Global Positioning System Reference
In-Depth Information
FIGURE 1.8.
Satellite
altimeter data showing
variations in sea level
across the Pacific Ocean in
December 2009. Warm water
is higher than cool water.
Image from NASA/JPL Ocean
Surface Topography Team.
navigation, but our deep understanding of the physics of tides is very sig-
nificant because it produces accurate predictions in the form of tide tables.
The influence of solar heating on the flow of seawater is apparent from
figure 1.8. This satellite altimeter image shows the variations in sea level
height across the Pacific Ocean. Water expands as it warms, and this ther-
mal expansion is responsible for height di√erences; water flows downhill,
and so solar heating influences currents (see ''Ocean Currents''). These
influences may be seasonal and periodic.
On the Surface
To a good first approximation, the earth is spherical—a ball. More accu-
rately, it is slightly squashed—an ellipsoid. If we seek greater precision of
expression, as geographers do, we must specify what we mean by ''the
earth.'' The natural (to a physicist) definition of the earth's shape is the
geoid
, which is specified not by the solid or liquid surface of our planet, but
by its gravitational contours. In this section, I will explain these increas-
ingly accurate perceptions of the earth's shape.
We will see that humans learned from observations, very early in history,
that their planet was round like a ball. From then until a few hundred years
ago, we thought of the earth as a sphere, and this approximation is good
enough even today for many purposes. In the seventeenth century, Newton
realized that it
is
an approximation: the true shape must be a little flattened
