NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: The Moon is a "nuisance"
From: Frank Reed CT
Date: 2006 Oct 12, 19:55 EDT
From: Frank Reed CT
Date: 2006 Oct 12, 19:55 EDT
Hi Bill. You wrote:
"When attempting to use the moon in a day sky for IE/IC checks--as Frank
suggested several moons ago"
I suggested the twilight moon (at least I think I did <g>). There is that moment, especially when the Moon is more than half full, in early twilight, where the Moon's edge stands out beautifully against a sky just darkened after sunset. It's then that I have had really excellent results measuring sextant index error using the Moon.
And just to re-iterate, I much prefer a table-top observation using a laser for a reliable measurement of index correction.
And also:
"--and in checking observed 4SD against the published SD it struck me that at times determining the moon's SD at the time of observation was at best problematic."
Yes, I agree, that this should make you worry. Honestly, there should be little doubt that a properly-adjusted sextant can measure the size of the Moon to excellent accuracy. If your results don't match good almanac data, then something is up. Do you use the highest available magnification for these observations? I prefer a 7x telescope for this sort of thing. And... do you read your micrometer correctly? That second question may sound ridiculous, but I've absent-mindedly read it wrong in this observation so many times that I think it's worth emphasizing. Specifically, when reading the off-arc diameter of the Moon, the micrometer might read, as an example, 29.5 minutes of arc. It is so easy to make the mistake of writing down that value "29.5" when, in fact, the actual observation off-arc is 30.5. Since the final result will be fairly close to the actual diameter of the Moon (assuming small I.C.) whether you read it wrong or right, it's an easy error to overlook. Most likely, Bill, you already do this, but if you don't, say it aloud when you measure off-arc, 'I have to count back from zero'. I realize that this is *really obvious* when the angle is small, like 5 minutes of arc, but in the case of the semi-diameter of the Sun and Moon, it's very easy to screw it up since the values are close 30.
-FER
42.0N 87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars
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"When attempting to use the moon in a day sky for IE/IC checks--as Frank
suggested several moons ago"
I suggested the twilight moon (at least I think I did <g>). There is that moment, especially when the Moon is more than half full, in early twilight, where the Moon's edge stands out beautifully against a sky just darkened after sunset. It's then that I have had really excellent results measuring sextant index error using the Moon.
And just to re-iterate, I much prefer a table-top observation using a laser for a reliable measurement of index correction.
And also:
"--and in checking observed 4SD against the published SD it struck me that at times determining the moon's SD at the time of observation was at best problematic."
Yes, I agree, that this should make you worry. Honestly, there should be little doubt that a properly-adjusted sextant can measure the size of the Moon to excellent accuracy. If your results don't match good almanac data, then something is up. Do you use the highest available magnification for these observations? I prefer a 7x telescope for this sort of thing. And... do you read your micrometer correctly? That second question may sound ridiculous, but I've absent-mindedly read it wrong in this observation so many times that I think it's worth emphasizing. Specifically, when reading the off-arc diameter of the Moon, the micrometer might read, as an example, 29.5 minutes of arc. It is so easy to make the mistake of writing down that value "29.5" when, in fact, the actual observation off-arc is 30.5. Since the final result will be fairly close to the actual diameter of the Moon (assuming small I.C.) whether you read it wrong or right, it's an easy error to overlook. Most likely, Bill, you already do this, but if you don't, say it aloud when you measure off-arc, 'I have to count back from zero'. I realize that this is *really obvious* when the angle is small, like 5 minutes of arc, but in the case of the semi-diameter of the Sun and Moon, it's very easy to screw it up since the values are close 30.
-FER
42.0N 87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
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