A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2015 Dec 12, 14:57 -0800
Hi Brad, you wrote:
"The lunar distance is calculated from the center of the earth, not from its surface! The observer is at the surface, which will cause parallax. Parallax moves the moon's position, from location as if viewed from the center of the earth, to location as viewed from the earth's surface."
Yes, that's right. For a standard lunar, we're talking about geocentric center-to-center lunar distance arcs. And the traditional process of clearing the lunar distance is all about removing the effects of parallax and refraction so we can compare them with the tabulated geocentric distances. Note that it was, and is, nearly universal to do a "pre-clearing" step first where we first add on the semi-diameters of the Moon and Sun (if the Sun is the other body) or occasionally we subtract the Moon's semi-diameter in the case of a far limb lunar. In the case at hand, we have Venus sitting right on the limb of the Moon. This implies that the measured lunar is zero. During the pre-clearing step add on the Moon's SD, on the order of 16' of arc. Next we "clear" the distance which would normally add on a couple of minutes of arc for refraction and up to a degree or so (positive or negative) for the Moon's parallax. In the case of an occultation, the refraction is identical for the two bodies so that's zero.
"Unless the occultation occurs right at the zenith, the lunar distance will never be equal to the SD of the moon."
Yes, that would be the geocentric center-to-center distance in that case except for the small issues which Peter Monta pointed out already. By the way, I wasn't at all sure what you were asking in your original question on this, which was why I didn't reply sooner!