Johnny5 Posted June 2, 2005 Posted June 2, 2005 Does anyone know what the celestial ephemeris origin is? Global rotation of the non-rotating origin
Johnny5 Posted June 4, 2005 Author Posted June 4, 2005 A couple of reasons. For one, i was hoping to discuss it with someone already familiar with it, and two I had some followup questions.
swansont Posted June 5, 2005 Posted June 5, 2005 A couple of reasons. For one' date=' i was hoping to discuss it with someone already familiar with it, and two I had some followup questions.[/quote'] Then you look it up and ask the interesting questions, instead of asking to be spoon-fed.
Johnny5 Posted June 5, 2005 Author Posted June 5, 2005 Then you look it up and ask the interesting questions, instead of asking to be spoon-fed. Very well... Here is the abstract from the link in the first post of this thread: Abstract: Global rotation of the non-rotating origin We demonstrate the global secular rotation of the Celestial Ephemeris Origin (CEO)' date=' the nonrotating origin (NRO) for Earth's equator adopted recently by the IAU. The current speed of this global rotation is as high as -4.15" yr-1, where the minus sign indicates that the CEO rotates clockwise with respect to the inertial frame when viewed from the north celestial pole. Such secular rotation does not exist for some geometrically defined longitude origins. We think that the existence of a global secular rotation means that the CEO, and the NRO in general, is not appropriate to be specified as the x-axis of a celestial coordinate system. [/quote'] Apparently, the celestial ephemeris origin, is the origin of a particular frame of reference. Apparently, the IAU (international astronomical union) adopted some location on earth's equator to be the origin of a reference frame which is special for some reason or other. So two questions. What is so special about what I'll go ahead and call "the celestial ephemeris frame"? That my first question. Next... The author of this paper cites that the origin of the CEO which is supposed to be "non-rotating", is actually rotating -4.15" per year. That seems like nonsense. The earth is moving in some complex manner. How in the world does anyone come up with rotating 4 inches. The sentence doesn't even make sense. Does he mean circumference of something is 4 inches. Just what is meant by rotating 4.15 inches?
swansont Posted June 5, 2005 Posted June 5, 2005 I don't think the CEO is on the earth. It is in a celestial coordinate system. You can translate that back to the earth's orientation, but the earth undergoes a bunch of different motions that affect the rotation rate and the orientation of the pole. (That's why earth rotation is no longer used as the basis for timekeeping - it's not a very good clock). So you use a celestial frame (using distant objects outside the galaxy) and measure the orientation of the earth with respect to that. The unit in 4.15" is, I believe, arcseconds, not inches.
swansont Posted June 6, 2005 Posted June 6, 2005 From the Coincidence Files: Prof. Fukushima is giving a colloquium here tomorrow. "Efficient Orbit Integration by Manifold Correction Methods."
Johnny5 Posted June 7, 2005 Author Posted June 7, 2005 From the Coincidence Files: Prof. Fukushima is giving a colloquium here tomorrow. "Efficient Orbit Integration by Manifold Correction Methods." I looked back at the site for "coincidence files" and didn't find them. However, I did find this: Abstract: At its 24th General Assembly held at Manchester last year, the IAU has adopted the Celestial Ephemeris Origin (CEO) as a new longitude origin of the celestial coordinate system (Capitaine et al. 2000, IAU 2001). The CEO is the application of Guinot's non-rotating origin (NRO) to the Earth's equator (Guinot 1979, Capitaine et al. 1986, Capitaine 1990). By using the current IAU precession/nutation theory, we integrated the global orbit of CEO. It is a slightly curved zigzag pattern of the amplitude of around 23o moving secularly along the ecliptic. Among its kinematical features, we note that CEO has a large secular component of rotation with respect to the inertial reference frame. The current speed of this global rotation is as large as around -4.15 ''/yr. The negative sign shows that CEO rotates clockwise with respect to the inertial frame when viewed from the north celestial pole. Unfortunately this is a general property of NROs. On the other hand, such secular rotation does not exist for some geometrically-defined longitude origins like K, H, and Sigma already discussed in Kovalevsky and McCarthy (1998). We think that the existence of a global secular rotaion means that the CEO, and NROs in general, is not appropriate to be specified as the x-axis of celestial coordinate systems. What are they referring to as "the inertial reference frame"??? As if there is only one. ? And also, what are/where are the coincidence files? Regards
swansont Posted June 7, 2005 Posted June 7, 2005 And also' date=' what are/where are the coincidence files?[/quote'] It was a joke; I observed that you referred to Fukushima's paper and he will be giving a talk at work on something way over my head. The local Earth Orientation experts with whom I would discuss this were not in today. So this will have to wait until i get a chance to ask the questions. I suspect, however, that this will have to do with the earth's motion not being entirely predictable, so there are still problems translating from the earth frame to the celestial frame .
Johnny5 Posted June 7, 2005 Author Posted June 7, 2005 It was a joke; I observed that you referred to Fukushima's paper and he will be giving a talk at work on something way over my head. The local Earth Orientation experts with whom I would discuss this were not in today. So this will have to wait until i get a chance to ask the questions. I suspect' date=' however, that this will have to do with the earth's motion not being entirely predictable, so there are still problems translating from the earth frame to the celestial frame .[/quote'] Ok, thank you very much for this answer Dr. Swanson. And now I get the joke. Are you really serious? This Dr. Fukushima will be where you are, or was where you are? i think thats awesome. Well ok thanks, we can talk about it more. I am still trying to figure out just what CEO is, where it is, how you switch frames properly, what is the significance of it, and why did Dr. Fukushima refer to it as "THE" inertial frame.
swansont Posted June 7, 2005 Posted June 7, 2005 Yes, he was here. I wasn't able to attend the colloquium, even if I had wanted to. I did check with some people and I was basically right. No matter what you do, the translation between an earth frame and a celestial frame requires earth orientation information that must be measured (using distant quasars and interferometers) because it cannot be modeled. Even though the rotation of the earth frame and the celestial frame are supposed to be related linearly through UT1 (earth time), UT1 is not predictable!
Johnny5 Posted June 7, 2005 Author Posted June 7, 2005 Even though the rotation of the earth frame and the celestial frame are supposed to be related linearly through UT1 (earth time), UT1 is not predictable! UT1?
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