Tides Lecture Notes (Topic 7A) – page 1
Tides Lecture Notes
Why There Are 2 High Tides & 2 Low Tides Each Day
(Why the Tides Changes Every 6 Hours)
What is a tidal “bulge”? How are the “bulges” related to high tide and low tide?
A “bulge” is a place on the Earth where there is ________________ water in the ocean.
In other words, it is _______________ tide where a bulge is present.
Outside the bulge, there is________________ water in the ocean
and it is _______________ tide.
How many “bulges” are there in the ocean? Where are they on the Earth relative to the Moon?
There are _______ bulges in the ocean.
One bulge is: __________________________________________________.
The other bulge is: __________________________________________________.
North Pole
Moon
Earth
Why are there 2 high tides and 2 low tides each day? Why does it take about 6 hours for sea level
to go from high tide to low tide? Explain using “bulge” theory of the tides.
It takes ______________________________ for the Earth to spin once all the way around.
When a location goes into a bulge, the location experiences ________________ tide.
When a location goes out of a bulge, the location experiences ________________ tide.
Since there are _____ bulges in the ocean, each location on Earth goes into a bulge (high tide)
and out of a bulge (low tide) twice each day. Since bulges are on opposite sides of the Earth,
a location goes from inside a bulge (high tide) to outside bulge (low tide) in _________ turn
of the Earth which takes ______ hours.
Tides Lecture Notes (Topic 7A) – page 2
Why There Are 2 Bulges in the Ocean
How Gravity Works
Is the gravitational attraction stronger between two objects
that are close together or far apart?
_______________________
Is the gravitational attraction stronger between
2 large, heavy objects or 2 small, light objects?
_______________________
Note: Both objects experience exactly the same attraction towards one another;
one is not more attracted than the other.
As the Earth orbits the Sun, it is pulled off
course a little bit by the gravitational pull of
the Moon. Since the Moon goes around the
Earth, sometimes the Earth is pulled ahead a
little faster, sometimes it is pulled back
slowing it down, sometimes it is pulled a little
towards the Sun, and sometimes it is pulled a
little away from the Sun. Overall, the Earth
makes a small circle around the place in the
Earth’s orbit around the Sun where it “should
be”: the barycenter.
barycenter
E
The Earth's path is shown
by the brown, solid line.
The barycenter's path
is shown by the
blue, dashed line:
This is the path that
the Earth would follow
IF the Moon's
gravitational pull
did not distort
the Earth's orbit
around the Sun.
Why does the Earth orbit
the Earth-Moon barycenter?
If you spin a bottle or bucket in a circle, what does the water in it do?
S
Circle or "Orbit"
Bottle
Center
of the
Circle
Like water is a spinning bottle or bucket, water on the Earth moves away from the center
of the Earth’s orbit around the barycenter – which means it also moves away from the Moon.
On the side of the Earth facing towards the Moon, the Moon’s gravitational pull on the water is
stronger than its tendency to move away from the barycenter and Moon, so water gets pulled
towards the Moon, creating the bulge on the side of the Earth facing the Moon.
On the side of the Earth facing away from the Moon, the Moon’s gravitational pull on the water
is weaker than its tendency to move away from the barycenter and Moon, so water moves away
from the Moon, creating the bulge on the side of the Earth away from the Moon.
Which bulge is larger, the bulge towards the Moon or the bulge away from the Moon?
Tides Lecture Notes (Topic 7A) – page 3
Sketching Bulges: Relationships between the Bulges, High and Low Tides, and the Moon
In each of the pictures below, sketch the bulges in the ocean for the arrangement
of the barycenter, Earth, Moon, and Sun shown below.
(“B” is the barycenter, “E” is the Earth, “M” is the Moon, and “S” is the Sun.)
Label the bulge(s). In other words, write the word “bulge” on or next to the bulge(s).
Label the locations of high tide and low tide. Write “HT” for high tide and “LT” for low tide.
You may use arrows to indicate
the specific location of a bulge,
a high tide, or a low tide.
Tides Lecture Notes (Topic 7A) – page 4
What if the Moon’s gravitational pull were stronger or weaker than it is right now?
