Transcript Document

Tides are generated by:
1.Gravitational pull of the moon
and sun
2.Centripetal force of the rotating
Earth
Tides are generated by:
• the gravitational pull of the moon and sun
- moon has 2x greater gravitational pull than the
sun
- sun is 10 million x more massive than the
moon and is 390 times farther away
Centripetal force
Halo
Why?
How?
• In any two body system, such as the Earth and moon or the Earth
and sun, one body does not orbit around the other. Rather the two
bodies orbit around a common balance point that is closer to the
larger body. For the Earth and moon, this balance point is beneath
the Earth’s surface but not at the Earth’s center. Similarly the
common point of rotation between the Earth and sun is inside, but
not at the center of, the sun.
• Any body in orbit must be held in that orbit by a centripetal force
that can be supplied by the gravitational attraction.
• Centripetal force varies with distance from the center of rotation.
All points within each rotating body follow the same diameter circle
of rotation, and centripetal force is the same at all points on and
within each of two orbiting bodies.
• The gravitational force varies with the square of the distance and
is slightly higher on the side of a body facing the other orbiting
body and slightly lower on the opposite side
GRAVITATIONAL FORCE
CENTRIPETAL
GRAVITATIONAL & CENTRIPETAL
Spring Tides
• During these times the two tide producing bodies
act together to create the highest and lowest tides
of the year. These spring tides occur every 14-15
days during full and new moons.
Neap Tides
• When the gravitational pull of the moon and Sun
are at right angles to each other, the daily tidal
variations on the Earth are at their least
Tidal Cycles
The geometric relationship of moon and Sun to
locations on the Earth's surface results in creation
of three different types of tides.
• Diurnal Tide:24 hr 50 min cycle
• Semi Diurnal Tide:12 hr 25 min
cycle
• Mixed Tide: 12 hr 25 min cycle
Diurnal Tide
• In parts of the northern Gulf of Mexico and
Southeast Asia, tides have one high and one low
water per tidal day
Semi-Diurnal Tide
• Semi-diurnal tides have two high and two low
waters per tidal day (Figure 8r-5). They are common
on the Atlantic coasts of the United States and
Europe.
Mixed Tides
• Many parts of the world experience mixed tides
where successive high-water and low-water stands
differ appreciably. In these tides, we have a higher
high water and lower high water as well as higher
low water and lower low water. The tides around
west coast of Canada and the United States are of
this type.
Global distribution of the three tidal
types. Most of the world's coastlines have
semidiurnal tides.
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Description of tides
High water: a water level maximum ("high tide")
Low water: a water level minimum ("low tide")
Tidal range: the difference between high and low
tide
Spring Tide: full moon and new moon (14.77 days)
Neap Tide: 1st quarter and 3rd quarter (14.77 days)
High tide
Intertidal zone
Low tide
The monthly tidal cycle
(29½ days)
• About every 7 days, Earth alternates
between:
– Spring tide
• Alignment of Earth-Moon-Sun system (syzygy)
• Lunar and solar bulges constructively interfere
• Large tidal range
– Neap tide
• Earth-Moon-Sun system at right angles
(quadrature)
• Lunar and solar bulges destructively interfere
• Small tidal range
Earth-Moon-Sun positions and
the monthly tidal cycle
Spring Tide
Highest high tide
and lowest low tide
Neap Tide
Moderate tidal
range
Tidal Patterns
Semidiurnal tides- two high and two low per day;
Cape Cod, MA (high latitudes)
Diurnal tides- one high and one low per day; Mobile,
AL (low latitudes)
Mixed pattern tides- Two high and two low tides per
day BUT with successive high tide levels that are
VERY DIFFERENT from each other; Hawaii (mid
latitudes)
Type of tide depends on:
• Position on the globe
• Water depth
• Contour- shape of ocean basins
Tidal Range
56 ft
6 ft
The Bay of Fundy: Site of the
world’s largest tidal range
• Tidal energy is
focused by shape
and shallowness of
bay
• Maximum spring
tidal range in
Minas Basin = 17
meters (56 feet)
Alma at High Tide
Alma at Low Tide
Tidal bore = a true tidal wave
• Wall of water that
moves upriver
• Caused by an
incoming high tide
• Occurs in some
low-lying rivers
• Can be large
enough to surf or
raft
Corals exposed to air at extreme low tide
Inquiry
1. Which has the greatest tidal effect–
sun or moon?
2. Where is the greatest tidal range
located?
3. Which lunar phase produces
moderate tides?
4. How is a tidal bore created?
Surf the tidal bore