1. TidesTides
What are they?What are they?
What are they caused by?What are they caused by?
Are there different kinds of tides?Are there different kinds of tides?

2. • Those of you who have spent some time onThose of you who have spent some time on
the beach have probably noticed that the waterthe beach have probably noticed that the water
level of the ocean does not stay the same alllevel of the ocean does not stay the same all
day long. Although waves continually wash theday long. Although waves continually wash the
shore, the actual water level changes as theshore, the actual water level changes as the
day passes.day passes.

3. • It rises for a period ofIt rises for a period of
hours until it reaches itshours until it reaches its
highest level, and thenhighest level, and then
it begins to fall, orit begins to fall, or
recede, to its lowestrecede, to its lowest
level. This rise and falllevel. This rise and fall
of the ocean is knownof the ocean is known
as theas the tidetide. Its highest. Its highest
and lowest levels are,and lowest levels are,
of course called highof course called high
and low tides.and low tides.

4. High TideHigh Tide Low TideLow Tide

5. Tides – What are they?Tides – What are they?
• Tides are daily changes in the level ofTides are daily changes in the level of
ocean water.ocean water.
• They are the periodic rise and fall of theThey are the periodic rise and fall of the
water level in the oceans and other largewater level in the oceans and other large
bodies of water.bodies of water.
• I wonder how often this periodic rise andI wonder how often this periodic rise and
fall happens each day?fall happens each day?

6. What causes tides?What causes tides?
What moves ocean water?What moves ocean water?
• WindWind
• EarthquakesEarthquakes
• Plate tectonicsPlate tectonics
• The dominant force that moves oceanThe dominant force that moves ocean
water is the gravitational forces from bothwater is the gravitational forces from both
thethe sunsun and theand the moonmoon..

7. Early thoughts about whatEarly thoughts about what
caused the tides.caused the tides.
• Some believed the earth was aSome believed the earth was a
living animal and the tides a resultliving animal and the tides a result
of its breathing.of its breathing.
• Others, who thought the ocean’sOthers, who thought the ocean’s
waters were the earth’s blood,waters were the earth’s blood,
decided the tides were its pulse,decided the tides were its pulse,
caused by the beating of its greatcaused by the beating of its great
heart.heart.

8. Early thoughts about whatEarly thoughts about what
caused the tides.caused the tides.
• Early followers of the religious prophetEarly followers of the religious prophet
Muhammad believed that the rise and fallMuhammad believed that the rise and fall
of the waters were caused when theof the waters were caused when the
“Angel of the Sea” placed his foot in and“Angel of the Sea” placed his foot in and
out of the ocean.out of the ocean.
• Primitive natives thought the tides werePrimitive natives thought the tides were
a sign of a sea god’s anger and madea sign of a sea god’s anger and made
human sacrifices to please him.human sacrifices to please him.

9. The Lure of the MoonThe Lure of the Moon
• The relationship between the phases ofThe relationship between the phases of
the moon and the tides was discoveredthe moon and the tides was discovered
more than 2,000 years ago!more than 2,000 years ago!
• People of the time knew the moon hadPeople of the time knew the moon had
something to do with the tides, but theysomething to do with the tides, but they
couldn’t explain why.couldn’t explain why.

10. Sir Isaac Newton to the Rescue!Sir Isaac Newton to the Rescue!
• Newton’sNewton’s UniversalUniversal
GravitationGravitation theorytheory
helped to explainhelped to explain
how the gravity ofhow the gravity of
the moon (and thethe moon (and the
sun) were relatedsun) were related
to tides.to tides.

11. What causes tides?What causes tides?
• The gravity of the moon pulls onThe gravity of the moon pulls on
everything on the Earth.everything on the Earth.
• The moon’s gravitational pull on liquids isThe moon’s gravitational pull on liquids is
much more noticeable than on solids.much more noticeable than on solids.
• Why? Because liquids move more easilyWhy? Because liquids move more easily
than solids.than solids.

12. What causes tides?What causes tides?
• How often tides occur and the differenceHow often tides occur and the difference
in tidal levels depend on the position of thein tidal levels depend on the position of the
moon as it revolves around the Earth.moon as it revolves around the Earth.
• The moon’s pull is strongest on the part ofThe moon’s pull is strongest on the part of
the Earth directly facing the moon.the Earth directly facing the moon.

13. High TideHigh Tide
• The part of the ocean directly facing theThe part of the ocean directly facing the
moon, bulges towards the moon.moon, bulges towards the moon.
• There is a corresponding bulge on theThere is a corresponding bulge on the
opposite side of the Earth.opposite side of the Earth.
• These bulges are calledThese bulges are called high tide.high tide.

