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 I have never surfed anything except the
internet, but I have watched plenty of people enjoy this sport. I have done
my fair share of bodysurfing, which may not be as exhilarating, but most certainly
beats the boogie boarders who just go out onto the waves and run people over.
The key to body surfing is to know when to get into the wave, how to streamline
with your head up and eyes open, and get all the way onto the beach, thus making
a favorable impression on anyone who might be watching. Closing your eyes is
bad, because you might inadvertently run into some foolish junior high kid who
is in your way. Then you jam your fingers into his gut, and dislocate the ring
finger of your left hand, as I did when I was in my thirties, riding the 10
foot waves of Huntington Beach, CA. Okay, back to tides. Oh, body surfing in
the total nighttime darkness is really cool. Huntington Beach has these campfire
pits. You go to some local business, buy a bunch of wooded pallets, and make
a big bonfire. Then you ride the waves in the dark. Hopefully, you do not mind
the sight of some local fisherman hauling in a small nurse shark, 50 yards from
where you are bodysurfing in the dark.
Now, underlying this minute to minute wave 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's gravity 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. If you will recall from
our history lesson, Isaac Newton was the first to give a mathematical description
of the force of Gravity. Gravity is this invisible force that acts between any
two bodies of mass. The force of gravity, otherwise called the force of attraction,
is proportional to the square of the distance between them. Understanding the
tides is an exceedingly complicated manner, well beyond the scope of this introductory
level Astronomy course. For those mathematical details, I would direct you to
the NOAA Tides
website. For now, I will give a more simplified explanation.
The Moon Tide
The earth and the moon are two great masses
that have a significant gravitational pull on each other. This is what keeps
the moon in orbit around the earth, and it is also what causes tides to occur
in the ocean. Picture the earth with a uniform level of water all around it.
The moonís gravity pulls on the earth, and pulls the rock and water towards
it. (Yes, I know that my fellow Physics teachers abhor the use of the word "pull"
in reference to tides and gravity, when we should be describing "mutual
attraction," but the word "pull" will suffice for our purposes
right now.) So, I did say that the gravitational force pulls the Earth's rock
toward the moon, slightly deforming the entire planet. But since the rock is
solid, obviously, the tidal distortion is barely perceptible. However, the liquid
water ocean can be deformed more markedly. The water moves up into a slight
bulge on the side of the earth that faces the moon. At the same time, there
is a force pulling water out in the opposite direction of the moon.
To understand this force,
you need to picture the earth and the moon as one unit. Picture two unequal
balls on the ends of a stick. The stick can represent the gravitational attraction
between the two objects of mass. The barycenter is the term used to described
the center of gravitational mass between the two bodies of unequal mass. By
plugging the appropriate values into Newton's gravity formula, you will discover
that the center of the earth-moon system is over 4700 km from the center of
the Earth ... still inside of the Earth's crust. Therefore, the Moon technically
does not simply orbit the Earth, but the Earth-Moon system is spinning around
this barycenter of mass, some 4700 km from the center of the Earth's core.
If you look at the system from
a distance, you will discern the Earth appearing to move back and forth 9400
km per complete lunar orbit. It is this wobble of the Earth that caused astronomers
to try looking for similar wobbles at distant stars, for if a star wobbles,
it means that there must be a planet or even planets orbiting it.
Now, back to the stick and two ball model. If you spin this stick around,
you can imagine the force that a particle might feel if it were on the far end
of either the moon or the earth. It would feel a force outward, away from the
center of the spin. This is called the centrifugal force. The water on the far
end of the earth, away from the moon is always being pulled out from the center
of the spinning earth-moon unit. The gravitational and centrifugal forces are
constant, always pulling water towards the moon and directly away from the moon.
The forces in either direction are equal to each other. The bodies of water
that feel these forces change constantly as the earth rotates within these forces,
but the force directions are always toward and away from the moon.
there is more to the story than simple centrifugal force, accounting for the
high tide opposite the Moon. There is a 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.
