OUR RESTLESS TIDES
A BRIEF EXPLANATION OF THE BASIC ASTRONOMICAL
FACTORS WHICH PRODUCE TIDES AND TIDAL CURRENTS
Chapter 2
The Astronomical Tide-Producing Forces: General Considerations
At the surface of the earth, the earth's force of gravitational attraction
acts in a direction inward toward its center of mass, and thus holds the ocean
water confined to this surface. However, the gravitational forces of the moon
and sun also act externally upon the earth's ocean waters. These external
forces are exerted as tide-producing, or so-called "tractive" forces.
Their effects are superimposed upon the earth's gravitational force and act to
draw the ocean waters to positions on the earth's surface directly beneath these
respective celestial bodies (i.e., towards the "sublunar" and "subsolar"
points).
High tides are produced in the ocean waters by the "heaping"
action resulting from the horizontal flow of water toward two regions of the
earth representing positions of maximum attraction of combined lunar and solar
gravitational forces. Low tides are created by a compensating maximum
withdrawal of water from regions around the earth midway between these two
humps. The alternation of high and low tides is caused by the daily (or
diurnal) rotation of the earth with respect to these two tidal humps and two
tidal depressions. The changing arrival time of any two successive high or low
tides at any one location is the result of numerous factors later to be
discussed.
Go to Top of page
Origin of the Tide-Raising Forces
To all outward appearances, the moon revolves around the earth, but in
actuality, the moon and earth revolve together around their common center of
mass, or gravity. The two astronomical bodies are held together by
gravitational attraction, but are simultaneously kept apart by an equal and
opposite centrifugal force produced by their individual revolutions around the
center-of-mass of the earth-moon system. This balance of forces in orbital
revolution applies to the center-of-mass of the individual bodies only. At the
earth's surface, an imbalance between these two forces results in the fact that
there exists, on the hemisphere of the earth turned toward the moon, a net (or
differential) tide-producing force which acts in the direction of the moon's
gravitational attraction, or toward the center of the moon. On the side of the
earth directly opposite the moon, the net tide-producing force is in the
direction of the greater centrifugal force, or away from the moon.
Similar differential forces exist as the result of the revolution of the
center-of-mass of the earth around the center-of-mass of the earth-sun system.
Go to Chapter 3 of "Our Restless Tides"
Return to Table of Contents
Go to Top of page
Revised: February, 1998