[time-nuts] Measuring speed of light or reproducing a metre

Jim Lux jimlux at earthlink.net
Mon Jun 24 18:04:17 EDT 2013


On 6/24/13 2:26 PM, Chris Albertson wrote:
> On Mon, Jun 24, 2013 at 1:55 PM, Jim Lux <jimlux at earthlink.net> wrote:
>
>> On 6/24/13 10:08 AM, Chris Albertson wrote:
>>
>>>
>>> Isn't that the Fizeau technique, which antedates Michelson's?
>>
>
>   Michelson got the precision good enough that it finally put the question
> to rest.  We used a miles-long baseline of very clear and still air.
>
>>
>> You need to know the rotation rate of the toothed cog or rotating mirror,
>> don't you?
>
>
> I think you have to count the number of rotations over a larger time
> interval.  And at the same time make sure the spin rate is constant.  They
> used a large hexagon with mirrors on all the faces and spun it up using
> some kind of clock work.

An Air turbine?, from the descriptions of "screaming" while in operation.

>
> You can see if the wheel is speeding up or slowing down because the slit of
> returned light should have constant offset.  So the experiment has a
> built-in check on the rate remaining constant.  And then you count the
> turns over some long interval using gears or what not.   They built a shed
> to house this thing on the side or Mt. Wilson.  All that is left no is the
> concrete foundation and concrete pier for the instrument.  This was a not
> small scale lab experiment.  It must have been well funded to be able to
> pour tons of concrete at a remote location like that.
>
> I think the harder part is knowing what the long baseline is.  How to
> measure 5 miles distance with the required accuracy in 1900?

Measuring the 22 miles from Mt Wilson to Baldy was the work of the US 
Geodetic survey. It would have been done by triangulation and reference 
to a precisely measured baseline (which I believe was in Pasadena 
somewhere).  1 ppm would have been achievable.


(Baseline street in the inland empire is based on San Bernardino peak, 
and is the reference for the state plane grid in Southern California)



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