Re: space time

On Tue, 22 Aug 2006 18:48:01 -0500, Dick <remdickhm@xxxxxxxxxxxxx>
wrote:

On Tue, 22 Aug 2006 16:58:28 -0400, r norman <r_s_norman@xxxxxxxxxxxx>
wrote:

On Tue, 22 Aug 2006 15:44:00 -0500, Dick <remdickhm@xxxxxxxxxxxxx>
wrote:

On 22 Aug 2006 08:25:38 -0700, "Dwib" <dwibdwib@xxxxxxxxx> wrote:

*** wrote:
Light-year is also strange because it is a limited measure. Only so
many kilometers in a earth-year, fewer but never more.

You are over-analyzing the definition of "light year"... nearly to the
point of comedy.

The definition "A light-year is the distance light travels in 1 year"
is an approximate definition... as you have correctly pointed out.

When confronted with the fact that a "year" changes with time, any
scientist worth their PhD will quickly switch from using light-years
(as a distance measurement) to mega-kilometers or some other
well-defined distance standard.

Dwib

If one doesn't recognize that gravity controls the radius of the orbit
thus the cycle distance, it is stable and would have been familiar to
a world used to using the sun for so many other events in their lives.
In early times it was worshipped, so not hard to believe it would be a
gold standard to compare earth orbital circumference with the distance
a light would travel in one earth orbit. The approximate speed of
186,000 miles/second also is relative to a fraction of the earth's
rotation. A second is a fraction of the time/distance of one period
of earth rotation at the equator.

Time measures seem to all come to standard measures of length ratios.
Clocks are standardized on energy entropy, uniform release of energy
counted and accumulated and displayed.


Has anyone yet pointed out that a "light year" is now defined as the
distance light travels in a vacuum in one "Julian year", where "Julian
year" is defined as 365.25 days and one day is defined as .....?

In other words, "light year" as used by astronomers is defined in
terms of the speed of light and the definition of the second and has
absolutely nothing at all to do with gravity and the orbit of the
earth.



I don't see your point. No matter what the measurement, it equates to
the distance the earth travels around the sun. Further, no such
measure can be a constant. The sun is consuming energy thus earth's
orbit is getting longer. Less sun mass, the greater the radius sun to
earth, the greater the circumference.

I presume light speed does not change, but the ratio of earth's travel
distance in one rotation is changing. The distance light travels
while earth makes one rotation is not changing.

The constant will eventually become significantly different from the
current ratio.

Better if speed of light did not reference one day or one year. How
far light travels could be expressed as a unit more reliable such as
the distance the hydrogen electron travels in one rotation.

I still don't believe there is a time factor.

Once again I fail to understand either your point or your argument.
Let me try to clarify.

The specific issue is on the supposed inconstancy of the notion of a
light-year since the time it takes the earth to orbit the sun is not
constant. The speed of light is assumed to be a universal constant.
If you want to argue that it varies, that is another story. However,
it is defined as a physical phenomenon quite separate from the
earth-sun relationship. So the constancy of a light-year depends on
the constancy of the year. As I indicated, the modern astronomical
definition of a light-year uses a definition of 'year' that is
standardized in terms of the definition of second.

I believe your misconception arises from how to define a second.
Originally, it was a fraction of a minute which was a fraction of an
hour which was a fraction of a day which was determined by the earth's
spin, one second was "1/86,400 of a mean solar day." It has been known
for a very long time that the spin is quite inconstant so the second
was first redefined in 1960 as "the fraction 1/31,556,925.9747 of the
tropical year for 1900 January 0 at12 hours ephemeris time". The
ephemeris year 1900 is constant. However more recently, atomic clocks
became more accurate than astronomical measurements so the second was
revised again in 1967. The SI unit of the second is "the duration of
9,192,631,770 periods of the radiation corresponding to the transition
between the two hyperfine levels of the ground state of the cesium 133
atom." That definition is intended to give the same result, within
experimental error, as the ephemeris second of 1960.

So the second is a physical constant depending on the properties of
the Cesium atom and the speed of light is a physical constant
depending on the properties of electromagnetic radiation or quantum
electrodynamics or whatever other physics you care to invoke. So the
light year is a physical constant, or at least as close to constant as
you can get.

The earth and the sun have absolutely nothing to do with the
light-year except in the choice of how many seconds to let the light
travel. That number was chosen to be quite close to the orbital
period of the earth around the sun, but that is irrelevant to the
definition.


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