Still more: > Subject: Re: [time-nuts] Result of Earth Quake speeds up earth? > From: "Chris Albertson" <[email protected]> > > 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 ================== _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