14. Low TideLow Tide
• When high tides occur, water is drawnWhen high tides occur, water is drawn
away from the area between the highaway from the area between the high
tides, which causestides, which causes low tideslow tides to occur.to occur.

15. How often is there a change in tides?How often is there a change in tides?
Try to figure it out . . .Try to figure it out . . .
• High tides happen on opposite sides ofHigh tides happen on opposite sides of
the Earth.the Earth.
• The Earth rotates once on its axis everyThe Earth rotates once on its axis every
24 hours.24 hours.
• How many high tides would any one placeHow many high tides would any one place
in the ocean experience each day?in the ocean experience each day?

16. AnswerAnswer
• The periodic rise andThe periodic rise and
fall of the water levelfall of the water level
in the oceans andin the oceans and
other large bodies ofother large bodies of
water happens . . .water happens . . .
• Twice each day!Twice each day!

17. What effect does the sunWhat effect does the sun
have on tides?have on tides?
• The sun is much larger than the moon.The sun is much larger than the moon.
• The sun is farther away from the EarthThe sun is farther away from the Earth
than the moon.than the moon.
Q: Which one (the sun or the moon) do youQ: Which one (the sun or the moon) do you
think has a bigger effect on tides?think has a bigger effect on tides?
A: The moon, just because it is closer.A: The moon, just because it is closer.

18. Tidal VariationsTidal Variations
• Tidal RangeTidal Range = the difference in levels of= the difference in levels of
ocean water at high tide and low tide.ocean water at high tide and low tide.
• Tidal ranges vary greatly due to theTidal ranges vary greatly due to the
combined forces of the sun and the mooncombined forces of the sun and the moon
on the Earthon the Earth
• It all depends on the position of the Earth,It all depends on the position of the Earth,
moon, and the sun in space.moon, and the sun in space.

19. Spring TideSpring Tide
• A tide with a largeA tide with a large
tidal range that occurstidal range that occurs
two times a monthtwo times a month
(new moon and full(new moon and full
moon)moon)
• Sun, Earth, and theSun, Earth, and the
moon are alignedmoon are aligned

20. Neap TidesNeap Tides
• A tide of minimumA tide of minimum
range that occursrange that occurs
during the first andduring the first and
third quarters of thethird quarters of the
moonmoon
• The sun, Earth, andThe sun, Earth, and
moon form a 90moon form a 90
degree angledegree angle

21. Weird Science!Weird Science!
• The moon also creates tides in ourThe moon also creates tides in our
atmosphereatmosphere
• They are calledThey are called lunar windslunar winds
• They can be detected by studying slightThey can be detected by studying slight
periodic fluctuations in weather pattersperiodic fluctuations in weather patters
• Very strange, indeed!Very strange, indeed!

22. ReviewReview
Q:Q: What are tides?What are tides?
• TideTide = the periodic rise and fall of the water level= the periodic rise and fall of the water level
in the oceans and other large bodies of water.in the oceans and other large bodies of water.
Q:Q: What is the dominant force that moves oceanWhat is the dominant force that moves ocean
water?water?
• The gravitational forces from both theThe gravitational forces from both the sunsun andand
thethe moonmoon..

23. Review (cont’d)Review (cont’d)
Q:Q: Where is the moon’s pull is strongest onWhere is the moon’s pull is strongest on
the Earth?the Earth?
• The moon’s pull is strongest on the part ofThe moon’s pull is strongest on the part of
the Earth directly facing the moon.the Earth directly facing the moon.

24. Review (cont’d)Review (cont’d)
Q:Q: Which one (the sun or the moon) has aWhich one (the sun or the moon) has a
bigger effect on tides?bigger effect on tides?
• The moonThe moon

25. Review (cont’d)Review (cont’d)
Q:Q: What is the difference in levels of oceanWhat is the difference in levels of ocean
water at high tide and low tide called?water at high tide and low tide called?
• Tidal RangeTidal Range
Q:Q: Tidal ranges vary greatly due to theTidal ranges vary greatly due to the
combined forces of the _________ and thecombined forces of the _________ and the
________ on the Earth________ on the Earth
• Sun, moonSun, moon

26. Review (cont’d)Review (cont’d)
Q:Q: How are the sun,How are the sun,
Earth, and moonEarth, and moon
situated in space sosituated in space so
that we experience athat we experience a
spring tide?spring tide?
• Sun, Earth, and theSun, Earth, and the
moon are alignedmoon are aligned

27. Review (cont’d)Review (cont’d)
Q:Q: How are the sun,How are the sun,
Earth, and moonEarth, and moon
situated in space sosituated in space so
that we experience athat we experience a
neap tide?neap tide?
• The sun, Earth, andThe sun, Earth, and
moon form a 90moon form a 90
degree angledegree angle