# [time-nuts] FW: Bulletin C number 30

Bjorn Gabrielsson bg at lysator.liu.se
Tue Jul 5 07:43:51 EDT 2005

Maybe being a newbie, means staying out of the way of giant..  well
I'll take the chance of embarrassing myself...

Magnus Danielson <cfmd at bredband.net> writes:

> > >> It will not be locked to any more or less random piece of geophysics,
> > >> anyone with a cesium clock and a set of gen-rel coordinates will be
> > >> able to figure out what time it is, and time intervals can be measured
> > >> and compared without weird gottchas.
> > >
> > >No. You are missing a detailed refinement in the definition of a second, it is
> > >assumed that the Cesium clock is at sea-level.
> >
> > Sea-level on this planet, yes.
>
> Whatever sea-level on this planet is. It's not at the sea-level at
> all.

A common definition, iirc, is some epoch of sea-level in the Rotterdam
harbour. From this starting point, MSL is simply the altitude where
the geopotiential has the same value as the "zero-point". MSL is often
called the geoid as opposed to various ellipsoids. One of the most
used ellipsoids are ofcause the WGS84 ellipsoid that GPS uses.

> You
> have to measure the gravity and compensate for that to some constructed and
> standardised sea-level.

There are elaborate models of the geopotentials, where the potential
is expressed with spherical harmonics. (GEMxx, JGM-S, EGM96S etc)
With coordinates for your Cs you should be able to compute the
geopotential (gravity) potential quite good! Would you be able to
measure the error the usage of a good model gives you? If you go into
real details, you need to measure continously, since the continents
are moving and there is landrise (&fall).

> The gravity and sea-level maps of the earth is
> interesting to study and when you think of it in this context you really start
> to wonder. There are pretty accurate gravity instruments that are among other
> things used to measure when vulcano bursts is due to happend and stuff like
> that.
>

What is the gain of having yet another expensive instrument, when
there are good models that can be fed with position/altitude.

> > If you are on a different planet in a different orbit and a differnet
> > rotation period (and axis!), general relativity takes a toll.
> >
> > If you bring a HP5071A to Mars, it will give you a wrong length of
> > seconds.
>
> Indeed. There is an 24 hour cycle which is due to the shift in gravity position
> relative to the sun as the earth turns. The amplitude is however quite small
> and I'd guess that the temperature shifts from the same cycle makes larger
> influence on most clocks.
>
> Relativity bites you many times when trying to do this.
>
> The Universal in UTC is to indicate our world, the earth, and not to be
> confused with the universe as in deep space. It was never meant to include
> Mars for instance.

There are relativistic time scales for such use. TT - time measured by
surface earth clocks. Geocentric coordinate time (TCG) - time wrt
center of Earth. Barycentric coordinate time (TCB) - time wrt center
of solar system.

Going to Mars you will supposedly end up relating your Cs to TCB and
then transform back and forth to TT to compare with measurements from
earth clocks.

I am sure there are listmembers who are knows relativity and its time
applications and can explain more, and maybe tell me if I completely
failed to understand.

--

Björn