[time-nuts] Result of Earth Quake speeds up earth?
jfor at quik.com
Wed Mar 16 02:11:09 UTC 2011
"Polar motion can be determined pretty well from ground-based observations
of GPS satellites, period. However, UT is not well determined from
observations of GPS satellites alone, because the entire GPS constellation
may rotate in longitude with respect to an inertial frame. This rotation
may be determined from observations of GPS satellites alone because the
satellites' geocentric orbits are perturbed by the sun and moon; however,
those perturbations are relatively small, so one's handle on longitude
with respect to an inertial frame is not ral strong.
So, to determine UT, it's best to observe quasars by VLBI, using a form of
double-differencing, that is, differencing of phase and/or group delay
both between ground stations (radiotelescopes) and between quasars widely
separated in the sky but nearly simultaneously observed. By differencing
between quasars, one cancels station-related errors such as clock sync.
>From an expert friend,
> Does anyone here know the current state of the art for timing the
> Earth's rotation? I know the outline, An instrument on a transit
> telescope notes the time when a star passes overhead. You take many
> of these observations and you can determine the period
> What is the instrument of choice? Is it still a transit telescope or
> do that track a star's motion over a longer span of time? I'd guess
> that getting a good rotational period would required tracking many
> stars over months and years.
> 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.
> 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.
> Chris Albertson
> Redondo Beach, California
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