Why are there no ocean tides at the equator?
"Tides are a very complex phenomenon. For any particular location, their height and fluctuation in
time depends to varying degrees on the location of the Sun and the Moon, and to the details of the
shape of the beach, coastline, coastline depth and prevailing ocean currents. The tidal bulge of
the Moon follows along the path on the earth's surface which intersects with the orbital plane of
the Moon. This plane is tilted about 23 degrees with respect to the equatorial plane of the earth.
The result is that near the equator, the difference between high tide and low tide is actually rather
small, compared to other latitudes. To see this, draw a circle inscribed in an ellipse, with the major
axis of the ellipse rotated by 23 degrees with respect to the circle's horizontal diameter. Now measure
the height of the elliptical contour just above the 'equator' of the circle. You will see that it is
quite small compared to other positions on earth, particularly at latitudes of 23 degrees or so.
Even larger differences can occur depending on the shape of a bay or inlet or continental shelf."
- Dr. Odenwald's ASK THE ASTRONOMER
Why are ocean tides so different everywhere?
"Because they depend on many factors including the geometry of your local coastline, and exactly
where the Sun and Moon are located. Also, like the surface of a vibrating drum, the world oceans
have vibratory modes that get stimulated in changing ways from minute to minute. Finally, there
are storms at sea and elsewhere which move large quantities of water. Detailed forecasts are
available for high and low tides in all sea ports." - Dr. Odenwald's ASK THE ASTRONOMER
Why aren't the Atlantic and Pacific coast tides the same?
"The nature of tides on the Earth's oceans is very complex. The oceans are, of course, being
periodically 'forced' by a number of tidal sources including the Moon and the Sun, but this
forcing has a number of different periods and harmonics. The two dominant periods are sue to
the Sun and Moon, these are referred to as the S1 and M2 'modes' which have roughly 12 hour
periods because they raise TWO water tides on the ocean diametrically opposite each other. But,
for a variety of reasons, any given port will not have two high and two low tides each day;
also called 'semi-diurnal tides'. A careful monitoring of the tides at any port for several
years will show that in addition to the major modes, there are as many as 300 minor or 'harmonic'
modes as well.
The World Ocean is a complex dynamical system. The natural velocity of a water disturbance depends
on the depth and salinity of the water at each point it passes. When bodies of land circumscribe
bodies of water, they produce a collection of resonating systems that favor water oscillations
with certain frequencies over others. From among the 300+ harmonics that can be measured, every
port and coastal location has its own unique signature depending on its latitude, longitude,
water depth and salinity. The result is that the 'two high two low' tide rule can be strongly
modified so that the time between successive high tides can be greater than or less that 12
hours in many cases. The result is that for some locations, there can be days when only one
high tide occurs. Looking at the Atlantic and Pacific Coast tide tables for 1995, the data for
the various 'Standard Ports' showed that virtually all days had two high tides and two low
tides in San Diego, San Francisco, New York and Charleston. There were, however a few days
every few months when only a single high tide occurred." - Dr. Odenwald's ASK THE ASTRONOMER
What is a Proxigean Spring Tide?
"The Moon follows an elliptical path around the Earth which has a perigee distance of 356,400
kilometers, which is about 92.7 percent of its mean distance. Because tidal forces vary as the
third power of distance, this little 8 percent change translates into 25 percent increase in the
tide- producing ability of the Moon upon the Earth. If the lunar perigee occurs when the Moon is
between the Sun and the Earth, it produces unusually high Spring high tides. When it occurs on
the opposite side from the Earth that where the Sun is located ( during full moon) it produces
unusually low, Neap Tides. The High, High Tide is called the Proxigean Spring Tide and it occurs
not more than once every 1.5 years. Some occurrences are more favorable that others.
A very interesting book "Tidal Dynamics" by Fergus J. Wood, published in 1986 by Reidel
Publishing Company, talks at great length about these tides, and their environmental consequences.
Because of the gravitational nature of the interaction between the Earth, the Moon, and the water on
the Earth, there is a curious amplification event called 'evection' that occurs when the Moon is at
its closest 'perigee' distance called its 'proxigee'. The Moon draws even closer to the Earth than
its ordinary perigee distance. Because of the complex dynamics of the Earth's oceans, their inertia,
friction with the ocean floor, internal viscosity and the distribution of the continents, the maximum
tides do not always coincide with the optimal times of proxigee. Still, these tides can produce
enormous damage when all factors come together optimally. There are many recorded instances of
unusually high storm or coastal flooding during the proxigean times. On January 9, 1974 the
Los Angeles Times reported 'Giant Waves Pound Southland Coast".
During the last 400 years, there have been 39 instances or 'Extreme Proxigean Spring Tides'
where the tide-producing severity has been near the theoretical maximum. The last one of these
was on March 7 1995 at 22:00 hours Greenwich Civil Time during a lunar Full Moon. There were,
in fact cases of extreme tidal flooding recorded during these particular spring tides which occur
once every 31 years." - Dr. Odenwald's ASK THE ASTRONOMER
If the Moon were to escape, what would happen to the Earth's oceans?
