[time-nuts] WWV Doppler Shift

[Steve Allen]

On Wed 2018-11-21T09:18:58-0800 jimlux hath writ:
> I'm not sure I understand why the slowing due to spin happens to exactly
> match the speedup from altitude.
>
> The spheroidness of the Earth is, indeed, mostly due to the rotation, but
> that would be related to the overall material properties and rotation rate,
> wouldn't it? Would a blob of water and a blob of rock both converge to the
> same shape (water would move faster, but over billions of years, rock would
> get there)
>

The important aspect is that mean sea level (or the geoid) is an
equipotential.  That is saying "from where I am there is no downhill".
Getting from any point on the equipotential to Infinity requires the
same amount of energy.

If the earth were alone in space and not rotating and of uniform
density everywhere then the surface of the ocean would be a sphere.
If it were rotating then an oblate sphere.

Add in continents and that becomes a distorted surface with geoid
undulations.

Add in sun and moon and there are diurnal tides which happen because
the water can see that "right now downhill is over there".

But as of last week TAI is not defined by sea level nor the geoid.
It is defined as the rate of the cesium resonance on an equipotential
surface at a depth of W_0 = 62636856.0 m^2 s^-2
This is the same change that the IAU made 18 years ago in response to
the IAG defining the potential of the geoid.  See pages 12 and 33
https://www.bipm.org/utils/en/pdf/CGPM/Convocation-2018.pdf

The fixed value of the potential means that the rate of TAI is now
defined not to change over geologic time scales as sea level changes.

--
Steve Allen                    <sla at ucolick.org>              WGS-84 (GPS)
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