Why No Predictions for Theoretical Tides?
by Duane Dunkerson
Why? There is the vagabond Moon, constant Sun, far
Jupiter, and an inconstant Earth.
The earthly and astronomical effects on the tides are
so complex that a theoretical finality is impossible.
Meanwhile, periods of tidal oscillations are
predictable. At least the oscillations could benefit the
business interests. Seagoing commerce had to have some
approach to prediction for the tides.
The first tide predicting machine was designed by Lord
Kelvin. It went into operation in 1873. By 1910 the US
Coast and Geodetic Survey had the Coast and Geodetic
Survey Tide-Predicting Machine No.2. It was eleven feet
long, two feet wide, and six feet high with a weight of
2,500 pounds. Thirty-seven elements bearing on tidal
prediction were represented by components of the
machine. This machine was used to attempt to predict the
tides. Accompanying the tides are currents. Finding the
velocities of these currents was also attempted by
Machine No. 2. These velocities are a sum of a series of
harmonics of periodic elements of the tides.
By 1965 the Table of Tides was still typed by hand.
Then in 1973 predictions were attempted by computer.
This involved two walk-in closets of punch cards. In
1979 was inaugurated an interactive phone dial-up
prediction service. PCs got into the act by 1987. Now
the seven volumes of the Tide and Current Prediction
Tables are on one CD.
The most recent water levels, within 6 minutes, can be
obtained by phone. One can also get the information via
satellite. A tidal gauge that finds unusual water
heights will automatically send such notice to the
satellite for retransmission to various ground stations.
This is as close as it gets to knowing the tides.
Michelson in 1913 got microscopically close to tidal
action. He used pipes, one set N-S, another set W-E, to
measure tides through a microscope. He saw changes as
little as a variance in water level of 1/1000 of an
inch. These water heights were about 69% of what theory
would have predicted.
Michelson observed the usual decreases in water levels,
the ebb tides, and the increases in water levels, the
flood tides. There are two tides per day, two high, two
low. Between a high and a low there is usually six
hours. The average interval between the same high tide
is about 24 hours and 51 minutes. This corresponds to
two successive southern passages of the Moon. One tide
comes ahead of the Moon on the earthly side towards the
Moon. Another tidal outward bulge of water is to be
found on the opposite side of the Earth.
The Moon pulls on the Earth. The water can freely flow
and does so, to and from the sublunar point. Because the
earth rotates, the bulges of water move in waves. The
bulge moves ahead of the Moon because of friction. That
is, the Earth rotates faster than the Moon revolves.
From high tide to high tide is called the establishment
of port. For New York, the e of p is on average at 8
hours and 13 minutes.
Even with a locale pinned down, the water heights of
tides varies since the Moon in its orbit is inclined to
the ecliptic. Because of this, for half a month the Moon
is north of the Earth's equator and for the other half,
it is south of the equator. There is then a smallish
second tide. Twice a month, as the Moon appears to cross
the equator, the second tide is not observed.
There are also brief periods when the water isn't going
anywhere. There is not ebbing or flooding during what is
called slack water. This doesn't mean that the Moon has
stopped working gravitationally. In any event the Moon
does not lift the water toward it. It can't do such
lifting when its lunar force is nine million times
weaker than the force of Earth's gravity at the Earth's
surface. The tides are produced by a component of the
tidal force of the Moon which draws water along the
earth's surface toward sublunar and antipodal points.
If the Earth's surfaces were completely submerged by an
ocean of unvarying depth and if the Earth were to face
the Moon at all times, there would be tides two feet
high. But, of course, the oceans are of varying depth.
Shallow water slows wave travel. Times for high water of
two ports 2/10 of a mile apart may differ if one has
shallower water than the other. Bigger tides arise in
water that is more shallow. The Bay of Fundy has tides
of 50 feet. Other high tides have been found on the east
coast of Patagonia, the Bristol Channel, the coast of
Normandy and in the Hudson Strait. In New Brunswick at
St. John, the river, also called St. John, flows
upstream during flood tide.
In addition to rivers, lakes are also affected by the
lunar tide action. These are effects usually easier to
detail. Lake high tides are opposite the Moon when the
Moon is below the horizon. If the Moon is up, the water
of the lake follows the Moon around the shore. Low tide
goes along the opposing shore. Lake Michigan, for
example, has a tidal difference of 1and 3/4 inches.
It is not only the Moon that rises above the lakes of
Earth. The Sun plays a role in the tides too. At new or
full moon there are higher tides as Sun and Moon act
along the same line. At first and third quarter
(quadrature) of the Moon, there are neap ("neap" - from
the Greek for "scanty") tides.
Neaped or not, the Moon comes in closer at perigee. It
is 10 per cent closer and then the tides can be 20%
greater. Now, by this time this should be enough to show
that the tides don't allow for prediction. Overcome
these various factors and then there is Jupiter. Yes,
Jupiter causes tides. They are tiny. Nevertheless we are
speaking of tides. Tides are tides. Jovian tides are
10,000,000 less effective than the familiar lunar
causation. The solar contribution, by the way, is 5/11
of the lunar one. The Moon, Sun, Jupiter, and other
contributors to celestial mechanics for accurately
locating all the generating forces are simply not
enough. You need to conduct observations per locale for
at least more than 18 years.
In some matters the past provides a comfort zone of
predictability. We know sometimes what happened first
then what came next. Unfortunately the lack of
prediction involving tides reaches back to ancient
times. Actually it is a subsidiary effect of the tides
that has thrown off the calculations for the occurrences
of ancient eclipses. The eclipses happen later than we
would suppose. This is because the earth's rotation is
slowing due to tidal friction. At the same time(s) the
Moon is increasing in speed in its orbit. The tidal drag
is two billion horsepower.
For more uncertainty as to the when or what tides were
or could do, one can go much, much further back in time
as Asimov did in his speculation that the tides played
the key role in the evolution of life. The tides
challenged some of sea life by tossing then up onto land
twice daily. To change you need change - like no credit
extended unless you have credit. The agents of change
became changed. With the Moon closer such a long time
ago, the tides were more pronounced. Asimov speculated
that perhaps one needs a sea and Moon and tides. Without
these in existence, evolution like ours could not come
It is like us, contrarily, to look ahead and not so far
behind. One can attempt to predict when there will be a
new moon by gathering your own tidal data. During new
and full moon high tides get higher and low tides get
lower. Measurement of your local tides can lead to a
prediction as to when the next new moon should appear.
If you could stick with the measurement for around 18
years, you could become fairly accurate in those