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## Should the Astronomical Unit be replaced by the Light Second/Light Minute Etc.?

 Quote by BadBrain Please understand that my ideas on this subject are still evolving, and respond in the light of that understanding.
There's no need to get defensive about it. I don't see twofish's statement as condescending or anything like that, merely informative.
 Given that 1 AU is almost equal to eight light minutes, we do have a 'scientific' conversion factor... IMHO, keeping the AU is handy because of the convenience when dealing with Sol-types and 'goldilocks zones'. It's a nice, round number for hand-waving. It's the same argument over LY vs Parsecs, really. One embraces a solar system out to the Oort cloud, is handy for long-period and common-motion binaries beyond easy AU reckoning. The other is a convenient yardstick for neighbouring systems. IMHO, LY are more convenient when you look at relative positions of the neighbours, but that's just my preference. Kilo-parsecs and their mega-parsec associates come into their own for galactic astronomy. Just don't get kilometres and nautical miles confused...
 Well, the astronomical unit is essential in parallax distance measurements of other stars. Namely, as the Earth revolves around the Sun, the apparent position of a star traces an elliptical trajectory on the night sky. If the angular size is $2\theta$, then the distance to the star d is: $$\tan \left( \frac{\pi}{180 \times 3600} \theta(") \right) \approx \frac{\pi}{180 \times 3600} \theta(") = \frac{r}{d} \Rightarrow d = r \frac{206265}{\theta(")}$$ where the small angle approximation was used. If r is chosen as a unit of lenght (1 A.U.), then this formula naturally gives the distance in those units. Thus, a distance of 1 parallax-second (parsec) corresponds to 206265 A.U. Of course, if you want to convert these distances into metric units, you must measure the radius of Earth's orbit by other methods.

 Quote by Nik_2213 IMHO, keeping the AU is handy because of the convenience when dealing with Sol-types and 'goldilocks zones'. It's a nice, round number for hand-waving.
It's also pretty essential for any sort of precision solar system astronomy. The good thing about the AU is that it is "local." If you try to convert from something local to something universal, you end up with a lot of conversion factors whose accuracy is uncertain, and that degrades your results.

 It's the same argument over LY vs Parsecs, really.
And you have similar issues. With a parsec, you can take a measurement of a star, and then instantly convert it into a distance, without going putting in any conversion factors whose numbers are uncertain. When you do light years, then you have to do conversions whose accuracy is not certain, and that introduces a lot of issues.

One funny thing is that when you talk with observational astronomers in the United States, they talk in terms of inches. The reason for that is that if you have build an instrument, you will go through all sorts of heck trying to get metric screws. And yes, this causes big, big problems (i.e. Mars Climate Orbiter failing).
 Recognitions: Gold Member What conversions are you referring to? Why are they inaccurate?

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 Quote by twofish-quant There's a problem, which is that if you use SI units, the conversion factors to and from celestial mechanical units have more uncertainty than the observations which are available. We can do celestial mechanics to something like nine significant digits, whereas G is known only to six.
That's overstating things with respect to G. G is known to a tad less than four significant digits, not six. The 2010 CODATA value for G is (6.67384±0.00080)x10-11m3kg-1s-2, or a relative uncertainty of one part in 1.2x104.

One way around this is to use the product G*M, which is known to almost ten significant digits for the Sun. Another way around this is to use IAU units, which essentially whitewashes away the problem of the lack of precision in G. In IAU units, the gaussian gravitational constant is a defined constant. Almost all of the uncertainty in the value of the AU is attributable to the uncertainty in the solar gravitational parameter, multiplied by 3/2.
 Blog Entries: 27 Recognitions: Gold Member Homework Help Science Advisor i think the basic unit of time should be called the tiny-tim

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