What Causes Tides?
What Are Tides?
Have you spent a day on an ocean beach and
noticed that the water comes in closer or goes
out farther during the day?
Have you wondered why the water does this?
What Are Tides?
The water rises and falls because of the tides.
There are two high tides and two low tides in a
little less than 25 hours—just a little more than
one full day.
You arrive at the beach at 9:00 A.M. You lay a towel on
the sand, and then you run 30 steps to reach the water’s
edge. By 3:00 P.M., the water has almost reached your
towel. What do you think happened?
The waves got bigger.
Rainwater filled up the sea.
The tide came in.
What Causes Tides?
Basically, tides are caused by the moon’s
gravity.
The moon pulls water on the Earth toward
itself, causing a bulge on the side of the Earth
near the moon.
Water bulges on the side of
the Earth nearest the moon.
But there’s more to it than that.
What Else Is Happening?
The Earth rotates one full turn in 24 hours, but
the bulge of water stays on the side of the Earth
facing the moon. The bulge stays in place as the
Earth moves under it.
The moon also orbits around the Earth about
every 27 to 28 days.
Earth rotates this way.
Moon orbits this way.
Draw an arrow to show which way the Earth rotates,
then draw another arrow to show which way the moon
orbits around the Earth.
How Else Does the Moon Affect
the Tides?
Earlier we said that it takes almost 25 hours for
a cycle of tides to finish. But why is this so, if a
day is only 24 hours long?
The moon is a little
farther East every day.
This also means that
the water bulge caused
by the moon’s gravity
arrives a little later
every day.
Moon and Earth Move
The Earth must rotate a little more before any
one place is as close to the moon as it was the
day before.
That’s why it takes more than one full
day for the tides to finish a full cycle.
Noon on Tuesday
Earth’s
North Pole
Noon on Monday
Moon orbits this way
On Saturday at the beach, the water is highest at 11:15 A.M.
On Sunday, the water is highest at 11:55 A.M.
Why does the tide come in later?
It’s really the same time, but your watch
is fast, so you think it’s earlier.
The moon has moved a little, so the beach
is closest to the moon at a later time.
The tide always comes in later on Sunday.
Why Are There Two High Tides and
Two Low Tides?
There are really two bulges of water on the Earth.
One bulge is toward the moon. The other bulge is
on the far side of Earth from the moon. Why?
Have you ever ridden a carousel?
Did you feel your body pulled to the outside
as the carousel spun?
The same thing happens to water
on Earth.
The Earth and Moon Really Orbit
Each Other
We say that the moon orbits the Earth, but in
fact, the Earth moves a little too, because of the
moon’s gravity.
Common center
of gravity
The Earth and moon both orbit a
common point.
Some Water is Spun into the
Opposite Direction from the Moon
As the Earth follows the moon’s gravity, some
water on the Earth is pulled in the opposite
direction.
Earth’s movement
Moon’s gravity
pulls some water
toward the moon.
pulls other water
in the opposite
direction.
Where the water bulges are, the tide is high.
Between the bulges, the tide is low. That’s why
there are two high tides and two low tides each day.
You’re on the beach at midnight. The moon is
right overhead. Will the tide be…
…high, because the moon is as close as it
can be to where you are?
…between low and high, because the
tide rises and sets with the Sun?
…low, because the tide is
caused by the Sun’s gravity?
Draw where you would expect the bulges of
water that cause high tides to be.
The sun is rising, but you can still see the moon in the
western sky. Would you expect the tide to be going out?
Yes
No