NOTE: When I ask “what if?” questions, I will only change ONE thing, and that is the only thing
that you should consider when considering your answer. For example, if the size of the Moon
changes, all other factors remain the same: the Moon’s distance from the Earth, the speed of the
Moon’s orbit, the speed of the Earth’s rotation, and so on do not change.
Suppose that the Moon was twice as large.
How would the height of the tides be different?
The (A)____________________________________ pull of
the (B) __________________________ would be (C) ______________________________,
so the high tides would (D) _______________________________________ they are now
because (F) _______________ water would be pulled (G) ____________________________
the places where it is high tide (the bulges), and
the low tides would (E) ______________________________________ they are now
because (F) ________________ water would be pulled (G) ___________________________
the place(s) where it is low tide (outside the bulges).
Suppose that the Moon was two times farther from the Earth.
How would the height of the tides be different?
The (A)____________________________________ pull of
the (B) __________________________ would be (C) ______________________________,
so the high tides would (D) _______________________________________ they are now
because (F) _______________ water would be pulled (G) ____________________________
the places where it is high tide (the bulges), and
the low tides would (E) ______________________________________ they are now
because (F) ________________ water would be pulled (G) ___________________________
the place(s) where it is low tide (outside the bulges).
Possible Answers
A: give the name of the force that pulls on the water in the ocean
B: Barycenter, Coriolis effect, Earth, Moon, Sun
C: stronger, weaker, about the same
D: be higher than, not be as high as, be about the same as
E: be lower than, not be as low as, be about the same as
F: more, less, about the same amount of
G: towards, away from
Tides Lecture Notes (Topic 7A) – page 5
The Timing of the Tides
The Moon orbits the Earth twice each day. We experience a “high tide” when the Moon is
overhead and pulls a “bulge” in the ocean to our coast.
What is wrong with this statement? How could you correct or improve it?
The Earth goes “in” and “out” of the “bulges” in the ocean twice each day.
What is wrong with this statement? How could you correct or improve it?
Given your answers to the questions above, which of the following would change the timing of
the tides? In other words, which of the following would change the amount of time it takes for the
ocean to go from high tide to low tide (or vice versa)?
a. Suppose that the Moon was twice as large.
b. Suppose that the Moon was two times farther from the Earth.
c. Suppose that the Earth rotated twice as fast.
In other words, suppose that the Earth made one complete revolution in 12 hours.
d. Suppose that the Moon orbited the Earth twice as fast. In other words, suppose that the
Moon completed one trip around the Earth in about 14 days.
e. Suppose that the Earth orbited the Sun twice as fast. In other words, suppose that the
Earth completed one trip around the Sun in about 6 months.
Tides Lecture Notes (Topic 7A) – page 6
Why the Tides Get a Little Later Each Day
About how much does the Moon move in a day? In other words, what fraction of its orbit does it
move through? (Answer with a fraction like 1/2 or 1/10.)
In a day, the Moon moves through __________ of its orbit around the Earth.
Do the tidal bulges in the ocean move?
Yes, because the tidal bulges are caused by ____________________________________,
and the ________________________ moves a little each day.
Explain why the time of the tides shifts a little each day using the bulge theory of tides.
Suppose that a location on the Earth is in a bulge, so it is experiencing a ____________ tide.
The location will be back in the same spot _____________ hours later after the Earth
has rotated all the way around 1 time. However, it will no longer be high tide at this spot
because during a day the Moon moves a little bit, so the ocean’s bulges _______________
a little as well. The Earth has to rotate a little longer and farther to catch up with the bulge.
Tides Lecture Notes (Topic 7A) – page 7
Why the Tides Change Over a Month: Spring Tides
Write the phase of the Moon (new moon, half moon, or full moon) and the tide conditions (spring
tide conditions or neap tide conditions) into each of the pictures below.
Write “HT” at the locations of high tides in the pictures, and “LT” at the locations of low tides.
In each picture, put a “M” at the locations where the Moon’s gravitational pull is creating
bulges in the ocean. Put a “S” at the locations where the Sun’s gravitational pull is creating
bulges in the ocean.
Explain what causes spring tides during these phases of the Moon
using the bulge theory of the tides.
During spring tide conditions, the Sun’s gravitational pull moves water towards
(A)__________________________________________________ the Moon.
