# NavList:

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

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Re: Lunar Distances with Alex's SNO-T
From: Bill B
Date: 2006 Nov 09, 02:08 -0500

```Left town unexpectedly for family matters.  I apologize for the delay in
response.

Frank wrote:

> So there's no
> problem calculating its exact angular size in the sky. BUT you can't use the
> value in the Nautical Almanac. Is that sacrilege? Isn't the Nautical Almanac
> nearly Holy Scripture for celestial navigation? Some people treat it that way
> sometimes, but in fact, the modern Nautical Almanac is no more than a
> well-honed tool for a particular class of celestial navigation observations,
> namely, ordinary altitude observations. And here and there in the tables of
> the Nautical Almanac, you will find that some quantities are inaccurate by as
> much as three-tenths of a minute of arc, because that level of error is not
> critical for ordinary altitude observations.
>
> To get the correct semi-diameter of the Moon, just take out the Moon's HP from
> the almanac for the correct hour of observation. Multiply that by 27.27%. Then
> apply the augmentation. You can calculate this or just use a short lookup
> table as follows: if the Moon's altitude is between 10 and 30 degrees, add 0.1
> minutes of arc, between 30 and 60 add 0.2', above 60 add 0.3' (see "Easy
> Lunars" on my web site). That's close enough in most cases. Note that if the
> Moon is lower than 15 degrees or so, you have to take refractional flattening
> into account, but you could just as well wait until the Moon's higher than
> that.

I'll take the word of a guy that knows the exact diameter of the moon and
its distance from the observer at any given time.  And I would bet the farm
you know how to determine that.

For those of us OCD in other areas, if I recall you stated your formula was
approximate.

You said 27.27%, Alex posted 27.277. Is his added digit meaningful?

How "approximate" is "approximate?" You wrote, "Actually, the correct
horizon SD at the time of the observation was 15.9 (calculated from 27.27%
of the HP --you should always use a calculated SD when doing lunars). Then
you need to augment it for altitude which gives 16.0 minutes of arc. The
result is then quite a bit closer, but yes, a little off --an error of 0.14
minutes of arc in the Moon's diameter.

"0.14 minutes of arc." Off what standard, please?

Thanks again

Bill

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