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From: Bill (no email)
Date: Fri Dec 09 2005 - 17:31:16 EST
> 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.
<G>
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
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