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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: venus
From: Frank Reed CT
Date: 2004 Oct 13, 07:01 EDT
From: Frank Reed CT
Date: 2004 Oct 13, 07:01 EDT
George H wrote:
"At the moment of Michael's observation their ecliptic latitudes differed by about 4 degrees, with their ecliptic longitudes differing by about 15 degrees. That is enough to seriously affect the angle of the line joining Moon and Venus, in relation to the direction of the Moon through the sky, and therefore to slow the changes in lunar distance, significantly. "
It's surprising, but this isn't really a big deal. The rate of change in the Moon-Venus distance at the time in question was 27.5 minutes per hour while the rate of change for star lunars (Aldebaran, Pollux) was 30.5 minutes. That 10% difference would have little practical impact. You might end up with an 11 minute error in longitude from your lunar instead of a 10 minute error.
Imagine the appearance of the objects through the sextant telescope when contact has been made. If the star or planet is directly in line with the Moon's motion, then it will be found, very nearly, sitting on the Moon's limb at a spot which is just halfway between the horns. If you see the other body at that location, 90 degrees from the line through the horns, then the rate of change in the lunar distance is as good as it gets (apart from whatever motion that body has on its own). Let's suppose that we never want to shoot lunars unless the rate of change of distance is at least 75% of that optimal value. Where would the star/planet rest on the Moon's limb, as seen through the sextant, for that less than optimal condition? Unless I've missed something, it could be nearly halfway to the Moon's horn (41 degrees off the central line) which is much farther out of line than you might suppose at first glance. Incidentally, that's why using Altair and Markab for lunars was not as crazy as it seems sometimes.
Frank R
[ ] Mystic, Connecticut
[X] Chicago, Illinois
"At the moment of Michael's observation their ecliptic latitudes differed by about 4 degrees, with their ecliptic longitudes differing by about 15 degrees. That is enough to seriously affect the angle of the line joining Moon and Venus, in relation to the direction of the Moon through the sky, and therefore to slow the changes in lunar distance, significantly. "
It's surprising, but this isn't really a big deal. The rate of change in the Moon-Venus distance at the time in question was 27.5 minutes per hour while the rate of change for star lunars (Aldebaran, Pollux) was 30.5 minutes. That 10% difference would have little practical impact. You might end up with an 11 minute error in longitude from your lunar instead of a 10 minute error.
Imagine the appearance of the objects through the sextant telescope when contact has been made. If the star or planet is directly in line with the Moon's motion, then it will be found, very nearly, sitting on the Moon's limb at a spot which is just halfway between the horns. If you see the other body at that location, 90 degrees from the line through the horns, then the rate of change in the lunar distance is as good as it gets (apart from whatever motion that body has on its own). Let's suppose that we never want to shoot lunars unless the rate of change of distance is at least 75% of that optimal value. Where would the star/planet rest on the Moon's limb, as seen through the sextant, for that less than optimal condition? Unless I've missed something, it could be nearly halfway to the Moon's horn (41 degrees off the central line) which is much farther out of line than you might suppose at first glance. Incidentally, that's why using Altair and Markab for lunars was not as crazy as it seems sometimes.
Frank R
[ ] Mystic, Connecticut
[X] Chicago, Illinois
