Tides are the rhythmic variations in sea level that we
observe day to day. These changes in sea level are caused
by the gravitational attraction of the sun and the moon
on the Earth. Many other factors influence the nature
and intensity of the tides, including the shape of the
ocean basin and the Coriolis force. Together, these factors
create high and low tides. Depending on the position of
the Earth with respect to the moon and the sun, differences
in the height of sea level during the high and low tides
may be great or small. Many organisms, especially the
California grunion, time their reproductive cycle to coincide
with the tides.

**Tide-Causing Forces**

Tides are the periodic variations in sea level caused
by the gravitational attraction of the sun and the moon
on the Earth. The force of this attraction creates very
predictable rises and falls in sea level as the Earth
rotates on its axis. When sea level is at its greatest
height, the tide is said to be high. When sea level is
at its lowest extent, the tide is said to be low.

High tides bring water far up on the shore. When wave
action is high, these high tides may damage homes and
undercut sea-side cliffs. On the other hand, low tides
expose great expanses of the beach. During these times,
marine organisms living on the shore may experience long
periods of time (up to 12 hours) exposed to the air.

The gravitational attraction of the sun and the moon
on the Earth depends on the distance of these bodies from
the Earth. The closer two planetary bodies, the greater
their gravitational attraction. Thus, the moon has a greater
effect on our tides than the sun even though the sun has
a greater mass (which also influences the degree of attraction).
In general, the moon has about twice the effect on our
tides than the sun.

Let's
look more closely at what these gravitational forces do
to the ocean. Consider first the simple effect of the
moon on the Earth. Because of the gravitational attraction
of the moon, the ocean on the side of the Earth facing
the moon bulges outwards. This bulge is represented by
Point A in the figure below. The Earth itself (Point
B) also is attracted towards the moon, but because the
center of mass of the Earth is slightly further away from
the moon than the ocean on the side facing the moon, the
gravitational attraction is not quite as strong. So the
ocean on the side facing the moon is pulled the farthest
and the Earth itself not quite as far.

This
distinction is very important when we consider the origin
of the bulge on the side *opposite* the moon. This
bulge is always present. Even if the sun and moon are
on the same side, there is a bulge of water on the opposite
side. Why?

As shown in the figure, the attraction of the gravitational
forces on Point C is not as strong as the forces on A
and B. Thus, this water gets left behind, so to speak.
The Earth is pulled away from the water on this side and
a bulge appears.

You can convince yourself of this by putting two springs
(or any material with elastic properties) between three
balls in a row. If you pull on the first ball (represented
by Point A), the second ball (represented by Point B)
moves forward, but not as far as the first ball. The third
ball (represented by Point C) also moves, but even less
than the first two balls.

The sun has the exact same effect on the tides, although
its attractive force is about half the attractive force
of the moon. The bulge created by the sun is sometimes
called the solar bulge while the bulge created by the
moon is sometimes called the lunar bulge. Together, they
add or subtract to create the tides.

Well, we've made two bulges. How does that translate
into tides? Recall that the Earth rotates around its axis.
(If you can't point in which direction the Earth rotates,
then you should go to bed without dessert. Think about
it. In which direction does the sun rise. Use your finger
to point and call it out loud. That is the direction that
the Earth is rotating towards.) As the Earth rotates on
its axis, ocean basins are alternately carried underneath
and away from these bulges. Where the forces that cause
the bulges exist, sea level rises, causing the high tide.
Where the gravitational forces don't exist, we have less
water and a lower sea level, causing low tides. Because
the Earth rotates beneath two bulges and two "troughs"
(not to be confused with wave troughs), we have, in general,
two high tides and two low tides every day.

In general, a complete tidal cycle takes 24 hours and
50 minutes. This is the time it takes for the Earth to
rotate on its axis back to its original position with
respect to the moon, the primary tide-causing force. Because
it takes the moon about 27.3 days to complete one orbit
around the Earth, the moon moves a little bit further
around the Earth each day. Thus, the time of the tides
advances about 50 minutes each day.

