Global Positioning System Reference
In-Depth Information
FIGURE 1.6.
Calculated tide height in feet, for Bridgeport, Connecticut, September
1991. The variation between neap and spring tides is clear.
important facets. Laplace, later in the eighteenth century, truly nailed
the phenomenon of tides mathematically—his equations are still used
today.
Modern analysis takes into account a lot more than could Laplace, of
course: computers can apply the math to accurate modern data that ac-
count for the depth and shape of the oceans, unknown in Laplace's day.
They can obtain numerical solutions that show detailed features of world-
wide tidal movements which physicists of earlier times could only dream
about. Such detailed analyses reveal the presence and shifting location of
amphidromes
; these are points about which the tides oscillate and at which
tidal fluctuations are zero. They are like the eye of a storm, in that they are
calm areas with large tidal movements swirling about them on all sides, in
either a clockwise or counterclockwise direction, depending on the action
of
Coriolis forces
and local bathymetry.
10
Tidal energy dissipates mostly as heat. That is, friction between the
moving water and ocean beds and shorelines dissipates much of the energy
possessed by tides. The degree of dissipation varies over the oceans, as can
10. More accurately, amphidromes are nodes in a two-dimensional standing wave,
analogous to the stationary point at the center of an oscillating string. There are long ocean
waves generated by tides that resonate at di√erent dominant wavelengths in di√erent
bodies of water. These resonances are sustained by energy that is sapped from the moon; as
we will soon see. Coriolis force arises from the earth's rotation; it is a sibling of the centrif-
ugal force but is more complicated. For example, it depends on the velocity of the mass that
it acts on.
