2. 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
3. Tides are generated by:
1.Gravitational pull of the moon
and sun
2.Centripetal force of the rotating
Earth
4. Tides are generated by:
• the gravitational pull of the moon and sun
- moon has 2x greater gravitational pull than the
sun
- sun is 400 x more massive than the
moon and is 400 times farther away
7. Why Two Tides?
• Tides are caused by the stretching of a planet.
• Stretching is caused by a difference in forces on the
two sides of an object.
• Since gravitational force depends on distance, there
is more gravitational force on the side of Earth
closest to the Moon and less gravitational force on
the side of Earth farther from the Moon.
8.
9. 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
10. Timing the Tides
The moon revolves around Earth much
more slowly than Earth rotates.
High and low tides at that place happen
50 min. later each day.
11. By the time the Earth has completed one revolution
(one day), the moon has moved slightly, so the Earth
needs to rotate a bit more to catch up with the moon…
12. How do Tides
Vary?
A tidal range is the difference between
levels of ocean water at high tide and low
tide.
The combined forces of the sun and the
moon on Earth produce different tidal
ranges.
15. Tidal Cycles
• Diurnal Tide:
24 hr 50 min cycle
• Semi Diurnal Tide:
12 hr 25 min cycle
• Mixed Tide:
12 hr 25 min cycle
16.
17.
18. 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)
22. Bay of Fundy – tide
gets huge (15 m) because
of resonance
High tide
Low tide
(∼6 hours later)
23. Earth’s Tides
• There are 2 high tides and 2 low tides per day.
• The tides follow the Moon.
24. 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
25. • St. Michel, N. coast of
France
• ~16.8 m highest tidal
range in Nova Scotia
26. Nova Scotia Tides
• Wolfville, NS (16 m tidal range)
• Diurnal Tides (one high and one low every 12 hours
and 25 minutes
27. Why the Moon?
• The Sun’s gravitational pull on Earth is much
larger than the Moon’s gravitational pull on
Earth. So why do the tides follow the Moon
and not the Sun?
28. Why the Moon?
• Since the Sun is much farther from Earth than
the Moon, the difference in distance across
Earth is much less significant for the Sun than
the Moon, therefore the difference in
gravitational force on the two sides of Earth is
less for the Sun than for the Moon (even
though the Sun’s force on Earth is more).
29. Why the Moon?
• The Sun does have a small effect on Earth’s
tides, but the major effect is due to the Moon.
30. Different kind of tides
• Spring tides: tides that have the largest
daily tidal range are spring tides.
• Neap tides: tides that have the smallest daily
tidal range are called neap tides.
31. Earth-Moon-Sun positions and
the monthly tidal cycle
Spring Tide
Highest high tide
and lowest low tide
Neap Tide
Moderate tidal
range
32. Tides
• The rise and fall in sea level is called a
tide.
• Caused by a giant wave.
• One low-tide/high-tide cycle takes about
12 hrs and 25 min.
• Tidal range is the difference in ocean
level between high-tide and low-tide
33. Gravitational Effect of the Sun
• Spring Tides
– Earth, Moon, and Sun are lined up
– High Tides are higher and Low Tides
are lower than normal
34. Gravitational Effect of the Sun
• Neap Tides
– Earth, Moon, and Sun form right
angles
– High Tides are lower and Low Tides are
higher than normal
35. Why Tides Happen?
The moon’s gravity pulls
on every particle on Earth
However, the gravity
doesn’t pull on every
particle with the same
length of strength.
The moon’s gravitational
pull on Earth decreases
with the distance from the
moon.
Therefore, the pull on
different parts of the
Earth is stronger than on
other parts.
37. The monthly tidal cycle
(27.3 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
Notes de l'éditeur
Gravity- pull or attraction between objects; varies with mass of object Centrifugal force- because the earth and moon are rotating simultaneously around a common center of mass, the water of the oceans shifts from the center of rotation, creating a 2 nd tidal bulge on the side of the earth that faces away from the moon
TIDES: Tides are the slow, periodic vertical rise and fall of the sea surface. They are usually described as being either diurnal or semi-diurnal. Diurnal tides have one high water and one low water in each lunar day (about 24.8 hours), while semi-diurnal tides have two high and two low waters in the same time period. While these tidal changes are easier to observe where land and water meet, they exist everywhere -- even in the middle of the ocean. Tidal ranges along the shoreline vary by location. For example, the tides in Canada's Bay of Fundy, an Atlantic Ocean inlet west of Nova Scotia, rise and fall as much as 50 feet, while the tidal range in Lake Superior is measured in inches. High and low tides are the result of the attractive forces (gravitational pull) of the moon and sun on a rotating Earth.
TIDES: Tides are the slow, periodic vertical rise and fall of the sea surface. They are usually described as being either diurnal or semi-diurnal. Diurnal tides have one high water and one low water in each lunar day (about 24.8 hours), while semi-diurnal tides have two high and two low waters in the same time period. While these tidal changes are easier to observe where land and water meet, they exist everywhere -- even in the middle of the ocean. Tidal ranges along the shoreline vary by location. For example, the tides in Canada's Bay of Fundy, an Atlantic Ocean inlet west of Nova Scotia, rise and fall as much as 50 feet, while the tidal range in Lake Superior is measured in inches. High and low tides are the result of the attractive forces (gravitational pull) of the moon and sun on a rotating Earth.
