A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Brad Morris
Date: 2013 Apr 25, 05:25 -0400
Telescope collimation introduces an error which is proportional to tan(LD/2). It is of "some concern" for lunars around 90 degrees and more so near 120 degrees. What happens with higher angles? Obviously, this error equation blows up at 180 degreees. I suppose for backsights, the error equation is different.
I've performed the only collimation adjustment that Cmdr Bauer tecommends. I aligned two stars, roughly 90 degrees apart, and kept them together across the FOV of the telescope.
I can also see that my horizon to horizon dip measurement (nearly 180 degrees) is close to the truth. Of course, I did not achieve the performance level of a theodolite. But it didn't blow up by any stretch of the imagination. Perhaps the optical principle of the reflecting circle changes the error equation?
Is there any other test available to determine collimation? I cannot imagine that this was the method used by manufacturers and calibrators of sextants.
Mr White, of Robert White Instruments - How did you fellows do it?
Yeah, I think many people thought he was joking."
With anybody else I might have thought the same thing but I spent way too much of my time trying to disabuse Mr. van Asten of his erroneous ideas about sunrise and sunset to think he was just joking about moonrise. It might be fun for the members of this list, people who do understand celestial navigation, to follow the extensive online conversations I had with him on the TIGHAR forum which discusses the disappearance of Amelia Earhart as she was approaching Howland island at about 1912 Z on July 2, 1937. The discussion starts at this link and continues on to the end of that thread.
It was finally suggested that the thread be shut down and I ended with this post:
"You don't have to, I'm done. I thought I was finally making some progress, but it's clear I wasn't.
I first communicated with Mr. van Asten after I read his two articles that are just full errors. I was making a friendly effort to help him understand how this navigation really works. I think that it is obvious to everyone that he is not interested in learning anything. He always goes back to his 0719:30 observation, the one that led to the position report that was received in Lae one minute and a half prior to that observation! I was always taught that time goes in only one direction, from earlier to later. Apparently in Mr. van Asten's parallel universe time can run in the opposite direction. Don't try to confuse him with the facts.
So, I give up, he will have to carry on without my help."
(If you do take the time to read though these posts please overlook the error I made in converting Lae local time to GMT (Lae is ten hours fast on GMT) in reply 198 since these are not significant.
"2:18 p.m., 3:19 p.m. and 5:18 p.m. (0218, 0319 and 0518 GMT.)"
2:18 p.m., 3:19 p.m. and 5:18 p.m. (0418, 0519 and 0718 GMT.))
--- On Wed, 4/24/13, Frank Reed <FrankReed---com> wrote:
From: Frank Reed <FrankReed---com>
Subject: [NavList] Re: Moonrise video
Date: Wednesday, April 24, 2013, 9:36 AM
Yeah, I think many people thought he was joking. But I have encountered this specific confusion before among people who have learned just enough positional astronomy or celestial navigation to get themselves in trouble. It is NOTHING BUT a confusion.
The Moon's "horizontal parallax" is just a convenient accounting trick. It's a means of pretending that the Moon is "really" infinitely distant on the celestial sphere but a "correction" has to be added to deal with its position in three-dimensional space. This is perfectly legitimate, of course, but it's merely accounting. Whenever one gets confused by concepts involving the Moon's HP, it's worth considering an artificial satellite in low orbit. While it is just barely possibly to pretend that a low orbit satellite is infinitely distant on the celestial sphere for positional calculations and then "correct" it by applying a huge parallax correction, it's much more obvious that the true position in space is really the more fundamental concept. A satellite is just "up there" at some point in space and you can see it if it's above your local geometric horizon (or conceivably a bit below and lifted by refraction). Like a satellite, the Moon has some position in space, which, if we wanted, we could give in Cartesian x,y,z coordinates relative to any chosen axes. Then from our position on Earth in the same coordinate system, we calculate its position in the sky directly. There's no HP then. It's just a straight-forward 3d calculation of the Moon's position relative to the observer's horizon.
The only reason we can't see the Moon rise on each and every day is because atmospheric extinction near the horizon, especially at sea level, is huge. Extinction is the reduction in apparent magnitude due to scattering and absorption by the atmosphere. At the zenith, the extinction is about 0.3 magnitudes under common conditions at sea level. If you were outside the atmosphere, stars would be about that much brighter (not much! ..except for the lack of "twinkling", the stars and the Milky Way in space look almost identical to their appearance from a high mountain and only slightly fainter than they do from sea level). Naturally, lower in the sky, the extinction is greater. At an altitude of 15 degrees, extinction rises to about 1.0 magnitudes. By 2.5 degrees altitude, it's around 5 magnitudes. That means a star like Vega would be visible to the naked eye as a very faint star at that altitude. Right at the horizon, the extinction is around 12 magnitudes which renders all the stars and planets invisible, and even the Full Moon is reduced to magnitude 0. Normally a magnitude 0 object can be seen easily, but when its light is spread out over a region half a degree wide, that's very difficult to see. But it certainly does happen. One of my favorite memories of a moonrise was seeing the Full Moon rising over Lake Michigan from the Chicago lakefront some years ago. I have photos somewhere... It was a strange, distorted, dusky dome climbing up over the horizon. I was not expecting it, and at first I did not realize what it was.
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