Thus, the Sun pulls water (B)________________________ the high tide spots (the bulges),
so there is (C)____________________ water in the high tide spots (the bulges),
making the high tides (D)________________________________________.
The Sun pulls water (B)______________________ the low tide spots (outside the bulges),
so there is (C)____________________ water in the low tide spots (outside the bulges),
making the low tides (E)_________________________________________.
A: the same places as, different places than
B: towards, away from
C: more, less
D: very high, not very high
E: very low, not very low
Tides Lecture Notes (Topic 7A) – page 8
Why the Tides Change Over a Month: Neap Tides
Write the phase of the Moon (new moon, half moon, or full moon) and the tide conditions (spring
tide conditions or neap tide conditions) into each of the pictures below.
Write “HT” at the locations of high tides in the pictures, and “LT” at the locations of low tides.
In each picture, put a “M” at the locations where the Moon’s gravitational pull is creating
bulges in the ocean. Put a “S” at the locations where the Sun’s gravitational pull is creating
bulges in the ocean.
Explain what causes neap tides during these phases of the Moon
using the bulge theory of the tides.
The (Z)_______________ has a stronger effect on the tides than the (Z)_______________
and thus determines where the bulges in the ocean are.
During neap tide conditions, the Sun’s gravitational pull moves water towards
(A)_________________________________________________ the Moon.
Thus, the Sun pulls water (B)________________________ the high tide spots (the bulges),
so there is (C)____________________ water in the high tide spots (the bulges),
making the high tides (D)________________________________________.
The Sun pulls water (B)______________________ the low tide spots (outside the bulges),
so there is (C)____________________ water in the low tide spots (outside the bulges),
making the low tides (E)_________________________________________.
Z: Moon, Sun
A: the same places as, different places than
B: towards, away from
C: more, less
D: very high, not very high
E: very low, not very low
Tides Lecture Notes (Topic 7A) – page 9
Why the Tides Change Over a Month: The Timing of Spring and Neap Tides
How long does it take to go from spring tide to neap tides conditions?
Why? Explain using the bulge theory of the tides.
Spring tides occur when the Moon and Sun try to pull water
to (A)____________________________________ places.
This happens during new moon when the Moon and Sun are on
the (B)_____________________ side(s) of the Earth and during full moon
when the Moon and Sun are on (B)_____________________ side(s) of the Earth.
Neap tides occur when the Moon and Sun try to pull water
to (A)____________________________________ places.
This happens when the Moon crosses the path of the Earth’s orbit around the Sun
(half moon, when then Moon, Earth, and Sun form an “L” shape).
It takes the Moon (C)____________________________ to go one time around the Earth.
The phase of the Moon changes each week, the time it takes the Moon to move through
(D)__________ of the Moon’s orbit around the Earth, so it takes (C)_________________
for the tides to switch between spring tide and neap tide conditions.
A: the same, different
B: same, opposite
C: enter an amount of time, like 6 hours, 1 day, 1 week, 2 weeks, 1 month, 6 months, etc.
D: enter a fraction, like 1/2,1/3, 1/4,1/5, etc.
Tides Lecture Notes (Topic 7A) – page 10
Practice Questions
Why are there two high tides and two low tides each day?
• due to the Earth rotation: it spins all the way around once in a day
• due to the orbit of the barycenter around the Earth each day
• due to the orbit of the Moon around the Earth each day
• due to the orbit of the Sun around the Earth each day
• There is a change in the gravitational pull on the ocean over the course of a day
What is the phase of the moon in each of the pictures below?
Is the Earth experiencing spring tide conditions or neap tide conditions?
E
New Moon
Spring Tides
Full Moon
Spring Tides
E
S
#1
S
#3
Half Moon
Neap Tides
E
#2
Half Moon
Neap
Tides
E
S
S
#4
The Moon and Sun pull the water in the ocean towards the same places (the bulges)
during (A)_________________________ tide conditions.
This makes the high tides (B)______________________ (more water in the bulges than normal)
and the low tides (C)_________________________ (less water outside the bulges than normal).
The Moon and Sun try to pull the water in the ocean towards different places
during (A)_________________________ tide conditions.