You can use two pieces of round fruit to convince yourself
of this (oranges or grapefruits work fine, bananas don't).
Mark a spot or insert a toothpick into a piece of fruit
we will call Earth. Start the Earth fruit rotating on
its axis and move the moon fruit forward about 1/27 of
a complete orbit. You will see that the toothpick must
travel that 1/27th further to catch up with the moon.
Repeat several times until this concept is clear to you.
(Then make yourself a fresh-squeezed fruit drink with
the Earth and the moon!)

On top of the movement of the moon around the Earth,
the Earth is also orbiting around the sun. In fact, the
Earth moves "forward" around the sun slightly
less than 1/12 of a complete orbit during one revolution
of the moon. This extra 1/12 distance means that it actually
takes the moon some extra time to line up again with the
Earth and sun. Thus, for the purposes of the tides, which
depend on the alignment of the Earth-moon-sun system,
the monthly tidal cycle takes 29.5 days to complete. You
will need three pieces of fruit and a friend (as an extra
pair of hands) to convince yourself of these motions.

All of these motions, the Earth's rotation on its axis,
the moon's orbit around the Earth and the Earth's orbit
around the sun influence the height of sea level and thus,
the height of the high and low tides at any time during
the 29.5-day period.

**Patterns in the Tides**

Recall that the shape of the ocean basin and the Coriolis
force also have an effect on the tides. We won't discuss
the Coriolis force and its effect in creating what are
known as amphidromic points, but I will mention that this
knowledge is necessary if you truly want to understand
tides.

However, the Coriolis force and the shape of ocean basins
create some fundamental differences in the tidal pattern
discussed above. For example, the shape of the Gulf of
Mexico creates a tidal pattern in which we have only one
high tide and one low tide every day. This tidal pattern
is called a diurnal tide.

Along the east coast of the United States, the two daily
high tides and low tides are about equal in height. We
wouldn't necessarily expect this because we wouldn't expect
the two bulges to be the same. However, these other factors
influence the tides and create the semidiurnal tide we
observe in places like Boston and New York.

In California and the west coast of the United States,
the two daily high tides and low tides are unequal in
height. We call this pattern a mixed tide. Mixed tides
can be broken down further on the basis of their height.
The highest of the high tide in a day is called the high
high tide. The lowest of the low tide in a day is called
the low low tide. The lowest of the high tide is called
a low high tide and the highest of the low tide is called
a high low tide. Got that?

There is one more important pattern to the tides that
we need to be aware of. As shown in the figure above,
the position of the moon varies throughout the tidal cycle
(every 29.5 days). The position of the moon with respect
to the Earth is what gives us the different phases of
the moon. When the moon is directly overhead at night,
we have a full moon. When the moon is directly behind
us at night, we have the new moon (or no moon). When the
moon is at a 90 degree angle to the Earth with respect
to the sun, then we have the first and third quarters
of the moon.

We might expect that when the sun and the moon are on
the same side or directly opposite sides of the Earth
(during the full and new moon), their influence on the
Earth will be the greatest because they will act together
to cause the tides. This is exactly the case, as shown
in the figure below.

During the full and new moon, the "size" of
the bulges are greater; the lunar and solar bulges add
together, creating a bigger difference between the high
and low tides. When this happens, the tidal range, the
difference between the highest and lowest tides in a day,
is greatest. The period of the month when tidal ranges
are at their greatest are called the spring tides. The
tides appear to "spring up" between their high
and low periods. (Now don't get confused, spring tides
have nothing to do with the season.)

When the moon-Earth-sun system is at a 90 degree angle,
the lunar bulge and the solar bulge tend to cancel each
other out. During these times, the tidal range is minimal.
The differences between the height of the high tide and
the height of the low tide are small. This period of the
month is called the neap tides.

Now, think about the effect of these planetary movements
on the tides. In one tidal cycle (29.5 days), what are
the phases of the moon? We have the new moon, the first
quarter, the full moon, the third quarter and back to
the new moon. So, how many spring tides and how many neap
tides do we have in a month? If you answered 2 each, you
can have your dessert now. Yes, we have two spring tide
periods and two neap tide periods, each of which last
a little over 7 days each (one-quarter of 29.5, technically).

