Chapter 9
Tides
http://www.bbso.njit.edu/espr/sci_images/composite_earth1_red.gif
Overview
Rhythmic rise and fall of sea level
 Very long and regular shallow-water waves
 Caused by gravitational attraction of Sun,
Moon, and Earth

Tide-generating forces
Barycenter between Moon
and Earth – center of gravity
of “Earth-Moon System”
 Mutual orbit due to gravity
and motion

Fig. 9.1
Gravitational forces
Every particle attracts every other particle
 Gravitational force proportional to product of masses,
toward center of gravity of Moon
 Inversely proportional to square of separation distance –
decrease the farther away from Moon (look at red
circles)

Fig. 9.2
Centripetal force
Center-seeking force – force equal at all points
 Tethers Earth and Moon to each other

Fig. 9.3
Tide-producing forces

Resultant forces =
 differences between centripetal and gravitational forces (blue arrows)
○ Near moon, gravitational forces are greater
○ Away from moon, centripetal forces are greater
 Tide-generating forces are horizontal components resulting in
“bulges”
Fig. 9.4
Tidal bulges (lunar)

Small horizontal forces push seawater into two
bulges on opposite sides of Earth
 One bulge faces Moon
 Other bulge opposite side Earth
Tidal bulges (lunar)
Moon closer to Earth so lunar tide-producing
force greater than that of Sun
 Ideal Earth covered by ocean

○ If the entire earth was covered by ocean and the ocean
depth and bottom was uniform
○ This is not the case (continents, etc)

Two tidal bulges
 Two high tides, 12 hours apart
 High tide, flood tide, seawater moves on shore
 Low tide, ebb tide, seawater moves offshore
Lunar Day



Moon orbits Earth
24 hours 50 minutes for observer to see subsequent
Moons directly overhead
High tides are 12 hours and 25 minutes apart
○ Tides are based on lunar cycle
Tidal bulges (solar)
Sun effects tides, also
 Similar to lunar bulges but much
smaller because sun is farther from
earth
 Understand this figure

Other complicating factors: declination
•
•
Angular distance Moon or
Sun above or below Earth’s
equator
• Sun to Earth: 23.5o N or S
of equator
• Moon to Earth: 28.5o N
or S of equator
Fig. 9.11
Shifts lunar and solar bulges
from equator  unequal tides
at given latitude
http://www.nos.noaa.gov/education/kits/tides/media/
Declination and tides

Unequal tides (unequal tidal ranges at given latitude)
Fig. 9.13
Other complicating factors: elliptical orbits



Tidal range greatest at perihelion (January) and perigee
 Perihelion – Earth closest to sun
 Perigee – Earth is closest to moon
Tidal range least at aphelion (July) and apogee
 Aphelion – Earth farthest from sun
 Apogee – Earth farthest from moon
Moon’s perigee and apogee cycle 27.5 days
Fig. 9.12
Idealized tide prediction
Two high tides/two low tides per lunar day
 Six lunar hours between high and low tides

http://www.aztecsailing.co.uk/theory/Ch2-Fig-2-tidal-cycle.gif
Real tides affected by many factors
Tides extreme example of shallow-water waves
 Earth not covered completely by ocean

 Continents and friction with seafloor modify tidal bulges
Tides are shallow water waves with speed
determined by depth of water
 Tidal bulges cannot form (too slow)

 Ocean is not deep enough
 Speed of tides (which are shallow water waves) cannot keep
up with rotation of Earth
 Therefore:
 Tidal cells “slosh” around amphidromic point in ocean basins
○ Crests and troughs of tides rotate around this point
○ Tidal range is zero at this point
Tidal cells in world ocean

Cotidal lines
 Connects all points experiencing the same phase of the
tide
 Radiates from amphidromic point
Tidal patterns



Semidiurnal
 Two high tides/two low
tides per day
 Tidal range about same
Diurnal
 One high tide/one low
tide per day
 Rarest
Mixed
 Two high tides/two low
tides per day
 Tidal range different
 Most common
http://www.fhwa.dot.gov/engineering/hydraulics/images/h25_b6.jpg
Global distribution of tides
www.nos.noaa.gov/.../ tides/media/
Tidal Pattern for Fort Pierce
Tidal Phenomena

Standing waves
 Tide waves reflected by coast
 Amplification of tidal range
 Example, Bay of Fundy maximum tidal range 17 m
(56 ft)
http://academics.sru.edu/GGE/pictures/Newfoundland/Images
Tides in coastal
waters

Tidal bore in low-gradient
rivers
○ Come in to shallow rivers
○ Example is Amazon River
 Can rise 5ft
Fig. 9A
Tides and coastal areas like the Indian River
Lagoon

Reversing current
 Flood current or tide – water rushes into harbor with
incoming tide
 Ebb current or tide – water drains out of harbor or bay
with out-going tide
 Slack tide – current stops at peak of high and low tides
 High velocity flow in restricted channels
Fig. 9.18

Tidal range is not a good predictor of tidal currents; altered
by:
○ Shape and volume of basin
○ Restriction of flow at basin mouth
○ Winds
○ Currents stronger at channel center than at edges
Coastal tidal currents

Whirlpool
 Rapidly spinning seawater
 Restricted channel
connecting two basins
with different tidal cycles
 Not as deadly as legend
suggests
Fig. 9.19
Tides and marine life
Tide pools and life
 Organisms living in
intertidal have to be
adapted to spending long
periods underwater and
long periods exposed to
sun and drying out

Fig. 9C
http://www.stripersonline.com/surftalk/attachment.php?attachmentid=30481&stc=1&d=1169261518
Tide-generated power



Renewable resource – mostly
hypothetical at this point
Does not produce power on
demand
Possible harmful environmental
effects – alternating water flow
http://www.bluenergy.com/technology.html
Misconceptions
Distance from the sun causes the seasons.
 All tides happen in bodies of water that are
very large.
 Outer space doesn’t really affect Earth.
 The sun revolves around the Earth.

Ocean Literacy Principles



1.c - Throughout the ocean there is one interconnected circulation system powered by
wind, tides, the force of the Earth’s rotation (Coriolis effect), the Sun, and water density
differences. The shape of ocean basins and adjacent land masses influence the path of
circulation.
1.d - Sea level is the average height of the ocean relative to the land, taking into account the
differences caused by tides. Sea level changes as plate tectonics cause the volume of ocean
basins and the height of the land to change. It changes as ice caps on land melt or grow. It
also changes as sea water expands and contracts when ocean water warms and cools.
5.h - Tides, waves and predation cause vertical zonation patterns along the shore,
influencing the distribution and diversity of organisms.
Florida Sunshine State
Standards

SC.6.P.13.2 - Explore the Law of Gravity by recognizing that every object exerts gravitational force on every
other object and that the force depends on how much mass the objects have and how far apart they are.

SC.8.E.5.9 - Explain the impact of objects in space on each other including: 1. the Sun on the Earth
including seasons and gravitational attraction 2. the Moon on the Earth, including phases, tides, and
eclipses, and the relative position of each body.
SC.912.L.17.3 - Discuss how various oceanic and freshwater processes, such as currents, tides, and waves,
affect the abundance of aquatic organisms.