This makes the high tides (B)_______________________ (less water in the bulges than normal)
and the low tides (C)________________________ (more water outside the bulges than normal).
A: spring, neap
B: higher, not very high
C: lower, not very low
Tides Lecture Notes (Topic 7A) – page 11
What if?
Complete the following sentences.
1. The height of high and low tides is determined by:
______________________________________________________________________.
2. How often the tides switch from high tide to low tide is determined by:
______________________________________________________________________.
3. How often the tides switch from spring tide conditions to neap tide conditions is determined by:
______________________________________________________________________.
Possible Answers:
how fast the Earth rotates (spins)
how fast the Earth orbits the Sun (goes around the Sun)
how fast the Moon orbits the Earth (goes around the Earth)
the strength of the gravitational pull of the Moon and Sun
Tides Lecture Notes (Topic 7A) – page 12
What if? Challenge Questions
Consider each situation below. How would the tides be different?
A. Suppose that the Moon was twice as large.
B. Suppose that the Moon was half as large.
C. Suppose that the Moon was two times closer to the Earth.
D. Suppose that the Moon was two times farther from the Earth.
E. Suppose that the Earth rotated twice as fast.
In other words, suppose that the Earth made one complete revolution in 12 hours.
F. Suppose that the Earth rotated half as fast.
In other words, suppose that the Earth made one complete revolution in 48 hours.
G. Suppose that the Moon orbited the Earth twice as fast. In other words,
suppose that the Moon completed one trip around the Earth in about 14 days.
H. Suppose that the Moon orbited the Earth half as fast. In other words, suppose
that the Moon completed one trip around the Earth in about 56 days.
I. Suppose that the Earth orbited the Sun twice as fast. In other words, suppose
that the Earth completed one trip around the Sun in about 6 months.
J. Suppose that the Earth orbited the Sun half as fast. In other words, suppose
that the Earth completed one trip around the Sun in about 6 months.
Possible Answers:
HT/H - High tides would be higher than they are now.
HT/Not H - High tides would not be as high as they are now.
LT/L - Low tides would be lower than they are now.
LT/Not L - Low tides would not be as low as they are now.
T/Faster - The tide would change from high tide to low tide and back again much more often.
T/Slower - The tide would change from high tide to low tide and back again much less often.
ST/NT/Faster - The tides would change from spring tide conditions to neap tides conditions
and back again much more often.
ST/NT/Slower - The tides would change from spring tide conditions to neap tides conditions
and back again much less often.
None - There would be little or no change in the tides.
Tides Lecture Notes (Topic 7A) – page 13
Challenge Questions
8. Suppose there was no Moon. How would the tides be different?
Explain your reasoning.
9. Suppose that the Earth had two moons, each with exactly
the same size and distance from the Earth, and that both
Moons orbit the Earth once per month and are on opposite
sides of the Earth. (See the diagram in the right.)
Moon
Moon
Earth
How many bulges would there be in the ocean? How would our tides be different?
Explain your reasoning. (For simplicity, ignore the influence of the Sun.)
10. Suppose that the Earth had two moons, each with exactly the same
size and distance from the Earth, and that both Moons orbit the Earth
once per month and one’s location is always 90 degrees behind the
other. (See the diagram on the right.)
Earth
Moons
How many bulges would there be in the ocean?
How would our tides be different?
Explain your reasoning. (For simplicity, ignore the influence of the Sun.)
Tides Lecture Notes (Topic 7A) – page 14
Real Tides
When is the safest time for a ship to enter an estuary (e.g., harbor),
near high tide or near low tide? Why?
What are the best tide conditions for going tide pooling? In other words, should you go
at high tide or low tide? Should you go during a full moon, a half moon, or a new moon?
Explain your reasoning.
You should go tide pooling at (A)__________________ tide
during (B)______________________ tide conditions,
which occur during a (C)__________________________________________ moon.
A: high, low
B: spring, neap
It is better to go tide pooling at this time because:
C: full, half, new
(Mark all that apply.)
• The animals will be more active. For example, you can watch them feed.
• It is easier to see the animals, because there is no water (or less water) in the way.
• It is safer. For example, you are less likely to get knocked over or fall down.
• You can see more animals.
• You will not get wet (or will not get as wet as you would at other times).