(This might be of some slight interest to coastal boaters.)

Lunatic Tides


"There must be a full moon!" We often hear that expression when someone is
amazed by bizarre human behavior. Theories abound that people are effected by
phases of the moon. The word "lunatic" (based on the Latin word for moon,
"luna") originally referred to individuals thought to be utterly and
inexplicably controlled by Earth's captive satellite.

A high percentage of the human anatomy is water. Although the question of the
moon's effect on the water in human organisms remains controversial,
astronomers and oceanographers all agree that the moon is the primary generator
of ocean tides on our planet.

Our solar system is an intricate balancing act. Planets remain in predictable
orbits around the sun, and moons in orbit around planets, due to the laws of
gravity, inertia, and centrifugal force. Over 300 years ago, Sir Isaac Newton
concluded that these same forces are responsible for the rising and falling
tides that we observe in our coastal waters.

Newton's law states that a gravitational attraction exists between any two
bodies. The force is directly proportional to the relative mass, and inversely
proportional to the square of the distance between the objects. While tides are
an expression of gravity, the complete math involved with tidal forces is
slightly skewed toward relative distance, rather than mass. Will all factors
considered, lunar and solar influence on Earth's tides varies inversely by the
cube, rather than the square, of the distances between our planet, the moon,
and the sun.

The sun is roughly 27 million times the size of the moon. If gravitational mass
were the sole factor in determining tides, the sun would create tides that
were so extreme that most of our planet would be uninhabitable. The sun is
about 390 times the average lunar orbit from Earth, reducing the sun's tidal
influence by 390 to the third power, or a factor of about 59 million. The sun's
tidal influence is therefore only about 27/59 that of the moon, or
approximately half. The moon determines tidal cycles, and the sun influences
the severity of the tidal fluctuations.

Basic Tidal Theory

The moon orbits around Earth in the same direction that the planet rotates on
its axis.
Our solar day is 24-hours in length, as any point on the earth's surface that
is directly under the sun, (local "noon"), will be under the sun again in
exactly 24 hours. We also have a lunar day, which is 24-hours, 50-minutes in
length. (It takes an additional 50- minutes to "catch up" to the moon's new
position as it continues to orbit Earth).

The gravitational effect of the moon creates a "bulge", or incredibly broad
standing wave of ocean water, cresting at the surface of the Earth at the point
closest to the moon. This wave follows the moon as it orbits the earth. A
second wave is created by a combination of inertial and centrifugal forces on
the opposite side of the planet, so as Earth rotates each location will pass
through two "bulges" of water everyday. The tidal effect increases closer to
continental masses. The peak of the tidal bulge in the center of the Pacific
Ocean may be only a few feet in height, while tides along the NE Pacific
Coastline often run 15 feet or more.

When our particular point on the earth is midway between the two standing
waves, there is less water to cover the shorelines. As we exit one of the
waves, depths decrease and we experience "low tide". We encounter more water as
we enter into one of the standing waves, with depths increasing and tides
"rising" until we reach the crest of the wave, (directly under the moon on one
side of the Earth), and depths decreasing again as we have passed beyond the
crest. It's interesting to consider that when the tide is "in" or "out" it is
actually as attributable to the Earth's surface rotating through increasing and
decreasing lunar gravity as it is to the ocean itself "going somewhere".

If there weren't some complex variables, (and there are) we would experience
two reasonably identical tide cycles every 24 hours and 50 minutes. The
theoretically perfect interval between one high tide and the next would be 12
hours, 25 minutes. This tidal cycle is referred to as a "semi-diurnal" (half a
lunar day) tide.

Continental landmasses interrupt and distort the standing waves created by the
moon, and create some interesting regional variations. Different ocean basins
develop tidal patterns that can be geographically unique. The Gulf of Mexico is
among a category of "diurnal" locations experiencing only one tide cycle every
lunar day. The Pacific Northwest experiences a category of tides known as
"mixed diurnal", with two tide cycles every lunar day that may be substantially
different in height.


Neaps, Springs, and Elliptics


The moon rotates around the earth, and the earth around the sun in orbits that
are elliptical, (more ovoid than perfectly circular). Once each solar year,
about January 2, the Earth is closer to the sun than at any other time during
the year. This point in the Earth's solar orbit is known a "perihelion" (near
"helios", a Greek word for the sun.) During perihelion, the sun's effect on
tide-generating forces is greater than at any other time of the year.
"Aphelion" is the point in the Earth's orbit that is farthest from the sun,
occurring six months later in early July. Solar tidal influences will be at
their lowest during aphelion.

The moon completes an orbit of Earth once each lunar month. The closest portion
of this orbit is known as "perigee" (near "geo", or "Gaia", an ancient name for
Earth) and the most distant portion of the orbit, reached two weeks later, is
considered "apogee".
The lunar gravitational influence on tides during perigee is greater than the
lunar influence during apogee.

Once each month the moon will be on a line between the sun and the Earth. This
position puts the "sunny side" of the moon away from Earth, and we observe the
phenomenon we call a "new moon". Two weeks later the moon's orbit will place
it in a position directly "beyond" the Earth relative to the sun, completely
exposing the sunny side of the moon and creating a full moon.

The changing dynamics of these orbital relationships and alignments form a
complex pattern that takes 18 years, six months to complete.

The term "spring tide" has nothing to do with tides occurring around the vernal
equinox, but instead refers to the highest tides that will occur during a
monthly lunar orbit. (Think of "spring" as a mechanism of potential tidal
energy, rather than a season). When the moon and the sun are aligned on the
same side of the earth, the gravitational influence of the sun combines with
the gravitational influence of the moon to create a larger "bulge" of ocean
water. During years when the moon at perigee happens align with the sun at the
same time Earth is at perihelion, conditions are ideal for extreme tides.

Spring tides can also occur when the moon is full, on the opposite side of
Earth as the sun. In such a case, the lunar gravity is attracting its customary
"bulge", while the corresponding bulge on the opposite side of the planet is
slightly enhanced by the gravitational influence of the sun.

Neap tides are the least variable during a monthly lunar cycle. Neap tides
occur during quarter moons, when the moon and the sun are perpendicular to
Earth's orbit.


Other considerations:

Man has been observing tides and the solar system for thousands of years. We
can record, calculate, and project tidal date for centuries to come. Even with
our sophisticated computer technology, observed tides will often vary from
official predictions.

Strong onshore or offshore winds can exaggerate or reduce the actual height of
a tide.
Drainage from a large river system during periods of heavy rainfall can result
in higher tides than predicted. Variations in local weather can effect tides as
well, with sunny, high pressure systems depressing the "bulge" and creating
lower tides. Tides that occur during stormy, low-pressure conditions can be
higher than forecast.

With a rudimentary appreciation of the forces that create tides, no boater in
tidal waters will want to leave port without a current tide book. Should a
boater happen to go aground in the worst possible location, on the worst
possible day, at the worst possible moment, at the highest possible tide, it
may not be at all comforting to consider that a mere 18 years, 6 months will
have to elapse before enough water is again available to set the vessel afloat.

Oh, those lunatic tides.