Tides

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

The Global Ocean

• The Earth is covered by more than 70% water!!

• All of the oceans are interconnected

• They can move as a “system”

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.

This is high tide at Douglas (in Juneau, Alaska).

                                                                                                                                                                                

Photo from Daniel Cornwall at Alaskan Librarian.

This is Douglas at low tide.

                                                                                                                                                      

Photo from Daniel Cornwall at Alaskan Librarian.

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

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.

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

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

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

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

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.

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.

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)

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.

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

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

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

Look for the phases!