# NavList:

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

**Re: venus**

**From:**George Huxtable

**Date:**2004 Oct 17, 01:18 +0100

I just sent the following message- 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. ============== Now I've unloaded my in-box and found Frank Reed's comment that the sin and cosine in my expression were swapped. That seems more than likely; I remember starting off by working in terms of zenith angles, and then switching to altitudes, and perhaps didn't make the necessary changes. The error didn't show up as obvious in Michael Dorl's example, because, by chance, the Moon altitude turned out to be 44 deg 11.8': in which case sin alt and cos alt are nearly the same, dammit! So thanks to Herbert and Frank for alerting me to that problem. Doesn't this list work well, in terms of the scrutiny that's given to postings? Not many errors survive that process, I bet. I haven't checked it out in detail, but I have little doubt that Herbert and Frank are indeed right. But now, it's time for bed, where I should have been long ago. 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. ================================================================