Re: Astronomical coordinates
- From: pausch@xxxxxxx (Paul Schlyter)
- Date: Fri, 13 Jan 2006 08:13:19 GMT
In article <43C70A4D.F2A6550B@xxxxxxxxxxxxxxxxx>,
Jake <jakmal@xxxxxxxxxxxxxxxxx> wrote:
> Jonathan Silverlight wrote:
>>
>> In message <IID+uCAi4RxDFwvE@xxxxxxxxxxxxxxxxxx>, Dr John Stockton
>> <jrs@xxxxxxxxxxxxxxxxxx> writes
>>>JRS: In article <1136866044.838100.246890@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>
>>>, dated Mon, 9 Jan 2006 20:07:24 local, seen in news:sci.astro,
>>>jgreen@xxxxxxxxxxx posted :
>>>
>>>>If you are an Einstein subscriber, and apply c=c+v to radar ranging of
>>>>the sun,(sic planets)
>>>>the astronomical unit varies up to 274,000kms whether measured at dawn
>>>>(radar approaching sun) or dusk (retreating).
>>>
>>>Since the Astronomical Unit is defined as the *mean* distance between
>>>earth and sun, your conclusion would be wrong even if your speed
>>>argument were otherwise correct, which it is not.
>>>
>>>The definition does not depend on the measurement method.
>>>
>>>Moreover, ISTR that the AU is not determined by ranging the Sun (which
>>>has a rather uncertain surface well away from its middle) but by ranging
>>>other planets, such as Venus.
>>>
>> A bit of rummaging around on the Web shows that the definition is a lot
>> more complex than that. This seems typical
>> " the radius of an unperturbed circular orbit a massless body would
>> revolve about the sun in 2*(pi)/k days (i.e., 365.2568983.... days),
>> where k is defined as the Gaussian constant exactly equal to
>> 0.01720209895." <http://neo.jpl.nasa.gov/glossary/au.html>
>> He's wrong about c'=c+v, too. Or "not even wrong".
>
> The AU is the average distance from the center of Earth to the center of
> Sol,
That was the original definition of the AU, true. But this was
changed some time in the 1800's or early 1900's, in order to not have
to adjust k (the Gaussian gravitational constant) when improved values
of the masses of the Sun and the planets were obtained. Therefore,
nowadays the average distance from the center of Earth to the center
of the Sun is considered to be slightly more than one AU -- I believe
it's something like 1.0000004 AU, so the difference from 1 AU exactly
is very small.
> not the distance from the surface of Earth to the surface of Sol.
....in particular since you then would have to specify the distance
from WHICH surface point on Earth to WHICH surface pont on the Sun... :-)
> Radar has nothing to do with measuring the AU,
That's wrong! Radar ranging in the 1960's gave a much better
precision in our measurements of the value of the AU. Nowadays I
believe radio ranging from interplanetary spacecraft is the preferred
method though - the difference is that the interplanetary spacecrafts
all have active radio transmitters, instead of merely being passive
reflectors.
> even if it was possible to radar-range the sun (which I don't believe
> for a second, and for reasons that have nothing to do with anti-Einsteinism).
Radar ranging of the Sun has its difficulties -- the Sun's radar
surface has no precise definition afaik. Therefore, in the 1960's,
one did radar ranging of nearby planets (Venus, Mars) instead, to
improve our value of the AU.
--
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e-mail: pausch at stockholm dot bostream dot se
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- From: Jake
- Re: Astronomical coordinates
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- Re: Astronomical coordinates
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