"What happens is that the lunar water tides on the Earth go away, but the solar water tides still
occur, but with about 1/3 or so the amplitude. There are still daily high and low tides, but they
would be noticeably smaller. There would be no 'Spring' or 'Neap' tides, however."- Dr. Odenwald's ASK THE ASTRONOMER
Why does the Moon produce TWO water tides on the Earth and not just one?
"It is intuitively easy to understand why the gravitational pull of the Moon should produce a water
tide on the Earth in the part of the ocean closest to the moon along the line connecting the center
of the Moon with the center of the Earth. But in fact not one but TWO water tides are produced under
which the Earth rotates every day to produce about two high tides and two low tides every day.
How come?
It is not the gravitational force that is doing it, but the change in the gravitational
force across the body of the Earth. If you were to plot the pattern of the Moon's 'tidal'
gravitational force added to the Earth's own gravitational force, at the Earth's surface,
you would be able to resolve the force vectors at different latitudes and longitudes into a
radial component directed towards the Earth's center, and a component tangential to the Earth's
surface. On the side nearest the moon, the 'differential' gravitational force is directed toward
the Moon showing that for particles on the Earth's surface, they are being tugged slightly
towards the Moon because the force of the Moon is slightly stronger at the Earth's surface
than at the Earth's center which is an additional 6300 kilometers from the Moon. On the far
side of the Earth, the Moon is tugging on the center of the Earth slightly stronger than it
is on the far surface, so the resultant force vector is directed away from the Earth's center.
The net result of this is that the Earth gets deformed into a slightly squashed, ellipsoidal
shape due to these tidal forces. This happens because if we resolve the tidal forces at each
point on the Earth into a local vertical and horizontal component, the horizontal components
are not zero, and are directed towards the two points along the line connecting the Earth and
the Moon's centers. These horizontal forces cause rock and water to feel a gravitational force
which results in the flow of rock and water into the 'tidal bulges'. There will be exactly two
of these bulges. At exactly the positions of the tidal bulges where the Moon is at the zenith
and at the nadir positions, there are no horizontal tidal forces and the flow stops. The water
gets piled up, and the only effect is to slightly lower the weight of the water along the vertical
direction.
Another way of thinking about this is that the gravitational force of the Moon causes the Earth
to accelerate slightly towards the Moon causing the water to get pulled towards the Moon faster
than the solid rock on the side nearest the Moon. On the far side, the solid Earth 'leaves behind'
some of the water which is not as strongly accelerated towards the Moon as the Earth is. This
produces the bulge on the 'back side' of the Earth."- Dr. Odenwald's ASK THE ASTRONOMER
What Causes Tides?
"There are several kinds of tides. The ones that break upon a beach every 10 seconds to a minute
are caused by sea level disturbances out in the ocean produced by such things as storms. Also,
the various circulation currents of sea water can have velocity components directed towards the
land which will bring water up onto the beach. As this water travels towards the beach from deep
water to shallow water, its amplitude will increase until it finally 'breaks' as a full-fledged
breaker, suitable for surfing etc.
Now, underlying this minute to minute activity is a slower water wave which causes an alternating
pattern of high-tide, low-tide, high-tide, low-tide in most places on the Earth that are directly
on the ocean. This roughly 6 hour cycle is caused by the gravitational tugging of the Moon upon
the Earth. This 'tidal' pull causes the shape of the solid Earth to be not perfectly round by
something like a few dozen yards over its entire 27,000 mile circumference. The Earth gets
distorted a small bit, but because it is solid rock its a small effect. The water in the oceans,
however, gets distorted into a roughly ellipsoidal ( football-like) shape with a much larger
amplitude. The orientation of this shape changes from minute to minute as the Moon orbits the
Earth, which is why the high and low tide times change all the time. The Moon causes these tides
by deforming the oceans, and as the Earth rotates under this ocean bulge, it causes a high tide
to propagate onto beaches. Because there are two bulges, we get two high tides, and also two low
tides each day.
The Sun also causes tides on the Earth because even though it is so far away, it is very massive.
These solar tides are about half as strong as the ones produced by the Moon, and they cause the
so-called Spring tides and the Neap Tides. When the bulge of ocean water raised by the Moon is
added the a similar tidal bulge raised by the Sun, you get a higher, high tide called the Spring Tide.
When the solar low tide is added to the lunar low tide, you get the Neap Tide.
There may be even weaker tides caused by the gravitational influences of the planets Mars and Venus,
but they are probably lost in the daily noise of individual tides."- Dr. Odenwald's ASK THE ASTRONOMER
When the Earth, Moon and Sun are aligned for Spring Tides, are they highest at Full or New Moon?
"Spring tides are about the same height whether at New or Full Moon, because the tidal bulge occurs
on both sides of the Earth...the side toward the Moon ( or sun) and the side away from the Moon
(or Sun). They will not be equally high because the distance between the Earth and Sun, and the
Earth and Moon both vary and so will their tide producing effectiveness. The highest Spring tides
occur when the Moon is at its closest to the Earth...the so-called Perigee Tide."- Dr. Odenwald's ASK THE ASTRONOMER
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