# NavList:

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

**Re: Correct way to calculate time of LAN?**

**From:**Gary LaPook

**Date:**2017 Feb 7, 06:57 +0000

The "v" correction is in excess to the adopted standard Moon rate used in the Nautical Almanac increments table of 14° 19.0' per hour. The Moon's "v" correction varies between 3.4' to 14.8' extra motion per hour. So the real rate of the Moon is never less than 14° 22.4' per hour and never more than 14° 33.8' per hour. The standard rate used in the Air Almanac Moon increments table is 14° 30.0' which is approximate but accurate enough for a ten minute period especially since the Air Almanac only tabulates the GHA of the Moon to a precision of whole minutes of GHA.

gl

**From:**David C <NoReply_DavidC@fer3.com>

**To:**garylapook---.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.

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

**From:**David C <NoReply_DavidC@fer3.com>

**To:**garylapook---.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

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.