[time-nuts] finding time astronomically.
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.
But what sort of precision are we looking for here?
1 second of earth rotation is 1/240th degree (15 arc seconds), about
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:
Local gravitational anomalies. (I seem to recall tens of arcseconds for
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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