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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Venus
From: Bill B
Date: 2005 Dec 9, 17:31 -0500
From: Bill B
Date: 2005 Dec 9, 17:31 -0500
> You may be looking at errors of up to 1.0', but you will hardly be > looking at Venus at inferior conjunction before June 6, 2012! First, I acknowledge I am out of my depth here, hence my request for assistance. My source, "The Night Sky Deck." Text by Martin Ratcliffe, star charts by Charles Nix. Published by Barns & Nobel 2005, ISBN 0-7607-7394-7. Perhaps semantics. The above source defines conjunction as, "Conjunction. A line-of-sight alignment of two objects in the sky presenting small angular separation." Not exactly on the same horizontal plane, but inline despite declination. Looking at the 2006 almanac, between 10 and 11 UT January 13 Venus and the sun will have identical GHA's. "The Night Sky Deck." states the next "transit" of Venus *across* the sun will be "June 5-6, 2012." Of course, Venus would not be visible then except as dark spot on the sun--which may or may not arc perfectly through the center of the sun. After that we wait until December 11, 2117 for the next transit. My hunch is because Venus's southern declination is nominally 16d and the sun's nominally 21.5d on January 13, 2006, that the sun is lower in the sky than Venus (40d N) and only the bottom sliver of Venus will be illuminated. > That said, in principle, you are right. But I don't understand your > point about the almanac correction being for altitude only. If the > almanac tabulates center of illuminated area instead of center of mass, > why would this not be applicable to all kinds of optical observation? Simple answer, the correction is relative to the natural horizon, not to splitting the edge of a moon at the same apparent elevation. Go back to a time prior to when we looked at Venus. At maximum eastern elongation Venus would be 23" diameter and split in half almost vertically relative to the natural horizon (3 Nov, 2005). By the time Venus reachesd what I called "inferior conjunction" the sliver would be (by my reckoning) almost parallel to the natural horizon and on the bottom of a 60"-or-more body. With a diameter over 60", the visible center would be almost 30"below the center of mass. About the date we are playing with (approx 7 December, 2005) Venus would be have been 40" diameter and the "horn-to-horn" line from the .26 illumination would be a diagonal. If I am doing cel nav, the additional correction (adjust for the bottom sliver illuminated below the center of mass--relative to the horizon on 13 Jan, 2006 is simple for 0-26d elevation. Raise the illuminated rim at the lower limb of Venus of by 30" (0.5') to determine the center of mass. But how do you handle a lunars shoot where Venus not directly above or below the moon? Not so bad given the resolution of the system, when Venus is at maximum eastern or western elongation, 14" diameter, 50% illuminated, is at the same elevation as the moon. Realistically, on 13 Jan, 2006 the pairing is unusable. The moon marches to its own drummer. If I recall it takes 19 years to repeat a pattern. It would appear the earth, Venus, and the sun line up on one plane approx. every 584 days, as Venus "laps" the slower earth. Orbital resonance has the five paths repeating every eight years. Given the 19 and 8 year patterns, it is unlikely that a similar moon/Venus pairing will repeat in a human lifetime. I don't know if or when worse-case has occurred over the past 200 years from any spot on earth, or might occur in the next 200 years. I imagine the probability of worse-case is extremely low, and calculations for error will have to be made on case-by-case basis. On the other hand, when you are doing deck shoot outs (and shots of whiskey in 20 F) with steady hand, eagle eye, SNO-T, high-power-inverted-scope Alex, you need every 0.1' you can get.I wonder if the 117d angle Herbert gave might related to the center of the lighted portion along the rim (North 0, measuring east)? If that angle is close to 0 (perhaps 90 in trig coordinates) on November 3, 2005 it could be. A great help in determining corrections for lunars with Venus between maximum elongations and inferior conjunction. Thanks again. It would appear the affects of phase will be relatively small, but did not know that until I wrestled with the problem. Bill