# 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 13, 23:56 +0100

Frank Reed wrote- "By the way, does anyone do oblateness the way Chauvenet does? That's how my calculator does it." And Michael Dorl wrote- "As far as I can tell there is nothing in the routines to allow for oblateness of the earth. One can specify a elevation. Looks like the earth is assumed to have a radius of 6378137 Meters, same as equatorial radius in my 1988 AA page K6. If I include an elevation of -9600 meters, I get a ctr/ctr distance of 16d20'25.88". Taking off a moon semi-diameter of 15'3.88" gives a ctr to limb distance of 16d5'22" or 16d5.37' which is close to Frank's 16d5.5'. I got the -9600 meters by considering the earth to be an oblate spheroid. I think Mosier considers the earth to have an equatorial radius so setting the elevation negative should compensate for the oblateness." Michael's use of an adjusted altitude to allow for the Earth's oblateness is new to me, and deserves a bit more thought before I comment on it. I'm aware of two ways in which the oblateness of the Earth comes in. 1. It affects the apparent ALTITUDE of the Moon, by an amount which was supplied in navigators' table for "Reduction of the Moon's horizontal parallax", which effectively multiplies the HP by (1- ((sin lat)squared /300)). This can change the Moon's altitude by up to 0.2', at the equator. This may already be a "hidden" part of Michael's clearing process; it would need to know the observer's latitude, very roughly. 2. It affects the AZIMUTH of the Moon, by a similar amount, of up to 0.2'. For ordinary navigation using Moon altitudes above the horizon, parallax in azimuth has no effect at all. Even for lunar distances, which ARE affected by a displacement in azimuth of the Moon, there would be no parallax in azimuth if the Earth was a sphere. It's the oblateness that gives rise to the change in azimuth of up to 0.2', greatest at latitudes of 45d, greatest for azimuth differences between Moon and body of ?90d. Some fraction of this parallax in azimuth should be applied as a correction to the lunar distance, depending on the angle the line between the Moon and the body makes with the horizontal. This correction is mentioned in Meeus, but no formula are given. Smart's Text-book on Spherical Astronomy, in para. 121, shows how to derive the correction in azimuth. Chauvenet treats the matter in some detail, and very thoroughly, as is his wont. If Frank is allowing for that azimuth correction to lunar distance (and for ultimate accuracy, that really should be done) then it's a step further than most others have taken their clearing of lunar distances. In my own clearing process, that term has been neglected, and it may explain at least part of the small divergence between Frank's "clearing" results and my own. Henning Umland has shown me his routine for making such a correction, which may now be included in the lunars section of his website. =============== I suggested, about that divergence between Frank's lunar "clearing" and my own- "It might be enlightening to track it down." And Frank replied- "Thanks for questioning this. It could easily be that I've got a sign wrong somewhere or something like that. I'll see what I can find." It's just as likely to be an error or approximation of my own, as one of Frank's, but it would be good to get to the bottom of it. What I suggest is that we take a simple situation that involves little or no trig and see if we agree then. Perhaps an on-land lunar distance taken between the Moon and Schedar, at a GMT of 19h 51m 35s on 29 Oct 2001, at lat N 56d 32.9', long E 34d 03.3', somewhere near Moscow. If I've made no error, then Schedar would be dead over the observer's head, and the Moon would be exactly to his South. In which case, the lunar triangle shrinks to a straight line, there's no correction for the star, the true lunar distance between centres would depend only on the Moon's dec., and the clearing process would be a matter of simply adding/subtracting the Moon's parallax and refraction. In that case, by the way, there would be no Moon azimuth correction. Perhaps, if Frank would kindly check those initial assumptions, and if he agrees they make for a suitable test, we might compare our results for lunar distance between centres. Whether it's best to take this comparison of fine-points off-list, or to leave it on in case it interests others, is up to Frank; I have no strong feelings either way. 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. ================================================================