7A_2 – Slide 1
Topic 7A:
Tides, Part II
Online Lecture:
The “Bulge” Theory of Tides
Overly Simplistic:
no continents,
ocean is the same
depth everywhere
The “Bulge Theory of Tides”
7A_2 – Slide 2
Low Tide
High
Tide
High
Tide
North Pole
Moon
Bulge =
High Tide
No Bulge =
Low Tide
Earth
Low Tide
If water goes up in one place, it
has to go down somewhere else:
the extra water has to
come from somewhere.
North Pole
Earth
The “Bulge Theory of Tides” II
#1
#3
North Pole
Earth
#2
High
Tide
Bulge
Earth
Bulge
7A_2 – Slide 3
Moon
Moon
High
Tide
Bulge
Low Tide
Bulge
#4
Bulge
Moon
Earth
Earth
Bulge
Moon
Bulge
Bulge
Low Tide
What is moving, the Earth or the Ocean?
Why 2 “Bulges?”: How Gravity Works
7A_2 – Slide 4
○ Every object in the universe is attracted
to every other object in the universe.
– the more massive (“heavier”) the 2 objects are,
the stronger the attraction
– the farther apart (the centers of ) the 2 objects are,
the weaker the attraction
– the pull on the lighter object is the same as
the pull on the heavier object
Which pulls on you more strongly, the Earth or the Sun? Why?
Warning: The Sun’s gravitational pull is stronger than the Moon’s, but
it has a weaker effect on the tides. There is more to it than strength…
Why 2 “Bulges?”: Moon’s Pull on the Ocean
7A_2 – Slide 5
○ The Moon's gravity pulls on the ocean,
making a “bulge” in the ocean on the side of the Earth
facing towards the Moon.
How many
high tides
will there be
each day
if this is
the only
important
factor?
North Pole
Moon
Earth
Bulge
Why 2 “Bulges?”: Moon’s Pull on the Earth I
7A_2 – Slide 6
○ The Moon's gravity pulls on the Earth,
making it orbit a place called the barycenter.
At First:
Earth
Moon
Barycenter
2 weeks later
Why 2 “Bulges?”: Moon’s Pull on the Earth II
7A_2 – Slide 7
○ When the Earth moves in a circle (an “orbit”),
the water is flung away from the center of the circle
(the location it “orbits,” the barycenter), creating a bulge
in the ocean on the side of the Earth facing away from
the Moon.
Bulge
Barycenter
Circle or "Orbit"
North Pole
Moon
Earth
Why doesn’t the water
get flung off the Earth?
Bottle
Center
of the
Circle
Why 2 “Bulges?”: Putting Everything Together 7A_2 – Slide 8
○ The Moon’s gravity pulls the ocean towards the Moon,
but the Earth’s small orbit around the barycenter (owing to
the Moon’s gravity pulling on the Earth) flings the water
away from the Moon.
Bulge caused
by the Earth's
orbit around
the barycenter
Caused by the Moon’s
gravitational pull on the Earth
North Pole
Moon
Earth
Which bulge is larger?
Bulge caused
by the Moon's
gravitational pull
on the ocean
These 2 effects do not
cancel, because
gravity gets weaker
with increasing
distance. The Moon’s
pull far side of the
Earth is too weak to
keep the water from
being flung outwards,
whereas the Moon’s
pull on the closer side
of the Earth
overcomes this
tendency.
Why Tides Get Later Each Day
7A_2 – Slide 9
○ During a day the Moon moves a little bit,
so the ocean’s bulges move a little as well.
How long does it take
the Moon to orbit the
Earth one time?
○ After 24 hours, each location on the Earth's surface will be
back where it started, but the bulges will have moved on.
○ The Earth
has to rotate
a little farther
before the
location is in
the “bulge”
in the ocean
beneath
the Moon.
At First
24 hours later
25 hours later
Moon
Earth
Earth
rth
a
E
High
Between High & Low:
TideTide
Rising
High
Tide
Tide
Why There Are Spring & Neap Tides I
7A_2 – Slide 10
Tidal Range
Moon Phase(s)
Spring Tides
Large
Full & New Moon
Neap Tides
Small
Half Moon
○ The Sun’s gravity tries to create 2 “bulges” in the ocean
too, one towards the Sun and one away from the Sun.
– Sun’s effect is weaker than
the Moon’s, so the Moon always
determines where and when high
and low tide will be.
But the Sun can affect
how high or low the tides get…
Remember:
The Sun’s gravity is
stronger than the
Moon’s. There is
more to causing tides
than strength…
Why There Are Spring & Neap Tides II
New Moon: Spring Tides
Full Moon: Spring Tides
E
7A_2 – Slide 11
S
#1
E
S
E
S
#3
Half Moon: Neap Tides
E
S
#2
#4
How long does it take to go
from spring tide conditions
to neap tide conditions?
Half Moon: Neap Tides
How do the Earth, Moon, and
Sun have to be positioned to
produce Spring Tides? Neap?
Why There Are Spring & Neap Tides III
7A_2 – Slide 12
○ Sometimes the Moon & the Sun both want high & low tides
to happen in the same places at the same time.
– makes high tides higher
and low tides lower
Moon & Sun work together,
pulling water from some
places to other places.
○ Sometimes the Sun wants it be high tide where
the Moon wants is to be low tide (and vice versa).
– Which will win?
The Moon
– But the Sun interferes, pulling up the low tide,
and pulling down the high tide.
Moon & Sun fight about where
high tide and low tide should be.