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Old 2012-01-10, 23:31   #34
Dubslow
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Quote:
Originally Posted by Graff View Post
It doesn't disagree substantially. The definition on the IAU page is
almost correct. My definition using k is exact. 2 * \pi / k = 365.2568983... days.
I'm an IAU member and my primary work is orbits. I will alert the
maintainers of that page to their imprecise definition.

Gareth
Ooooohhhh, cool. Sorry about that. Any chance they could put the right definition on there, instead of the less-useful... wrong definition? If the page says "define", I'd like it to actually be right.

:P

Last fiddled with by Dubslow on 2012-01-10 at 23:33
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Old 2012-01-11, 04:13   #35
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http://en.wikipedia.org/wiki/Light_year

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Before 1984, the tropical year (not the Julian year) and a measured (not defined) speed of light were included in the IAU (1964) System of Astronomical Constants, used from 1968 to 1983.[4]
So my old Texas TI-85 calculator has the exact value for the old tropical light year (although the calculator is newer than 1984) while online conversion rounds it up to the nearest 100 miles/km.

Wikipedia has the exact current Julian lightyear while online conversion again rounds it.
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Old 2012-01-11, 17:49   #36
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Quote:
Originally Posted by Dubslow View Post
Ooooohhhh, cool. Sorry about that. Any chance they could put the right definition on there, instead of the less-useful... wrong definition? If the page says "define", I'd like it to actually be right.

:P
I contacted the maintainer of that page and the imprecise statement has been replaced.

Gareth
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Old 2012-01-11, 18:01   #37
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Originally Posted by xilman View Post
Ok, one Gaussian year. Thanks for the clarification. Only goes to show how complex this situation has become over the centuries. I was clearly thinking of some other quantity which is/was measured on the basis of the J1900.0 siderial year.
And I've now discovered why I though it was the J1900.0 year (though it's the tropical year).

Quote:
Originally Posted by http://en.wikipedia.org/wiki/Light_year#Other_values
Before 1984, the tropical year (not the Julian year) and a measured (not defined) speed of light were included in the IAU (1964) System of Astronomical Constants, used from 1968 to 1983.[4] The product of Simon Newcomb's J1900.0 mean tropical year of 31,556,925.9747 ephemeris seconds and a speed of light of 299,792.5 km/s produced a light-year of 9.460530Γ—1015 m (rounded to the seven significant digits in the speed of light) found in several modern sources[5][6][7] was probably derived from an old source such as a reputable 1973 reference[8] which was not updated until 2000.[9]
Ok, so I'm seriously out of date. That's what comes of relying on old memories instead of checking with the definition du jour.

Paul
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Old 2012-01-11, 18:06   #38
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Quote:
Originally Posted by Graff View Post
One AU is the heliocentric distance at which a massless particle in a circular, unperturbed orbit would have a mean motion of k ( = 0.01720209895) radians/day.
Which would imply that the solar mass is constant.

As the mass isn't constant, at which date was the mass measured, or does it have another arbitrary value which may or may not match the true solar mass at least once?

I'm nit-picking, of course, as the mass loss from EM and neutrino radiation and solar wind, offset by the mass gain from infalling matter such as comets is an exceedingly tiny fraction of a solar mass over reasonably short time scales.


Much easier, IMO, would be to define the AU as a particular number of metres.


Paul
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Old 2012-01-11, 20:50   #39
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Quote:
Originally Posted by xilman View Post
Which would imply that the solar mass is constant.

As the mass isn't constant, at which date was the mass measured, or does it have another arbitrary value which may or may not match the true solar mass at least once?

I'm nit-picking, of course, as the mass loss from EM and neutrino radiation and solar wind, offset by the mass gain from infalling matter such as comets is an exceedingly tiny fraction of a solar mass over reasonably short time scales.


Much easier, IMO, would be to define the AU as a particular number of metres.


Paul
In the 1976 IAU System of Astronomical Constants, the length of the AU
is a derived constant, derived by multiplying one defining constant
(c, the speed of light in a vacuum) by one primary constant ($\tau_{A}$, the light-time for unit distance).

Gareth
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Old 2012-01-11, 21:04   #40
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Quote:
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I contacted the maintainer of that page and the imprecise statement has been replaced.

Gareth
oooooooooooohhh thank you thank you this is so cool
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Old 2012-01-13, 05:40   #41
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Quote:
Originally Posted by xilman View Post
Quote:
Originally Posted by Graff View Post
Urm, no. One AU is the heliocentric distance at which a massless particle in a circular, unperturbed orbit would have a mean motion of k ( = 0.01720209895) radians/day.
Which would imply that the solar mass is constant.
Since the orbiting particle is massless, wouldn't the solar mass be irrelevant?

The massless particle is not being held in circular orbit by gravity, so we're using a bit of magic rather than Gm1m2 here. There may have been a moment when k = 0.01720209895 had a physical meaning, but it's just a defined constant now.

Last fiddled with by cheesehead on 2012-01-13 at 05:46
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Old 2012-01-13, 13:14   #42
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Quote:
Originally Posted by cheesehead View Post
Since the orbiting particle is massless, wouldn't the solar mass be irrelevant?
By "massless" the definition presumably means that the mass of the orbiting body "tends to zero" so that only the sun's gravitational field should be considered. I guess there must be some complicating factor which would affect the orbital speed of a body of significant mass but I am unsure what this factor might be. Perhaps the definition is simply seeking to avoid the hypothetical situation where the body has comparable mass to the sun and then the two bodies are orbiting each other, complicating the measurements of radial speed? Or is there some other more subtle complicating effect if the orbiting body has significant mass?
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Old 2012-01-22, 02:13   #43
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Quote:
There are other options, including the Gaussian year which, I've now learned or re-learned, is the basis for the A.U. - Xilman
This was an informative thread. I think we all learned quite a bit from it.

DarJones
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