since the days of Flamsteed and the founding of the Royal Observatory, so if you go there and stand on the line they have marked, you will actually be in, iirc, a few arcseconds east. The new prime meridian is in the parking lot.

Some say it's a result of a more precise geoid. I subscribe to the "England is tipping over towards Ireland" theory.
Been there in Greenwich and didn,t know it had moved:
That last suggestion is well out of order
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Old 04-06-2011
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Quote:
Originally Posted by LandLocked66c View Post
So, this has to do with what exactly? Celestial navigation?

It has to do with astronomy. Celestial Navigation makes use of it to determine Longitude.

Local Time for any point on the globe is the difference between greenwich time and the local longitude; 4 minutes of time for every degree or 4 seconds of time for every minute of angle. Add for East longitudes, subtract for West longitudes.

Determing Longitude with CN is reciprocal.
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Old 04-06-2011
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Quote:
Originally Posted by Tea-Rex View Post
It has to do with astronomy. Celestial Navigation makes use of it to determine Longitude.

Local Time for any point on the globe is the difference between greenwich time and the local longitude; 4 minutes of time for every degree or 4 seconds of time for every minute of angle. Add for East longitudes, subtract for West longitudes.

Determing Longitude with CN is reciprocal.
Slightly over my head, very interesting though. Carry on gents!
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