[time-nuts] finding time astronomically.

Jim Lux jimlux at earthlink.net
Mon Jan 23 17:06:11 EST 2012


On 1/23/12 1:20 PM, Doug Millar wrote:
> A mercury mirror is better than a plumb bob.
>    Doug

Or Gallium?

But what sort of precision are we looking for here?

1 second of earth rotation is 1/240th degree (15 arc seconds), about 
0.07 milliradian.

So on a plumb bob a meter long, you're looking for a displacement of 
0.07 mm... Seems a bit challenging.

Even with an optical scheme looking for the reflection coming back from 
your mirror a meter away, that's just 70 microns..  Well, at least it's 
not a few wavelengths of light.

But I can see a lot of practical problems at that level of precision:

Vibration isolation?
Local gravitational anomalies.  (I seem to recall tens of arcseconds for 
this)

difference between local gravity vector and normal of the ellipsoid or 
geoid due to non spherical earth, etc.  (this one is calculatable)





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