As the earth turns upon its own axis in about 24 hours, a point on the earth
moves through areas with these different forces acting on it. In one rotation
(one day), a point on earth travels from an area of high tide (where there is
a force pulling water outward), through an area of low tide, through an area
of high tide again (the opposite pull), and through another area of low tide,
before it returns to the point of origin at high tide. This results in two high
tides and two low tides in a day (called semidiurnal tides).
The Tidal Day
The moon does not stay put, but rotates around the earth at a rate of about
12į a day, or one rotation a month. The rotation is in the same direction as
the earthís spin, so by the time the earth has done one rotation, the moon has
shifted 12į further, and it takes an extra 50 minutes for the moon to be in
the same position relative to a point on the earth. Therefore, the tidal cycle
is not 24 hours long, but 24 hours and 50 minutes. Because of this, high and
low tides are about 50 minutes later every day.
The Sun Tide
are caused mainly by the gravitational attraction of the moon and the earth,
but there is also a gravitational attraction between the earth and the sun.
The effect of the sun upon the tides is not as significant as the moonís effects.
Basically, the sunís pull can heighten the moonís effects or counteract them,
depending on where the moon is in relation to the sun. In one month, the moon
rotates around the earth. When the moon is between the sun and the earth (at
new moon), the sunís gravitational pull is in the same direction as the moonís.
During these days the high tides are higher and the low tides are lower than
they'd be with just the moonís pull alone. This is called spring tide. The term
has nothing to do with a season of the year, but is derived from the German
"der springen" which means to leap up. The larger tides leap up more
than other tides. The same thing happens when the moon is on the direct opposite
side of the sun (full moon). The two gravitational forces work together to make
high high tides and low low tides.
When the moon
is in its first quarter or its last quarter, the sunís gravitational pull is
in perpendicular direction to that of the moon. The sun pulls water away from
the areas of high tide to the areas of low tides, resulting in lower high tides
and higher low tides. These are called neap tides.
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
The moon does not rotate around the earthís equator, but follows an orbit
that is inclined in relation to the earthís axis. Because of this, northern
and southern latitudes commonly face only one high tide and one low tide in
a day, called diurnal tides. The inclination of the moon changes in relation
to the earth on a 19 year cycle. The earthís inclination in relation to the
sun also effects the tides. The sunís inclination follows a year-long cycle,
and is in highest inclination in the summer and winter months. During these
months the "bulges" in the ocean are offset the most from the equator, and it
is most likely to encounter only one tide cycle per day, or diurnal tides.
References: Duxbury and Duxbury (1994) An introduction to the World's Oceans,
Wm. C. Brown Publishers,4th edition: Dubuque: Iowa. Pinet, Paul (1998) Invitation
to Oceanography, Jones and Bartlett Publishers: Sudbury, Massachusetts. Check
out this great site: NOAA's
tide page - explanation of the tides, and a great glossary of terms related
to tides. Home | OceanInfo | Career Info | Ask a Scientist/Answer Archive AquaFacts
| OceanNews | Records | Links
Source for this tide information is OceanLink
The OceanLink Coordinator is Anne Stewart .
Questions or Comments about the OceanLink Project should be directed to the
This page was created April 14, 1996, and last updated December 11, 2001.
Questions/Comments about the web pages should be directed to firstname.lastname@example.org
Permission is granted by OceanLink for classroom teachers to make copies of
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works, or resale.
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
Something if great interest to Astronomy students living along coastal areas
is knowledge of the tides in greater detail. This Perigee Tide can be amplified
to produce the maximum possible high tide. In a very rare occurrence, New Moon
at Perigee, as well as Perihelion will produce the ultimate high tide. It is
this particular tide which Patrick Swazy escaped from the tail of Keannu Reeves
in the movie "Point Break." Patrick went to Australia, because it
is summer down under when Earth is at perihelion. The great wave was amplified
by a New Moon at perigee. You can watch the movie to see what happens. Although
I have no certainty about the movie director's knowledge of tides, or where
the director was aware of this chance alignment, but it makes for good storytelling.
