Transcript Slide 1

Why Are There Tides?
 The tides are caused by the gravitational pull of the
moon and sun and by the rotations of the earth, moon,
and sun.
 The Earth and moon both rotate around a common
point
 This rotation produces centrifugal force
Why Are There Tides?
 Centrifugal force and the moon’s gravity are not in
perfect balance.
 On the side of the earth nearest the moon, the moon’s
gravity is strongest and pulls the water towards the
moon
 On the other side away from the moon, centrifugal
force predominates, pushing the water away from the
moon.
Why Are There Tides?
 In addition to the rotation of the earth and moon the
earth is spinning on its axis.
 High tide occurs when the point is under a bulge
 Since the earth takes 24 hours to complete a rotation,
the point will have 2 high tides and 2 low tides every
day.
 The moon advances in its orbit too. It takes the point
on earth an extra 50 minutes to catch up and be under
the moon
High tide – low tide
Cont.
 A full tidal cycle takes 24 hours and 50 minutes
 The sun produces tidal bulges too.
 Although the sun is larger than the moon, it is 400
times further away therefore, the pull is about half as
much.
 When the sun and moon are in line, which happens at
full and new moons, their effects add together.
Spring Tides
 At full and new moons, when the moon and sun are in
line, the difference in eater level between successive
high and low tides is large.
 Spring tides occur throughout the year about once
every two weeks
Neap Tides
 When the sun and moon are at right angles their
effects partially cancel each other.
 During these neap tides the tidal range is small
 Neap tides occur when the moon is in first and third
quarter
Tides In The Real World
 Tides vary from place to place depending on the
location and on the shape and depth of the basin
 As predicted, most places do have two high tides and
two low tides a day : Semidiurnal tides
 Some places have mixed semidiurnal tides, with
successive high tides of different height (western US,
Canada)
Cont.
 Diurnal tides occur when there is only one high tide
and one low tide every day
 Diurnal tides are uncommon. They occur in the Gulf
of Mexico, on the coast of Antarctica, and in parts of
the Caribbean and Pacific.
end