Posted by David G on August 15, 2015 at 17:53:26 from (75.167.202.204):
In Reply to: Continental Drift posted by David G on August 15, 2015 at 17:44:01:
Not to bring up a global warming debate, I always wondered if people took wobble in the earth into the equation.
I found this:
The motion of the Earth's axis has two parts -- precession and nutation. Precession is the motion of the axis in a large circle over a roughly 26,000 year period. Nutation is a harmonic with a roughly 18.6-year period, superimposed on top of the precession circle.
Although the axis moves, the equator always remains fixed with respect to the axis. The equator does move with respect to the ecliptic, but since GPS coordinates are referenced to an Earth-centered, Earth-fixed plane, this movement of the equator with respect to the Earth-Sun line is transparent to GPS.
NGA issues a new map datum every 5-7 years. This is simply a function of measurements being refined enough to warrant the issue of a new datum. It's not tied to any natural event.
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