# NavList:

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

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Correct way to calculate time of LAN?
From: David C
Date: 2017 Feb 6, 19:33 -0800

I am getting very confused about calculating the time of LAN for a celestial body. I will describe two methods. The first I will call "My Method" because it is intuative (to me) and is what I have been using. The second is from the web site  https://thenauticalalmanac.com/Formulas.html. The methods give completely different results.

I would like someone to cast an eye over what I am doing. The thenauticalalmanac.com method gives a nonsensical result so maybe I am not following the instructions correctly?

Body The Moon

Date 2017 Feb 5 UTC

Longitude  175° 05.2' E

Using the Air Almanac

My Method

At LAN LHA = 0

therefore  long + GHA = 360

From which  GHA =  184° 54.8'

From the Air Almanac   GHA =  182° 42'   at 0710

GHA increment  2°  12.8'

From Moon interpolation table in the Air Almanac delta time =  9min  09 sec

Therefore  time of Moon LAN = 071909 UTC.

Daytime observations of the Moon on 2017 Feb 5 showed that it was at its maximum altitude sometime between 0714 and  0721 so the calculated value of LAN makes sense.

thenauticalalmanac method

GHA less than longitude is 173° 04' at 0630

long - GHA  = 2° 1.2'

From Moon interpolation table in Air Almanac  delta time = 8 min 23 sec

Therefore time of Moon LAN is 063823 UTC.

phone app

The Sun Facts Pro app on my phone give a time of 0718 UTC for Moon LAN on 2017 Feb 5.

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