Global Positioning System Reference
In-Depth Information
be seen from figure 1.7. In some parts of the world, such as the North
Atlantic, energy dissipation is very large, as much as 30 kW km
-2
; else-
where, it can be negative—meaning that parts of the ocean
absorb
energy.
The net flow, over all the earth's oceans, is outward: energy is lost as heat.
Analysis reveals that the source of this energy is the moon. Thus, the force
of lunar gravity moves large bodies of water; the energy required for these
tidal movements reduces the gravitational potential energy of the moon
(actually, of the earth-moon system), which causes it to drift away from the
earth at an rate of about 3.8 cm yr
-1
. Tidal friction applies a brake to the
earth's rotation, so that the spin of our planet is slowing down. This means
that the length of an earth day is increasing slowly; over the last century,
the length of a day has increased by 2.3 ms. Such a tiny increase may be
entirely unnoticed by a person during her lifetime, but over geological
timescales the change is significant. Geological data show that, 620 million
years ago, one earth day lasted for about 22 hours, and there were approxi-
mately 400 days in a year.
It was the astronomer Edmond Halley, of comet fame, who first noticed
the energy dissipation phenomenon. Comparing new and old data, he saw
a change in the moon's speed across the sky. In fact, what he had noticed
was due to the slowing of the earth's rotation speed. The full explanation
had to wait until the nineteenth century. Today, we understand the major
e√ects—and a myriad of minor e√ects—of tidal movements very well.
Observations are exquisitely accurate and show, for example, that tides
cause the rotation rate of our planet to vary from day to day. This variation
is only a few tens of microseconds but is well within the accuracy of our
instruments to measure. Such minor details have no consequences for
FIGURE 1.7.
Tidewater energy dissipation. The darker areas indicate regions of higher
energy flux (mostly dissipation or loss of energy from the oceans).
Image by Richard
Ray, NASA Goddard Space Flight Center.