Gravity Newton’s law states that all particles of mass have a gravitational attraction for all other particles, and that the gravitational force is proportional to the sum of the two masses and inversely proportional to the square of the distance between their centers of mass. The gravitational attraction between the moon and the Earth is very small compared to gravity felt by an object at the Earth’s surface due to the Earth itself. Although the sun has a much greater mass than the moon it is much further from the Earth and the gravitational attraction between the sun and the Earth is smaller than that between the moon and the Earth. Orbital Motions and Centripetal Force 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. The Balance between Centripetal Force and Gravitational Force The small imbalance between centripetal force and gravity at different points on the Earth is responsible for tides. The Earth’s own gravity is millions of times larger than the gravitational attraction of the moon or sun at the Earth’s surface and, therefore, the imbalance between centripetal force and gravity can be compensated by an immeasurably small change in an object’s weight if the force imbalance is directed in the same direction as Earth’s gravity (vertically toward Earth’s center). Distribution of Tide-Generating Forces The moon’s gravitational attraction and the required centripetal force are exactly balanced at all points along a ring around the Earth drawn almost exactly midway between the point closest to and farthest away from the moon At the points directly toward the moon and directly on the opposite side of the Earth from the moon, Earth’s gravity and the imbalance between the moon’s gravitational attraction and the required centripetal force is completely compensated by an immeasurably small change in an object’s weight. At all other points on the Earth there is a component of the imbalance between the moon’s gravitational attraction and the required centripetal force that acts parallel to the Earth’s surface and so cannot be compensated. This component is the tidal force. The tidal force acts toward the moon on the moon’s side of the Earth and away from the moon on the opposite side. The tidal force is zero at the points directly toward and directly away from the moon, increases away from these points, and then decreases to zero again at points along the ring around the Earth drawn almost exactly midway between the point closest to and the point farthest away from the moon, where the moon’s gravitational attraction and required centripetal force are balanced.
The closeness of the moon to Earth (238,857 miles), and the distance to the sun (92,955,770 miles), accounts for the moon having a tide-raising force nearly 2.5 times greater than the sun. The position of these celestial bodies results in significant variations in pulling forces causing above or below normal tidal ranges. The range between a high and a low tide is greatest when the sun, moon and Earth are in alignment. These are spring tides. When the sun and moon are at right angles to the Earth, their gravitational forces significantly reduce each other. This causes the neap tide, a period of decreased tidal range. The term neap is an acronym for near even as possible. Bulges are about the same size Tide-generating forces are a result of the gravitational attraction between the Earth, sun, and moon. It was not until Sir Isaac Newton (who lived from 1642-1727) discovered the law of gravity that the effect of the sun and the moon on the tides was fully understood. All surfaces of the Earth are pulled toward the moon and sun. This force has little effect on land masses, but it does have a very great and obvious effect on the water of the Earth's oceans. Twice each month the tidal range reaches a maximum and these large tides are called the spring tides. Halfway through the monthly cycle the range is much smaller, and these weak tides are called neap tides. As the moon rotates around the Earth, it pulls the water on the nearest side of the Earth outward into a bulge. A similar bulge on the opposite side of the Earth is caused by the water being thrown outward by the planet's spin. These two bulges travel around the globe, producing two high tides each day. During time of the new moon and full moon, when the sun and moon are in a straight line, their gravitational pull combine and produce spring tides; at this time the high tides are very high and the low tides are very low. When sun and moon are at right angles from the Earth, during the quarter phases of the moon, the gravitational pull on the oceans is less producing a smaller difference between high and low tide known as a neap tide. Some locations have much bigger tides than others. Tidal ranges are usually small in the middle of the ocean but can be very large where tidal waters are funneled into a bay or river estuary. Hawaii has hardly any tidal range at all while the water in the Bay of Fundy, in Canada, has a range of about 40 feet.
Flood tide directs water landward; Ebb tide directs water seaward TIDAL CURRENTS: The rise and fall of the tide is accompanied by the horizontal flow of water called a tidal current. The usual terms used to describe the direction of this horizontal movement are ebb and flood. Ebb currents occur when tidal currents are moving away from the coast. Flood currents move toward the coast. In a purely semi-diurnal current, the flood and ebb each last about 6 hours. Speed of tidal currents depends upon the shape and dimensions of the harbor, coastal areas and ocean bottom. The configuration also influences vertical range of the tide itself. Under certain conditions, tidal currents can move more than 10 knots.
Countor- can distort the rotary motion of circular waves Additional distortions occur near the equator where coriolis deflection changes direction, and along continental margins around large islands, where the sea bottom is shallow and the water is lowed down. These combined effects result in considerable variation in the type of tide and the tidal range along the world’s coastlines Shouldn’t the tides be 12 and 24 hours? Explanation: after 1 earth day, the moon has moved slightly forward in its orbit, so an additional 50 minutes or so is necessary for a spot on the earth to region its postion relative to the moon each day.Hence, the high tide and low tide shift forward each day
TIDES: Tides are the slow, periodic vertical rise and fall of the sea surface. They are usually described as being either diurnal or semi-diurnal. Diurnal tides have one high water and one low water in each lunar day (about 24.8 hours), while semi-diurnal tides have two high and two low waters in the same time period. While these tidal changes are easier to observe where land and water meet, they exist everywhere -- even in the middle of the ocean. Tidal ranges along the shoreline vary by location. For example, the tides in Canada's Bay of Fundy, an Atlantic Ocean inlet west of Nova Scotia, rise and fall as much as 50 feet, while the tidal range in Lake Superior is measured in inches. High and low tides are the result of the attractive forces (gravitational pull) of the moon and sun on a rotating Earth.
