# NavList:

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

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Re: Correct way to calculate time of LAN?
From: Gary LaPook
Date: 2017 Feb 7, 05:47 +0000

From: David C <NoReply_DavidC@fer3.com>
To: garylapook@pacbell.net
Sent: Monday, February 6, 2017 7:43 PM
Subject: [NavList] Correct way to calculate time of LAN?

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
=========================================================

HERE IS WHERE YOU MADE YOUR MATH ERROR

184° 54.8' - 173° 04' = 11° 50.8' NOT 2° 1.2'

The Air Almanac has increment tables only up to ten minutes of time since the data is tabulated every ten minutes. If you look at the increment table in the Nautical Almanac, which has a table for sixty minutes, you find that the time for the Moon to move 11° 50.8' is 00:49:39 so the time that the Moon should be on your meridian is 07:19:39. The standard speed for the Moon used in the increments table is 14° 19.0' per hour. You can get the same result by doing the division yourself.

Why the discrepancy? The Nautical Almanac also has a "v" correction to the increments for the movement of the Moon for when it varies from the standard 14° 19.0' rate per hour. If we applied the "v" correction then we would have found the same time for meridian passage of the Moon using both methods.

BTW, it is "meridian passage" for the Moon (and all bodies except the Sun) since LAN "local apparent noon" only applies to the Sun.
======================================================

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.

gl

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