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PostPosted: Sat, 05-07-14, 17:33 GMT 
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chuft-captain wrote:
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But clearly in Deep Space or in the regime of Cosmology with relevant length scales of many Megaparsecs, it would be idiotic to ask about individual stars.
I would suggest that there are very limited circumstances in which we would take an interest in individual objects at these distances, and those are when they are special classes of rare objects such as Quasars, which are not stars even though they appear to be similar to stars and emit huge amounts of energy, because they are "useful reference points in establishing a measurement grid on the sky", and because of what they might tell us about the history and formation of galaxies.
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Agreed.

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PostPosted: Thu, 31-07-14, 23:33 GMT 
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chuft-captain wrote:
I would suggest that there are very limited circumstances in which we would take an interest in individual objects at these distances, and those are when they are special classes of rare objects such as Quasars, which are not stars even though they appear to be similar to stars and emit huge amounts of energy, because they are "useful reference points in establishing a measurement grid on the sky", and because of what they might tell us about the history and formation of galaxies.


In this IERS site you can see an application of the Very Large Baseline Interferometer and other devices. For example in the plot below is shown the polar motion (x,y) for the month of July current year and its deviation from the "non-rotating origin" of the CIP (Celestial Intermediate Pole, aka UT1-UTC and x,y = 0). The Earth's "axis of figure" is not the CIP (X,Y) but it is closest to it within 20 microarcseconds/century. The x,y values are measured, tabulated and inserted within the IERS/IAU precession-nutation models and their conversion matrices through a series of reference quasars and DSOs, being the polar motion basically unpredictable. (red&green mine)

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IERS_pole.png
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