# NavList:

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

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Re: venus
From: George Huxtable
Date: 2004 Oct 17, 00:53 +0100

```Concerning the correction to make for Moon parallax in the "backwards"
direction, taking the Moon's true altitude TA and correcting it to give
what would be the "observed" altitude OA, I wrote-

>> The angle to be subtracted from TA to obtain OA is, in this routine-
>>
>> Arc-tan ( cos TA / ((23455 * D) - sin TA))

and Herbert Prinz has responded-

>This does not look right to me. Could there be a typo?

===========

Response from George-

Herbert, I don't THINK there's any typo, but if it worries you, then that
worries me!

What makes you worry? Are you getting silly answers?

Remember, the moon's radial distance from the Earth is expressed in AU
(astronomical units, in which the Sun's distance is 1.

The Moon's mean radial distance from Earth's centre is .00257 AU, so the
denominator of the expression above always comes to somewhere near 60,
which is, roughly speaking, the ratio of the Moon's radial distance to the
Earth's radius. This means that the correction for Moon's HP always comes
out to be a bit less than 1 deg, as it should. So the order-of-magnitude
seems OK.

If you would like an explanation of how the above expression was derived,
just ask, and I will do my best.

George.

================================================================
contact George Huxtable by email at george---.u-net.com, by phone at
01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
================================================================

```
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