Global Positioning System Reference
In-Depth Information
contributions: one is a slow drift and two are circular periodic motions, of
different periods.
The North Pole has drifted about 20 m over the last century; the cause is
not well understood. The periodic motions consist of an annual cycle and a
larger
Chandler wobble
with a period of about 433 days (the period is not
quite constant). Isaac Newton predicted the Chandler wobble, and later in the
1700s Leonhard Euler worked out the theoretical details, assuming a solid
earth. Euler predicted a period of 305 days. Later, when the wobble was mea-
sured experimentally by American astronomer Seth Chandler, the longer pe-
riod was a puzzle until Simon Newcomb (a Canadian-American astronomer)
repeated the calculations in 1891 while allowing for the fluid nature of the
earth's core. This explained the discrepancy.*
Polar motion has been measured with exquisite accuracy since 1962. In
the figure you can see a five-year section of the data. The largest component
of this motion is Chandler wobble, with a radius of about 0.3
&
(corresponding
to a diameter of about 9 m).
In addition to lunar and solar tidal wobbles and free nutations, there are
daily and twice-daily wobbles due to the ocean tides. This contribution to the
earth's wobble was unearthed only within the last couple of decades. These
movements are measurable despite being very small—less than 3 cm.
* The source of power driving the Chandler wobble was discovered only in 2001: the
wobble is perpetuated by fluctuating pressure in the ocean bed and atmosphere.
four basic ways that time can be measured: by reference to the earth's
rotation, by reference to the orbits of earth and of other planets and satel-
lites within the solar system, by reference to pulsars, and finally—rejecting
celestial and planetary observations—by reference to atomic clocks.
The first of these is the oldest: for millennia, farmers and priests have
watched the sun rise and move across the sky as the earth turns. Greenwich
Mean Time (GMT) was based on the earth's rotation. But we now know
that the rotation rate changes with time: we saw in chapter 1 that it drifts
over centuries and oscillates minutely over an interval of a few hours. GMT
has now been discontinued; its successor is Universal Time (UT1), which
incorporates some of the known rotation modifiers, such as polar motion
(see ''Earth Wobbles''). Even so, the rate at which a UT1 clock ticks is not
constant. The second method of measuring time, by observing the orbital
