[time-nuts] Result of Earth Quake speeds up earth?

[J. Forster]

Still more:

> Subject: Re: [time-nuts] Result of Earth Quake speeds up earth?
> From:    "Chris Albertson" <albertson.chris at gmail.com>
>
> Does anyone here know the current state of the art for timing the
> Earth's rotation? ...
>
> What is the instrument of choice?

The instrument of choice, chosen by the U S Naval Observatory (USNO)
decades ago, is Very Long Baseline Interferometry (VLBI) observations of
quasars at microwave radio frequencies, typically 8 to 15 GHz.

The axial rotation of the Earth is conventionally measured by an angle
called UT1, and expressed in hours, minutes, and seconds, where one hour
equals 15 degrees.  The letters "UT" stand for Universal Time, and UT
(which differs slightly from UT due to polar motion or "wobble") has
always been expressed in units of time notwithstanding its definition as
an _angle_, because prior to the invention of artificial clocks, the
"universal" standard of time was the Earth's rotation.

In the USA, the legal determiner of UT1 is the U S Naval Observatory; and
the values so determined are disseminated in International Earth Rotation
Service (IERS) Bulletin A
<http://maia.usno.navy.mil/ser7/ser7.dat>
and electronically via means listed at the top of the first page of this
Bulletin.

The "International" in "International Earth Rotation Service" refers to
the fact that observations are collected from around the world to
determine UT1.  In the first page of Bulletin A it is stated:

  The contributed observations used in the preparation of this Bulletin
are available at <http://www.usno.navy.mil/USNO/earth-orientation/
eo-info/general/input-data>. The contributed analysis results are based
on data from Very Long Baseline Interferometry (VLBI), Satellite Laser 
Ranging (SLR), the Global Positioning System (GPS) satellites, Lunar
Laser Ranging (LLR), and meteorological predictions of variations in
Atmospheric Angular Momentum (AAM).

Among the types of data mentioned, the data believed to have the best
long-term accuracy, i.e., the smallest systematic errors in the long run,
are the VLBI data -- because quasars, the objects observed by VLBI, are
the most distant of all the objects mentioned.  (See below.)  Thus they
have the smallest possible proper motions with respect to an inertially
nonrotating frame.

Abundant information about how the Earth's rotation is determined is
available on the Web.  Use Google.  I would give you the URL of the
horse's mouth, but the USNO's main web servers (e.g., tycho.usno.navy.mil)
are unresponsive tonight.

> What about effects like parallax due to the Earth's orbit around the
> sun,?  Do they only use very distant stars?  Or do they use radio
> telescopes now.

They don't use the sun and they don't use stars.  They use quasars, whose
distances are billions of light-years.

<http://en.wikipedia.org/wiki/Quasar>

Observations of the Moon and artificial Earth satellites are useful for
rapid measurement of short-term variations in Earth rotation, including
polar motion; but the long-term motions of these objects with respect to
an inertially nonrotating frame have substantial uncertainties.

> If I were doing this in my backyard on a budget I'd mount a small
> telescope nearly straight up so that a bright star would pass through
> the field on several nights.  I'd measure the light of the star
> through a slit and time the peak of the light each night.  I bet I
> could get to about a microsecond.   I'm wondering what professionals
> are doing in this field.

By observing stars you can't get accuracy or precision anywhere close to
what VLBI provides.  The best you could do in your back yard would be to
observe GPS satellites.  Then you'd need to learn a helluva lot about how
to analyze your observations.

-John

==================