# NavList:

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

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Re: Longitude at noon
From: Philippe Posth
Date: 2020 Sep 9, 11:56 -0700

Hi Tracy,

Thanks for your example, it will help me explain what I am trying to say.

The culmination time you have determined is 19h23min37s UT on September 7, 2020.

The "normal" calculation I've seen everywhere is:

On that day, the Greenwich Passage Time was 11h57min51s

The difference between the 2 times is 7h25min46s.

By multiplying this time by 15 we get your longitude: 111°26.5'W

The "second" calculation is:

GHA at 19h: 105°33.9'; Increment for 23min37s : 5°54.3'

GHA at 19h23min37s = Longitude = 111°28.2'W

Which calculation is correct? Why this (small) difference?

Thank you all (from France)

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