Tides and Tide Raising Forces

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Tides and Tide Raising Forces
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This presentation addresses five points about
tides

• What causes tides?
• Why do tide times change?
• Why do tide heights vary?
• What are spring and neap tides?
• In what ways are tides important?

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• There two high tides approximately 12 hours apart
  each day. They are separated by two low tides.
• How does this happen?
• One tide raising force is widely known - the
  moon.
• The moons mass causes gravitational pull all
  over the surface of the earth.
• This creates a bulge of water (a high tide) under
  the moon (the sublunar point) at all times.
• As the earth rotates on its axis once every 24
  hours, it therefore follows that a place on the
  earth (eg., an island) would pass through the
  bulge once a day.
• But causes the other high tide?

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Low Tide
Moon
Earth
High Tide
High Tide
Low Tide
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• To understand this, we first must understand the
  nature of the relationship of the earth and the
  moon.
• It is commonly held that the moon revolves (goes
  around) the earth once every 28 days, which is
  the period from new moon to new moon.
• Reality is a little more complex ..

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Assume there are two objects in space ..
M 10
M 10
Balance or center of gravity for two objects with
the same mass. (Mass M)
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Now, a bit of a change ..
M 10
M 5
Balance or center of gravity for two objects
where one object has a mass twice the others.
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Finally, the earth and moon ...
Earth
Moon

• Because of the much larger mass of the earth
  compared to the moon, the center of gravity of
  the earth-moon system is actually inside the
  earths crust by about 1100 km.
• This means that the earth and moon actually
  revolve around a common point of gravity once
  every 28 days.

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Common center of gravity of earth-moon system.
Direction of revolution
Moon
Earth
Direction of revolution
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How does this explain the two high tides daily?

• This revolution of the earth about the common
  center of gravity causes centrifugal force all
  over the surface of the earth.
• This causes there to be a bulge of water (another
  high tide) on the opposite side of the earth as
  the moon.
• Therefore, a place on the earth (eg., an island)
  would in the course of the earths rotation pass
  through two high water bulges in a 24 hour period.

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• On the earths hemisphere facing the moon, the
  main tide raising force is in the direction of
  the moons gravitational attraction, or toward
  the moon.
• On the opposite side, the main tide raising force
  is in the direction of the centrifugal force, or
  away from the moon.
• In between, where there is a low tide, the two
  forces balance each other out.

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• Why?
• Gravitational pull gets weaker as distance is
  increased and so the farther from the moon the
  less tide-raising force it has.
• However, centrifugal force gets stronger as the
  radius of rotation increases (think of crack the
  whip).

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Moon
Therefore, G1gt G2gt G3 and C1gtC2gtC3, where G
gravitational force and C centrifugal force.
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• With G2 C2, it follows that on the moon side
  G1gtC3 and therefore there is a high tide at the
  sublunar point caused by the gravitational pull
  of the moon.
• Conversely, on the opposite side C1gtG3 and so
  there is a high tide at the antipodal position
  caused by centrifugal force.

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Gravitational forces
Centrifugal forces
Common center of gravity of the earth-moon system.
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But the high tides dont occur at the same time
each day. Why?

• As the earth rotates on its axis, the earth and
  the moon revolve around their common center of
  gravity.
• So as a place on the surface of the earth makes
  one complete rotation (in 24 hours), it is not in
  the same position with respect to the moon.
• If the moon was directly overhead at hour zero,
  24 hours later the moon will not be directly
  overhead as the moon has moved or revolved in
  its orbit.
• Actually it takes another 52 minutes
  (approximately) for the earth and moon to aligned
  again.
• This accounts for the (on average) 12 hour and 26
  minute between each high (and low) tide.

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Day One - 0800 hours
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Day Two - 0800 hours
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In order for Passamaquoddy Island to be at high
tide takes another 52 minutes.
Moon on Day Two
Rotation
Moon on Day One
Day Two - 0852 hours
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The two high tides each day are not the same
size. Why?

• This is due to the fact that the earths axis is
  on a tilt.
• It is not at right angles to the plane of its
  orbit about the earth-moon system.

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North Pole
High Tide
Latitude line
High Tide
South Pole
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Spring and Neap Tides Caused by the
gravitational attraction of the sun adding to or
subtracting from the effects of the moon. Spring
tides are slightly higher than normal high tides
at the new and full moon phases. Spring tide
full moon
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Spring tide new moon phase
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Neap Tides Occur when the effects of the moon are
slightly reduced by the gravitational pull of the
sun.
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It is important to note that while the suns mass
is many times greater than the mass of the moon,
its gravitational attraction (that is, its tide
raising force) is much less because of the great
distance. The moon is about 400 000 km from the
earth the sun about 160 million km. Light
travels from the moon to the earth in less than 2
seconds light travels from the sun to the earth
in 8 minutes.
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Importance of tides Impact on navigation in
coastal waters and harbours, have to be taken
into account when construction harbour
facilities. In some areas tides create tidal
flats and salt marshes important for wildlife
and also source of nutrients for fisheries. Some
high tides can be harnessed for hydroelectric
power. Many problems, however, exist with this
source of power location, cost, engineering
difficulties, environmental concerns.