**Reading a Tide Chart**

A tide chart is a graphical representation of the tidal
cycle. Because the laws of planetary motion, the forces
of gravitational attraction and the other tide-causing
forces are so well known, we can predict the height of
sea level with pretty good accuracy. These predictions
are published by various governmental and private agencies
into little booklets, called tide tables, or calendars,
called tide charts.

The tide chart shown below represents the tidal pattern
for one day of tides in Moss Landing, California. Time
appears along the X-axis (the horizontal axis) and the
height of the tide appears along the Y-axis (the vertical
axis). The hills and valleys shown on the chart represent
the height of the tide at a given time. Studying this
chart carefully, we can see the presence of two "hills"
or high tides and two "valleys" or low tides.
As mentioned above, we name the high tides and low tides
according to their relative height. Thus, we have the
the low low, the low high, the high low and the high high
tides from midnight until midnight.

So far, we have taken for granted this concept of a tide
height. But what is the height of a tide and how do we
figure it out? The tide height is computed from a standard
tide height called the zero tide height. The zero tide
height is the average height of all the low tides or lowest
tides in a given period of time for a given location.
Without going into all the details, suffice it to say
that there is a zero tide height from which all the other
tide heights can be gauged.

In the figure above, the zero tide height is indicated
by a white line. Tides above this line have positive values
for their heights and tides below this line have negative
vales for their height. For example, what is the height
of the low low tide for this day? Because the dark blue
graph touches the zero line at the low low tide, the height
of this tide is zero feet. Using the same process, calculate
the height of the lo hi, hi lo and hi hi tides. You should
be able to see that they are slightly less than 4 feet,
a little more than 2.5 feet and a little more than 5 feet,
respectively.

Note also that the times of the 4 tides are given at
the top of the graph. For example, what is the time of
the high low tide? It is 1:12 PM.

Another useful way to determine the times of tides is
a tide table. The time and height of the tides are listed
in columns for each date. What is the time of the hi hi
tide on March 11? What is the height of the lo hi tide?
What is the time and height of the lo lo tide? When is
the hi lo tide?

Did you note that the times given are in GMT? What are
the local times for the tides you just identified?

Note that the low low tide on March 16 (at
22:36) is a minus tide, i.e. -0.7 feet. That means that
sea level at the low low tide is 0.7 feet below the zero
tide height.

Calculate the tidal range for this day. Recall that the
tidal range is the difference between the highest and
lowest tide of the day. We've already said that the low
low tide is -0.7 feet. Looking at the tide table, we can
see that the high high tide (at 16:01) is 5.8 feet. Setting
up the calculation:

Height (hi hi) - Height (lo lo) = Tidal
Range, or

5.8 feet - (-0.7) feet = ????

If you did not get 6.5 feet, then check
again. Recall that when you subtract a negative, you add.
Thus,

5.8 + 0.7 = 6.5 feet.

Here's an easy way to think about it. Imagine
you are standing on the beach. Towards the land, another
person is standing 5.8 feet away. Below you, towards the
ocean, another person is standing 0.7 feet. What is the
distance between the two people? Take yourself away and
measure and you find that 6.5 feet is the distance between
them. Try it with pencils and different positive and negative
values.

Knowing the time and height of tides can
make your beach-going experience much more pleasurable.
If you want to see the many fascinating organisms living
on rocky shores, you will want to visit during the lowest
tides. If you are hiking along the coasts (see the article
in Backpacker this month), you will need to calculate
the times and heights of the tides to gain access around
the headlands. If you own a boat, you will want to know
the time and height of the tides to make sure you stay
afloat in whatever location you are boating. If
you are a surfer, the time and height of the tides are
vitally important for the type of waves you will surf
at any given location.

There are many practical reasons for knowing
something about the tides. With a little practice, it
can be quite fun reading a tide calendar.