Global Positioning System Reference
In-Depth Information
Earth Wobbles
There are two types of earth wobbles—by which I mean shifts in the orienta-
tion of the earth's rotation axis. The first is due to the tidal forces of the moon
and the sun. This wobble moves the orientation of the earth's rotation axis
relative to the stars and consists of several components. The largest and
slowest component is the precession of the equinoxes, which has an angular
amplitude of 23
1
⁄
2
\
and a period of about 26,000 years, as we have seen.
The second type of wobble is quite separate—it would occur even with-
out lunar and solar tidal forces. The largest component in this second type of
wobble is a consequence of the fact that the earth is not quite spherical.
Physicists call this wobble
free nutation
; geodesists refer to it as
polar motion
or as
variation of latitude
because it moves the lines of latitude. Happily,
this movement is small and slow. Polar motion is known to consist of three
Polar motion. The movement of the true North Pole over a five-year period from
January 1, 2004. The coordinate system aligns the
x
-axis east-west, such that move-
ment to the right is toward Greenwich. One arc second corresponds to a distance of
about 30 m.
Data from the Earth Orientation Center, International Earth Rotation Service.