In reality, the Billabong Surfing Championship is always held in Australia in
December or January, during their summer, and when the Moon is New ... a simple
and predictable event to schedule. I am not downplaying the Bonzai Pipeline
of Hawaii's North Shore, but Australia is the place to go. The wide Pacific
Ocean has lots of time for the tidal bulge to build before it rams into the
East Coast and creates big surf ... Kowabunga, dudes!
Although the Sun exerts a gravitational force 180 times as strong as does
the Moon on Earth, because the Moon is so much closer, the variation in Moon's
force across Earth's diameter is about 2.2 times larger than the variation in
the Sun's force. As noted above, it is this variation that produces tides, thus
the pair of bulges raised by the Moon are considerably larger than the pair
of tidal bulges get in and out of step, combining in step to produce "spring"
tides (no connection with the season) when the Moon is new or full, and out
of step to produce "neap" tides when the Moon is at first or last
quarter. Another factor having a substantial influence on tidal ranges is the
elliptical shape of the Moon's orbit. Although the Moon is only 9 to 14% closer
at its close point to Earth (perigee) than at its far point (apogee), because
the variation in its gravitational force varies inversely as the cube of its
distance (the force itself varies inversely as the square of the distance),
the Moon's tidal influence is 30 to 48% greater at perigee than at apogee. In
the Bay of Fundy the perigee-apogee influence is greater than the spring-neap
influence. Although the variation of the Moon's distance is not readily apparent
to observers viewing the Moon directly, to observers near the shores of Minas
Basin, the three to six meter increase in the vertical tidal range makes it
obvious when the Moon is near perigee, clear skies or cloudy!
The Best Places on Earth to See the Highest Tides!! Minas Basin and Bay of Fundy
in Nova Scotia. Click on the map to learn more!!!
Anchorage Alaska boasts the world's second highest tides: varying over 40
feet, low to high tide! Bore tide (one of the three highest in the world, and
a weird phenomenon: capillary action on a gigantic scale!) occurs 2 hours 15
minutes after low tide; best viewed between Mileposts 101 and 90 Seward Highway
(26 to 37 miles from Anchorage). For details see the website http://hometown.aol.com/akvisinfo/data/general/scak.htm.
I have been up in Alaska twice in my life, and one of those times was a memorable
18 day wilderness camping experience with my best friend who I grew up with
in Milwaukee. We were looking out over the Anchorage bay on the first day in
Alaska, and saw a flock of shorebirds on the gray silt. Curious as to whether
these birds were puffins (my absolute favorite bird, but one I have never seen),
I began to walk onto the tidal flats to get a closer look. Within a few steps,
my foot was sucked into the silt to a depth of 1 foot (no play on words intended).
I was stuck, and Marc had to get a long stick to yank me out of the mud. I thought
nothing more of the experience until the end of our trip, when we were down
by the Portage Glacier. I noticed a sign posted by the tidal flats warning people
to stay of the silt, and if they were somehow stuck in the silt, as I was earlier,
they were to use the nearby emergency phone to dial 911 and get immediate help.
The tides there come in so fast and furiously that people who get their feet
stuck in the mud often will drown in place before a rescue team can extract
Here are two questions I found while surfing the net for tide information.
How is it that the Sun tide is 46 percent of the Moon tide?
The tidal force difference is the mass ratio times the cube of the distance
ratio. For M(sun) = 2 x 10^30 kilograms, M(Moon) = 7.4 x 10^22 kilograms and
d(moon) = 380,000 kilometers and d(sun) = 150 million kilometers, this gives
(2x10^30)/(7.4x10^22) X (380,000)^3/(150,000,000)^3 = 5/11 = 0.46
Copyright 1997 Dr. Sten Odenwald
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 dynamic 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.
Copyright 1997 Dr. Sten Odenwald
Okay, you are finished with tides, and have perhaps "had it up to here"
with this information, but hopefully you found some of the material interesting.
It is now time to move on to more practical learning opportunities about the
Moon. You have covered Structure,
Phases, and Tides. Please
move forward to Eclipses to learn
the most fantastic celestial sights visible to the naked eye, or return to the
or the Syllabus.
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