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Author
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Topic: Twilight, Sunset, Sunrise
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billstrath
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posted January 25, 2011 04:27 AM
Forgive me if I have missed this somewhere in the text, but why don't we use the equation of time adjustment when calculating the time of twilight, sunrise and sunset while we do when calculating LAN?
From: Lakewood, CO
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David Burch
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posted January 25, 2011 08:00 AM
we do not really use the equation of time for anything. all we need in navigation is the correct UTC, for any application.
the equation of time is just a way to specify the time the sun crosses the greenwich meridian. we could use the EqT for that, or we could just look up this time.
From: Starpath, Seattle, WA
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billstrath
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posted January 25, 2011 09:55 AM
I guess I'm confused. The meridian passage of the sun can be off 1200 by as much as 15 minutes. It was off 15'58" on 10/26/78 which brought this question up. Won't this difference follow thru the day and make sunset off by a similar amount?
From: Lakewood, CO
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David Burch
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posted January 25, 2011 10:07 AM
there is on each daily page a time of mer pass for the sun. that is what you use for LAN sights. on the other side of the page are values for sun set twilight, etc.
the equation of time has no role to play in anything we do in routine cel nav (it can be used in some emergency nav apps).
It is in our glossary but we do not use it for anything. therefore it cannot influence anything.
let me give an analogy. we can say the time is 745 or we can say it is 15 min before 8. think of the latter as somehow analogous to using the EqT to say what time it is.
From: Starpath, Seattle, WA
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David Burch
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posted January 25, 2011 10:18 AM
Bill.... i just spotted something. You abbreviated time with angle units. that should be avoided at all cost. 100%. never done.
it has a huge potential for causing error. way more important than the equation of time.
--david
From: Starpath, Seattle, WA
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