Tides are caused by the gravitational pull of the moon and sun on the Earth's oceans. This creates bulges of water on opposite sides of the Earth that result in regular rises and falls of sea level called tides. The moon has a stronger influence than the sun due to its closer proximity. Tides are strongest in coastal areas and weakest in the open ocean. There are typically two high tides and two low tides each day, known as semidiurnal tides, though some locations only experience one of each, called diurnal tides.

1. Tides

2. What are Tides?
• One of the most reliable phenomena in the world.
• Tides are very long period waves that move through the
ocean in response to forces exerted by the moon and to
a lesser extent the sun.
• Originate in the oceans and progress towards the
coastline where they appear as the regular rise and fall
of the ocean surface

3. The Global Ocean
• The Earth is covered by more than 70%
water!!
• All of the oceans are interconnected
• They can move as a “system”

4. High and Low Tide
• When the highest part, or the crest, of the
wave reaches a particular location
high tide occurs.
• When the lowest part, or the trough, of
the wave reaches a particular location
low tide occurs.
• Difference in height between high and low
tide: tidal variation.

5. This is high tide at Douglas (in Juneau, Alaska).
Photo from Daniel Cornwall at Alaskan Librarian.

6. This is Douglas at low tide.
Photo from Daniel Cornwall at Alaskan Librarian.

7. Ebb and Flood Currents
• A horizontal movement of water often
accompanies the rising and falling of the tide:
tidal current
• Incoming tides = flood current
• Outgoing tides = ebb current
• Strongest before or near the time of high/low
tide
• Weakest between high/low tide: slack tides

8. Where are tides strong or weak?
• In the open ocean tidal currents are weak
• Near estuary entrances, narrow straits and
inlets the currents are strong and the
tidal variation is high.

9. Greatest Tidal Variation in the
World: Bay of Fundy, Nova Scotia

11. What causes tides?
• In 1687 Isaac Newton stated that tides
are caused by the gravitational pull of the
moon and sun.
• In regard to tides, the distance between
objects is more critical than their masses.
• FGravity= GM1M2
r2

12. The Sun’s mass is 27 million times that
of the moon, but it is also 390 times
further away from the earth
So although the sun affects our tides, the
moon exerts the greater gravitational
attraction because of its proximity to our
planet.
Solar Tides: tides influenced by
the sun

13. The Main Culprit
• The gravity of the moon “tugs” at the
global oceans 2x more than the sun.
• Lunar Tides: tides affected by the moon

14. Gravity, Inertia and Tidal Bulges
• Inertia is the resistance of any physical
object to a change in its state of motion or
rest.
• The gravitational attraction between the
Earth and Moon is strongest on the side of
the Earth facing the moon (it’s closer)
• This causes water on the “near side” to be
pulled towards the moon
• Inertia attempts to keep the water in
place, but gravity wins creating a bulge.

15. What about the Far Side?
• On the opposite side of the earth the
gravitational attraction is less, it is further
away.
• Here, inertia exceeds the gravitational
force and the water “tries to keep going in
a straight line” moving away from the
Earth forming a bulge on the far side.

16. Tidal Variations
Enhanced Solar Tides: Perihelion (1x per year)
Enhanced Lunar Tides: Perigee (1x per month)
Reduced Solar Tides: Aphelion (1x per year)
Reduced Lunar Tides: Apogee (2x per month)

17. How many tides per day?
• Semidiurnal: 2 high
and 2 low tides every
24 hours and 50
minutes.
• Diurnal: 1 high and 1
low tide, caused by
the a blockage of the
tidal bulge by
continents.

22. Spring Tide
• When the Earth, moon and Sun are in line twice a month
at new and full moons
• Solar Tide pulls in the same direction as the Lunar Tide
• Extremely High Tides and Extremely Low Tides (more
extreme (about 20%) differences)
• Hint: “S”pring tides = “S”traight line up

23. Neap Tide
• When the sun and the moon are at 90 degrees/right
angles
• Solar tide partially cancels out the Lunar Tide
• High tides are not as high and low tides are not as low
(less extreme differences- 20% lower)
• Hint: “N”eap Tides = “N”inety degree angles

24. Spring and Neap Tides
-At what moon phases do these occur?

25. Look for the phases!