NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
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.
