Tides
Dana Desonie, Ph.D.
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Printed: January 5, 2015
AUTHOR
Dana Desonie, Ph.D.
www.ck12.org
C HAPTER
Chapter 1. Tides
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Tides
• Describe types of tides.
• Explain what causes tides.
Did you ever build a sandcastle?
A sandcastle only lasts until the next tide comes in. To build a sand castle the sand must be moist enough for the
grains to stick together. So it has to be near the water. But that puts the castle in position to be buried by the next
high tide. Oh well. You can build another one when the tide goes back out!
Tides
Tides are daily changes in the level of ocean water. They occur all around the globe. High tides occur when the
water reaches its highest level in a day. Low tides occur when the water reaches its lowest level in a day. Tides keep
cycling from high to low and back again. In most places the water level rises and falls twice a day. So there are two
high tides and two low tides approximately every 24 hours.
Below, you can see the difference between high and low tides ( Figure 1.1). This is called the tidal range.
Why Tides Occur
The figure below shows why tides occur ( Figure 1.2). The main cause of tides is the pull of the Moon’s gravity on
Earth. The pull is greatest on whatever is closest to the Moon. Although the gravity pulls the land, only the water
can move. As a result:
• Water on the side of Earth facing the Moon is pulled hardest by the Moon’s gravity. This causes a bulge of
water on that side of Earth. That bulge is a high tide.
• Earth itself is pulled harder by the Moon’s gravity than is the ocean on the side of Earth opposite the Moon.
As a result, there is bulge of water on the opposite side of Earth. This creates another high tide.
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FIGURE 1.1
Where is the intertidal zone in this picture?
• With water bulging on two sides of Earth, there’s less water left in between. This creates low tides on the other
two sides of the planet.
FIGURE 1.2
High and low tides are due mainly to the
pull of the Moon’s gravity.
Spring Tides and Neap Tides
The Sun’s gravity also pulls on Earth and its oceans. The Sun is much larger than the Moon, so is its pull greater
than the Moon’s? The pull of the Sun’s gravity is much less because the Sun is much farther away. The Sun’s gravity
strengthens or weakens the Moon’s influence on tides.
The figure below shows where the Moon is relative to the Sun at different times during the month ( Figure 1.3). The
positions of the Moon and Sun relative to each other affects the tides. This creates spring tides or neap tides.
• Spring tides occur during the new moon and full moon. The Sun and Moon must either be in a straight line
on the same side of Earth, or they must be on opposite sides of Earth. Their gravitational pull combines to
cause very high and very low tides. Spring tides have the greatest tidal range.
• Neap tides occur during the first and third quarters of the Moon. The Moon and Sun are at right angles to each
other. Their gravity pulls on the oceans in different directions so the highs and lows are not as great. Neap
tides have the smallest tidal range.
This animation shows the effect of the Moon and Sun on the tides: http://www.onr.navy.mil/focus/ocean/motion/t
ides1.htm .
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Chapter 1. Tides
FIGURE 1.3
The Sun and Moon both affect Earth’s
tides.
A detailed animation of lunar tides is shown here: http://www.pbs.org/wgbh/nova/venice/tides.html .
Here is a link to see these tides in motion: http://oceanservice.noaa.gov/education/kits/tides/media/tide06a_450.gif
.
A simple animation of spring and neap tides is found here: http://oceanservice.noaa.gov/education/kits/tides/media/
supp_tide06a.html .
Studying ocean tides’ rhythmic movements helps scientists understand the ocean and the Sun/Moon/Earth system.
This QUEST video explains how tides work, and visits the oldest continually operating tidal gauge in the Western
Hemisphere.
Watch it at: http://www.kqed.org/quest/television/science-on-the-spot-watching-the-tides .
MEDIA
Click image to the left or use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/116518
Summary
• The primary cause of tides is the gravitational attraction of the Moon. This causes two high and two low tides
a day.
• When the Sun’s and Moon’s tides match, there are spring tides. When the two tides are opposed, there are
neap tides.
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• The difference between the daily high and the daily low is the tidal range.
Explore More
Use the resource below to answer the questions that follow.
• Tides at http://www.teachertube.com/viewVideo.php?video_id=655&title=The_Mystery_of_Earth_s_Tides
1.
2.
3.
4.
How often do high tides occur?
What are tides?
What is a tidal bulge?
How is the tidal bulge created?
Review
1. How does the Moon make two high and two low tides a day?
2. How does the Sun make two high and two low tides a day?
3. What causes spring tides? What causes neap tides?
References
1. Dylan Kereluk. High tide and low tide in the Bay of Fundy . CC BY 2.0
2. Hana Zavadska, using Earth image courtesy of NASA, Moon image courtesy of NASA/GSFC/Arizona State
University. Diagram of how the moon causes tides . CC BY-NC 3.0 (Earth and Moon: Public Domain)
3. Zappy’s. The Sun and Moon both affect Earth’s tides . CC BY-NC 3.0
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